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Network Working Group T. Dierks Request for Comments: 4346 Independent Obsoletes: 2246 E. Rescorla Category: Standards Track RTFM, Inc. April 2006 The Transport Layer Security (TLS) Protocol Version 1.1 Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document specifies Version 1.1 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. Dierks & Rescorla Standards Track [Page 1]
RFC 4346 The TLS Protocol April 2006 Table of Contents 1. Introduction ....................................................4 1.1. Differences from TLS 1.0 ...................................5 1.2. Requirements Terminology ...................................5 2. Goals ...........................................................5 3. Goals of This Document ..........................................6 4. Presentation Language ...........................................6 4.1. Basic Block Size ...........................................7 4.2. Miscellaneous ..............................................7 4.3. Vectors ....................................................7 4.4. Numbers ....................................................8 4.5. Enumerateds ................................................8 4.6. Constructed Types ..........................................9 4.6.1. Variants ...........................................10 4.7. Cryptographic Attributes ..................................11 4.8. Constants .................................................12 5. HMAC and the Pseudorandom Function .............................12 6. The TLS Record Protocol ........................................14 6.1. Connection States .........................................15 6.2. Record layer ..............................................17 6.2.1. Fragmentation ......................................17 6.2.2. Record Compression and Decompression ...............19 6.2.3. Record Payload Protection ..........................19 6.2.3.1. Null or Standard Stream Cipher ............20 6.2.3.2. CBC Block Cipher ..........................21 6.3. Key Calculation ...........................................24 7. The TLS Handshaking Protocols ..................................24 7.1. Change Cipher Spec Protocol ...............................25 7.2. Alert Protocol ............................................26 7.2.1. Closure Alerts .....................................27 7.2.2. Error Alerts .......................................28 7.3. Handshake Protocol Overview ...............................31 7.4. Handshake Protocol ........................................34 7.4.1. Hello Messages .....................................35 7.4.1.1. Hello request .............................35 7.4.1.2. Client Hello ..............................36 7.4.1.3. Server Hello ..............................39 7.4.2. Server Certificate .................................40 7.4.3. Server Key Exchange Message ........................42 7.4.4. Certificate request ................................44 7.4.5. Server Hello Done ..................................46 7.4.6. Client certificate .................................46 7.4.7. Client Key Exchange Message ........................47 7.4.7.1. RSA Encrypted Premaster Secret Message ....47 7.4.7.2. Client Diffie-Hellman Public Value ........50 7.4.8. Certificate verify .................................50 7.4.9. Finished ...........................................51 Dierks & Rescorla Standards Track [Page 2]
RFC 4346 The TLS Protocol April 2006 8. Cryptographic Computations .....................................52 8.1. Computing the Master Secret ...............................52 8.1.1. RSA ................................................53 8.1.2. Diffie-Hellman .....................................53 9. Mandatory Cipher Suites ........................................53 10. Application Data Protocol .....................................53 11. Security Considerations .......................................53 12. IANA Considerations ...........................................54 A. Appendix - Protocol constant values ............................55 A.1. Record layer .........................................55 A.2. Change cipher specs message ..........................56 A.3. Alert messages .......................................56 A.4. Handshake protocol ...................................57 A.4.1. Hello messages .....................................57 A.4.2. Server authentication and key exchange messages ....58 A.4.3. Client authentication and key exchange messages ....59 A.4.4.Handshake finalization message ......................60 A.5. The CipherSuite ......................................60 A.6. The Security Parameters ..............................63 B. Appendix - Glossary ............................................64 C. Appendix - CipherSuite definitions .............................68 D. Appendix - Implementation Notes ................................69 D.1 Random Number Generation and Seeding ..................70 D.2 Certificates and authentication .......................70 D.3 CipherSuites ..........................................70 E. Appendix - Backward Compatibility With SSL .....................71 E.1. Version 2 client hello ...............................72 E.2. Avoiding man-in-the-middle version rollback ..........74 F. Appendix - Security analysis ...................................74 F.1. Handshake protocol ...................................74 F.1.1. Authentication and key exchange ....................74 F.1.1.1. Anonymous key exchange ...........................75 F.1.1.2. RSA key exchange and authentication ..............75 F.1.1.3. Diffie-Hellman key exchange with authentication ..76 F.1.2. Version rollback attacks ...........................77 F.1.3. Detecting attacks against the handshake protocol ...77 F.1.4. Resuming sessions ..................................78 F.1.5. MD5 and SHA ........................................78 F.2. Protecting application data ..........................78 F.3. Explicit IVs .........................................79 F.4 Security of Composite Cipher Modes ...................79 F.5 Denial of Service ....................................80 F.6. Final notes ..........................................80 Normative References ..............................................81 Informative References ............................................82 Dierks & Rescorla Standards Track [Page 3]
RFC 4346 The TLS Protocol April 2006 1. Introduction The primary goal of the TLS Protocol is to provide privacy and data integrity between two communicating applications. The protocol is composed of two layers: the TLS Record Protocol and the TLS Handshake Protocol. At the lowest level, layered on top of some reliable transport protocol (e.g., TCP[TCP]), is the TLS Record Protocol. The TLS Record Protocol provides connection security that has two basic properties: - The connection is private. Symmetric cryptography is used for data encryption (e.g., DES [DES], RC4 [SCH] etc.). The keys for this symmetric encryption are generated uniquely for each connection and are based on a secret negotiated by another protocol (such as the TLS Handshake Protocol). The Record Protocol can also be used without encryption. - The connection is reliable. Message transport includes a message integrity check using a keyed MAC. Secure hash functions (e.g., SHA, MD5, etc.) are used for MAC computations. The Record Protocol can operate without a MAC, but is generally only used in this mode while another protocol is using the Record Protocol as a transport for negotiating security parameters. The TLS Record Protocol is used for encapsulation of various higher- level protocols. One such encapsulated protocol, the TLS Handshake Protocol, allows the server and client to authenticate each other and to negotiate an encryption algorithm and cryptographic keys before the application protocol transmits or receives its first byte of data. The TLS Handshake Protocol provides connection security that has three basic properties: - The peer's identity can be authenticated using asymmetric, or public key, cryptography (e.g., RSA [RSA], DSS [DSS], etc.). This authentication can be made optional, but is generally required for at least one of the peers. - The negotiation of a shared secret is secure: the negotiated secret is unavailable to eavesdroppers, and for any authenticated connection the secret cannot be obtained, even by an attacker who can place himself in the middle of the connection. - The negotiation is reliable: no attacker can modify the negotiation communication without being detected by the parties to the communication. One advantage of TLS is that it is application protocol independent. Higher level protocols can layer on top of the TLS Protocol Dierks & Rescorla Standards Track [Page 4]
RFC 4346 The TLS Protocol April 2006 transparently. The TLS standard, however, does not specify how protocols add security with TLS; the decisions on how to initiate TLS handshaking and how to interpret the authentication certificates exchanged are left to the judgment of the designers and implementors of protocols that run on top of TLS. 1.1. Differences from TLS 1.0 This document is a revision of the TLS 1.0 [TLS1.0] protocol, and contains some small security improvements, clarifications, and editorial improvements. The major changes are: - The implicit Initialization Vector (IV) is replaced with an explicit IV to protect against CBC attacks [CBCATT]. - Handling of padding errors is changed to use the bad_record_mac alert rather than the decryption_failed alert to protect against CBC attacks. - IANA registries are defined for protocol parameters. - Premature closes no longer cause a session to be nonresumable. - Additional informational notes were added for various new attacks on TLS. In addition, a number of minor clarifications and editorial improvements were made. 1.2. Requirements Terminology In this document, the keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and "MAY" are to be interpreted as described in RFC 2119 [REQ]. 2. Goals The goals of TLS Protocol, in order of their priority, are as follows: 1. Cryptographic security: TLS should be used to establish a secure connection between two parties. 2. Interoperability: Independent programmers should be able to develop applications utilizing TLS that can successfully exchange cryptographic parameters without knowledge of one another's code. Dierks & Rescorla Standards Track [Page 5]
RFC 4346 The TLS Protocol April 2006 3. Extensibility: TLS seeks to provide a framework into which new public key and bulk encryption methods can be incorporated as necessary. This will also accomplish two sub-goals: preventing the need to create a new protocol (and risking the introduction of possible new weaknesses) and avoiding the need to implement an entire new security library. 4. Relative efficiency: Cryptographic operations tend to be highly CPU intensive, particularly public key operations. For this reason, the TLS protocol has incorporated an optional session caching scheme to reduce the number of connections that need to be established from scratch. Additionally, care has been taken to reduce network activity. 3. Goals of This Document This document and the TLS protocol itself are based on the SSL 3.0 Protocol Specification as published by Netscape. The differences between this protocol and SSL 3.0 are not dramatic, but they are significant enough that TLS 1.1, TLS 1.0, and SSL 3.0 do not interoperate (although each protocol incorporates a mechanism by which an implementation can back down prior versions). This document is intended primarily for readers who will be implementing the protocol and for those doing cryptographic analysis of it. The specification has been written with this in mind, and it is intended to reflect the needs of those two groups. For that reason, many of the algorithm-dependent data structures and rules are included in the body of the text (as opposed to in an appendix), providing easier access to them. This document is not intended to supply any details of service definition or of interface definition, although it does cover select areas of policy as they are required for the maintenance of solid security. 4. Presentation Language This document deals with the formatting of data in an external representation. The following very basic and somewhat casually defined presentation syntax will be used. The syntax draws from several sources in its structure. Although it resembles the programming language "C" in its syntax and XDR [XDR] in both its syntax and intent, it would be risky to draw too many parallels. The purpose of this presentation language is to document TLS only; it has no general application beyond that particular goal. Dierks & Rescorla Standards Track [Page 6]
RFC 4346 The TLS Protocol April 2006 4.1. Basic Block Size The representation of all data items is explicitly specified. The basic data block size is one byte (i.e., 8 bits). Multiple byte data items are concatenations of bytes, from left to right, from top to bottom. From the bytestream, a multi-byte item (a numeric in the example) is formed (using C notation) by: value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) | ... | byte[n-1]; This byte ordering for multi-byte values is the commonplace network byte order or big endian format. 4.2. Miscellaneous Comments begin with "/*" and end with "*/". Optional components are denoted by enclosing them in "[[ ]]" double brackets. Single-byte entities containing uninterpreted data are of type opaque. 4.3. Vectors A vector (single dimensioned array) is a stream of homogeneous data elements. The size of the vector may be specified at documentation time or left unspecified until runtime. In either case, the length declares the number of bytes, not the number of elements, in the vector. The syntax for specifying a new type, T', that is a fixed- length vector of type T is T T'[n]; Here, T' occupies n bytes in the data stream, where n is a multiple of the size of T. The length of the vector is not included in the encoded stream. In the following example, Datum is defined to be three consecutive bytes that the protocol does not interpret, while Data is three consecutive Datum, consuming a total of nine bytes. opaque Datum[3]; /* three uninterpreted bytes */ Datum Data[9]; /* 3 consecutive 3 byte vectors */ Variable-length vectors are defined by specifying a subrange of legal lengths, inclusively, using the notation <floor..ceiling>. When Dierks & Rescorla Standards Track [Page 7]
RFC 4346 The TLS Protocol April 2006 these are encoded, the actual length precedes the vector's contents in the byte stream. The length will be in the form of a number consuming as many bytes as required to hold the vector's specified maximum (ceiling) length. A variable-length vector with an actual length field of zero is referred to as an empty vector. T T'<floor..ceiling>; In the following example, mandatory is a vector that must contain between 300 and 400 bytes of type opaque. It can never be empty. The actual length field consumes two bytes, a uint16, sufficient to represent the value 400 (see Section 4.4). On the other hand, longer can represent up to 800 bytes of data, or 400 uint16 elements, and it may be empty. Its encoding will include a two-byte actual length field prepended to the vector. The length of an encoded vector must be an even multiple of the length of a single element (for example, a 17-byte vector of uint16 would be illegal). opaque mandatory<300..400>; /* length field is 2 bytes, cannot be empty */ uint16 longer<0..800>; /* zero to 400 16-bit unsigned integers */ 4.4. Numbers The basic numeric data type is an unsigned byte (uint8). All larger numeric data types are formed from fixed-length series of bytes concatenated as described in Section 4.1 and are also unsigned. The following numeric types are predefined. uint8 uint16[2]; uint8 uint24[3]; uint8 uint32[4]; uint8 uint64[8]; All values, here and elsewhere in the specification, are stored in "network" or "big-endian" order; the uint32 represented by the hex bytes 01 02 03 04 is equivalent to the decimal value 16909060. 4.5. Enumerateds An additional sparse data type is available called enum. A field of type enum can only assume the values declared in the definition. Each definition is a different type. Only enumerateds of the same type may be assigned or compared. Every element of an enumerated must be assigned a value, as demonstrated in the following example. Since the elements of the enumerated are not ordered, they can be assigned any unique value, in any order. Dierks & Rescorla Standards Track [Page 8]
RFC 4346 The TLS Protocol April 2006 enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te; Enumerateds occupy as much space in the byte stream as would its maximal defined ordinal value. The following definition would cause one byte to be used to carry fields of type Color. enum { red(3), blue(5), white(7) } Color; One may optionally specify a value without its associated tag to force the width definition without defining a superfluous element. In the following example, Taste will consume two bytes in the data stream but can only assume the values 1, 2, or 4. enum { sweet(1), sour(2), bitter(4), (32000) } Taste; The names of the elements of an enumeration are scoped within the defined type. In the first example, a fully qualified reference to the second element of the enumeration would be Color.blue. Such qualification is not required if the target of the assignment is well specified. Color color = Color.blue; /* overspecified, legal */ Color color = blue; /* correct, type implicit */ For enumerateds that are never converted to external representation, the numerical information may be omitted. enum { low, medium, high } Amount; 4.6. Constructed Types Structure types may be constructed from primitive types for convenience. Each specification declares a new, unique type. The syntax for definition is much like that of C. struct { T1 f1; T2 f2; ... Tn fn; } [[T]]; The fields within a structure may be qualified using the type's name, with a syntax much like that available for enumerateds. For example, T.f2 refers to the second field of the previous declaration. Structure definitions may be embedded. Dierks & Rescorla Standards Track [Page 9]
RFC 4346 The TLS Protocol April 2006 4.6.1. Variants Defined structures may have variants based on some knowledge that is available within the environment. The selector must be an enumerated type that defines the possible variants the structure defines. There must be a case arm for every element of the enumeration declared in the select. The body of the variant structure may be given a label for reference. The mechanism by which the variant is selected at runtime is not prescribed by the presentation language. struct { T1 f1; T2 f2; .... Tn fn; select (E) { case e1: Te1; case e2: Te2; .... case en: Ten; } [[fv]]; } [[Tv]]; For example: enum { apple, orange } VariantTag; struct { uint16 number; opaque string<0..10>; /* variable length */ } V1; struct { uint32 number; opaque string[10]; /* fixed length */ } V2; struct { select (VariantTag) { /* value of selector is implicit */ case apple: V1; /* VariantBody, tag = apple */ case orange: V2; /* VariantBody, tag = orange */ } variant_body; /* optional label on variant */ } VariantRecord; Variant structures may be qualified (narrowed) by specifying a value for the selector prior to the type. For example, an orange VariantRecord is a narrowed type of a VariantRecord containing a variant_body of type V2. Dierks & Rescorla Standards Track [Page 10]
RFC 4346 The TLS Protocol April 2006 4.7. Cryptographic Attributes The four cryptographic operations digital signing, stream cipher encryption, block cipher encryption, and public key encryption are designated digitally-signed, stream-ciphered, block-ciphered, and public-key-encrypted, respectively. A field's cryptographic processing is specified by prepending an appropriate key word designation before the field's type specification. Cryptographic keys are implied by the current session state (see Section 6.1). In digital signing, one-way hash functions are used as input for a signing algorithm. A digitally-signed element is encoded as an opaque vector <0..2^16-1>, where the length is specified by the signing algorithm and key. In RSA signing, a 36-byte structure of two hashes (one SHA and one MD5) is signed (encrypted with the private key). It is encoded with PKCS #1 block type 1, as described in [PKCS1A]. Note: The standard reference for PKCS#1 is now RFC 3447 [PKCS1B]. However, to minimize differences with TLS 1.0 text, we are using the terminology of RFC 2313 [PKCS1A]. In DSS, the 20 bytes of the SHA hash are run directly through the Digital Signing Algorithm with no additional hashing. This produces two values, r and s. The DSS signature is an opaque vector, as above, the contents of which are the DER encoding of: Dss-Sig-Value ::= SEQUENCE { r INTEGER, s INTEGER } In stream cipher encryption, the plaintext is exclusive-ORed with an identical amount of output generated from a cryptographically secure keyed pseudorandom number generator. In block cipher encryption, every block of plaintext encrypts to a block of ciphertext. All block cipher encryption is done in CBC (Cipher Block Chaining) mode, and all items that are block-ciphered will be an exact multiple of the cipher block length. In public key encryption, a public key algorithm is used to encrypt data in such a way that it can be decrypted only with the matching private key. A public-key-encrypted element is encoded as an opaque vector <0..2^16-1>, where the length is specified by the signing algorithm and key. Dierks & Rescorla Standards Track [Page 11]
RFC 4346 The TLS Protocol April 2006 An RSA-encrypted value is encoded with PKCS #1 block type 2, as described in [PKCS1A]. In the following example, stream-ciphered struct { uint8 field1; uint8 field2; digitally-signed opaque hash[20]; } UserType; the contents of hash are used as input for the signing algorithm, and then the entire structure is encrypted with a stream cipher. The length of this structure, in bytes, would be equal to two bytes for field1 and field2, plus two bytes for the length of the signature, plus the length of the output of the signing algorithm. This is known because the algorithm and key used for the signing are known prior to encoding or decoding this structure. 4.8. Constants Typed constants can be defined for purposes of specification by declaring a symbol of the desired type and assigning values to it. Under-specified types (opaque, variable length vectors, and structures that contain opaque) cannot be assigned values. No fields of a multi-element structure or vector may be elided. For example: struct { uint8 f1; uint8 f2; } Example1; Example1 ex1 = {1, 4}; /* assigns f1 = 1, f2 = 4 */ 5. HMAC and the Pseudorandom Function A number of operations in the TLS record and handshake layer require a keyed MAC; this is a secure digest of some data protected by a secret. Forging the MAC is infeasible without knowledge of the MAC secret. The construction we use for this operation is known as HMAC, and is described in [HMAC]. HMAC can be used with a variety of different hash algorithms. TLS uses it in the handshake with two different algorithms, MD5 and SHA- 1, denoting these as HMAC_MD5(secret, data) and HMAC_SHA(secret, data). Additional hash algorithms can be defined by cipher suites Dierks & Rescorla Standards Track [Page 12]
RFC 4346 The TLS Protocol April 2006 and used to protect record data, but MD5 and SHA-1 are hard coded into the description of the handshaking for this version of the protocol. In addition, a construction is required to do expansion of secrets into blocks of data for the purposes of key generation or validation. This pseudo-random function (PRF) takes as input a secret, a seed, and an identifying label and produces an output of arbitrary length. In order to make the PRF as secure as possible, it uses two hash algorithms in a way that should guarantee its security if either algorithm remains secure. First, we define a data expansion function, P_hash(secret, data) that uses a single hash function to expand a secret and seed into an arbitrary quantity of output: P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) + HMAC_hash(secret, A(2) + seed) + HMAC_hash(secret, A(3) + seed) + ... Where + indicates concatenation. A() is defined as: A(0) = seed A(i) = HMAC_hash(secret, A(i-1)) P_hash can be iterated as many times as is necessary to produce the required quantity of data. For example, if P_SHA-1 is being used to create 64 bytes of data, it will have to be iterated 4 times (through A(4)), creating 80 bytes of output data; the last 16 bytes of the final iteration will then be discarded, leaving 64 bytes of output data. TLS's PRF is created by splitting the secret into two halves and using one half to generate data with P_MD5 and the other half to generate data with P_SHA-1, then exclusive-ORing the outputs of these two expansion functions together. S1 and S2 are the two halves of the secret, and each is the same length. S1 is taken from the first half of the secret, S2 from the second half. Their length is created by rounding up the length of the overall secret, divided by two; thus, if the original secret is an odd number of bytes long, the last byte of S1 will be the same as the first byte of S2. Dierks & Rescorla Standards Track [Page 13]
RFC 4346 The TLS Protocol April 2006 L_S = length in bytes of secret; L_S1 = L_S2 = ceil(L_S / 2); The secret is partitioned into two halves (with the possibility of one shared byte) as described above, S1 taking the first L_S1 bytes, and S2 the last L_S2 bytes. The PRF is then defined as the result of mixing the two pseudorandom streams by exclusive-ORing them together. PRF(secret, label, seed) = P_MD5(S1, label + seed) XOR P_SHA-1(S2, label + seed); The label is an ASCII string. It should be included in the exact form it is given without a length byte or trailing null character. For example, the label "slithy toves" would be processed by hashing the following bytes: 73 6C 69 74 68 79 20 74 6F 76 65 73 Note that because MD5 produces 16-byte outputs and SHA-1 produces 20-byte outputs, the boundaries of their internal iterations will not be aligned. Generating an 80-byte output will require that P_MD5 iterate through A(5), while P_SHA-1 will only iterate through A(4). 6. The TLS Record Protocol The TLS Record Protocol is a layered protocol. At each layer, messages may include fields for length, description, and content. The Record Protocol takes messages to be transmitted, fragments the data into manageable blocks, optionally compresses the data, applies a MAC, encrypts, and transmits the result. Received data is decrypted, verified, decompressed, reassembled, and then delivered to higher-level clients. Four record protocol clients are described in this document: the handshake protocol, the alert protocol, the change cipher spec protocol, and the application data protocol. In order to allow extension of the TLS protocol, additional record types can be supported by the record protocol. Any new record types SHOULD allocate type values immediately beyond the ContentType values for the four record types described here (see Appendix A.1). All such values must be defined by RFC 2434 Standards Action. See Section 11 for IANA Considerations for ContentType values. If a TLS implementation receives a record type it does not understand, it SHOULD just ignore it. Any protocol designed for use Dierks & Rescorla Standards Track [Page 14]
RFC 4346 The TLS Protocol April 2006 over TLS MUST be carefully designed to deal with all possible attacks against it. Note that because the type and length of a record are not protected by encryption, care SHOULD be taken to minimize the value of traffic analysis of these values. 6.1. Connection States A TLS connection state is the operating environment of the TLS Record Protocol. It specifies a compression algorithm, and encryption algorithm, and a MAC algorithm. In addition, the parameters for these algorithms are known: the MAC secret and the bulk encryption keys for the connection in both the read and the write directions. Logically, there are always four connection states outstanding: the current read and write states, and the pending read and write states. All records are processed under the current read and write states. The security parameters for the pending states can be set by the TLS Handshake Protocol, and the Change Cipher Spec can selectively make either of the pending states current, in which case the appropriate current state is disposed of and replaced with the pending state; the pending state is then reinitialized to an empty state. It is illegal to make a state that has not been initialized with security parameters a current state. The initial current state always specifies that no encryption, compression, or MAC will be used. The security parameters for a TLS Connection read and write state are set by providing the following values: connection end Whether this entity is considered the "client" or the "server" in this connection. bulk encryption algorithm An algorithm to be used for bulk encryption. This specification includes the key size of this algorithm, how much of that key is secret, whether it is a block or stream cipher, and the block size of the cipher (if appropriate). MAC algorithm An algorithm to be used for message authentication. This specification includes the size of the hash returned by the MAC algorithm. compression algorithm An algorithm to be used for data compression. This specification must include all information the algorithm requires compression. master secret A 48-byte secret shared between the two peers in the connection. Dierks & Rescorla Standards Track [Page 15]
RFC 4346 The TLS Protocol April 2006 client random A 32-byte value provided by the client. server random A 32-byte value provided by the server. These parameters are defined in the presentation language as: enum { server, client } ConnectionEnd; enum { null, rc4, rc2, des, 3des, des40, idea, aes } BulkCipherAlgorithm; enum { stream, block } CipherType; enum { null, md5, sha } MACAlgorithm; enum { null(0), (255) } CompressionMethod; /* The algorithms specified in CompressionMethod, BulkCipherAlgorithm, and MACAlgorithm may be added to. */ struct { ConnectionEnd entity; BulkCipherAlgorithm bulk_cipher_algorithm; CipherType cipher_type; uint8 key_size; uint8 key_material_length; MACAlgorithm mac_algorithm; uint8 hash_size; CompressionMethod compression_algorithm; opaque master_secret[48]; opaque client_random[32]; opaque server_random[32]; } SecurityParameters; The record layer will use the security parameters to generate the following four items: client write MAC secret server write MAC secret client write key server write key The client write parameters are used by the server when receiving and processing records and vice-versa. The algorithm used for generating these items from the security parameters is described in Section 6.3. Dierks & Rescorla Standards Track [Page 16]
RFC 4346 The TLS Protocol April 2006 Once the security parameters have been set and the keys have been generated, the connection states can be instantiated by making them the current states. These current states MUST be updated for each record processed. Each connection state includes the following elements: compression state The current state of the compression algorithm. cipher state The current state of the encryption algorithm. This will consist of the scheduled key for that connection. For stream ciphers, this will also contain whatever state information is necessary to allow the stream to continue to encrypt or decrypt data. MAC secret The MAC secret for this connection, as generated above. sequence number Each connection state contains a sequence number, which is maintained separately for read and write states. The sequence number MUST be set to zero whenever a connection state is made the active state. Sequence numbers are of type uint64 and may not exceed 2^64-1. Sequence numbers do not wrap. If a TLS implementation would need to wrap a sequence number, it must renegotiate instead. A sequence number is incremented after each record: specifically, the first record transmitted under a particular connection state MUST use sequence number 0. 6.2. Record layer The TLS Record Layer receives uninterpreted data from higher layers in non-empty blocks of arbitrary size. 6.2.1. Fragmentation The record layer fragments information blocks into TLSPlaintext records carrying data in chunks of 2^14 bytes or less. Client message boundaries are not preserved in the record layer (i.e., multiple client messages of the same ContentType MAY be coalesced into a single TLSPlaintext record, or a single message MAY be fragmented across several records). Dierks & Rescorla Standards Track [Page 17]
RFC 4346 The TLS Protocol April 2006 struct { uint8 major, minor; } ProtocolVersion; enum { change_cipher_spec(20), alert(21), handshake(22), application_data(23), (255) } ContentType; struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[TLSPlaintext.length]; } TLSPlaintext; type The higher-level protocol used to process the enclosed fragment. version The version of the protocol being employed. This document describes TLS Version 1.1, which uses the version { 3, 2 }. The version value 3.2 is historical: TLS version 1.1 is a minor modification to the TLS 1.0 protocol, which was itself a minor modification to the SSL 3.0 protocol, which bears the version value 3.0. (See Appendix A.1.) length The length (in bytes) of the following TLSPlaintext.fragment. The length should not exceed 2^14. fragment The application data. This data is transparent and is treated as an independent block to be dealt with by the higher-level protocol specified by the type field. Note: Data of different TLS Record layer content types MAY be interleaved. Application data is generally of lower precedence for transmission than other content types. However, records MUST be delivered to the network in the same order as they are protected by the record layer. Recipients MUST receive and process interleaved application layer traffic during handshakes subsequent to the first one on a connection. Dierks & Rescorla Standards Track [Page 18]
RFC 4346 The TLS Protocol April 2006 6.2.2. Record Compression and Decompression All records are compressed using the compression algorithm defined in the current session state. There is always an active compression algorithm; however, initially it is defined as CompressionMethod.null. The compression algorithm translates a TLSPlaintext structure into a TLSCompressed structure. Compression functions are initialized with default state information whenever a connection state is made active. Compression must be lossless and may not increase the content length by more than 1024 bytes. If the decompression function encounters a TLSCompressed.fragment that would decompress to a length in excess of 2^14 bytes, it should report a fatal decompression failure error. struct { ContentType type; /* same as TLSPlaintext.type */ ProtocolVersion version;/* same as TLSPlaintext.version */ uint16 length; opaque fragment[TLSCompressed.length]; } TLSCompressed; length The length (in bytes) of the following TLSCompressed.fragment. The length should not exceed 2^14 + 1024. fragment The compressed form of TLSPlaintext.fragment. Note: A CompressionMethod.null operation is an identity operation; no fields are altered. Implementation note: Decompression functions are responsible for ensuring that messages cannot cause internal buffer overflows. 6.2.3. Record Payload Protection The encryption and MAC functions translate a TLSCompressed structure into a TLSCiphertext. The decryption functions reverse the process. The MAC of the record also includes a sequence number so that missing, extra, or repeated messages are detectable. Dierks & Rescorla Standards Track [Page 19]
RFC 4346 The TLS Protocol April 2006 struct { ContentType type; ProtocolVersion version; uint16 length; select (CipherSpec.cipher_type) { case stream: GenericStreamCipher; case block: GenericBlockCipher; } fragment; } TLSCiphertext; type The type field is identical to TLSCompressed.type. version The version field is identical to TLSCompressed.version. length The length (in bytes) of the following TLSCiphertext.fragment. The length may not exceed 2^14 + 2048. fragment The encrypted form of TLSCompressed.fragment, with the MAC. 6.2.3.1. Null or Standard Stream Cipher Stream ciphers (including BulkCipherAlgorithm.null, see Appendix A.6) convert TLSCompressed.fragment structures to and from stream TLSCiphertext.fragment structures. stream-ciphered struct { opaque content[TLSCompressed.length]; opaque MAC[CipherSpec.hash_size]; } GenericStreamCipher; The MAC is generated as: HMAC_hash(MAC_write_secret, seq_num + TLSCompressed.type + TLSCompressed.version + TLSCompressed.length + TLSCompressed.fragment)); where "+" denotes concatenation. seq_num The sequence number for this record. hash The hashing algorithm specified by SecurityParameters.mac_algorithm. Dierks & Rescorla Standards Track [Page 20]
RFC 4346 The TLS Protocol April 2006 Note that the MAC is computed before encryption. The stream cipher encrypts the entire block, including the MAC. For stream ciphers that do not use a synchronization vector (such as RC4), the stream cipher state from the end of one record is simply used on the subsequent packet. If the CipherSuite is TLS_NULL_WITH_NULL_NULL, encryption consists of the identity operation (i.e., the data is not encrypted, and the MAC size is zero, implying that no MAC is used). TLSCiphertext.length is TLSCompressed.length plus CipherSpec.hash_size. 6.2.3.2. CBC Block Cipher For block ciphers (such as RC2, DES, or AES), the encryption and MAC functions convert TLSCompressed.fragment structures to and from block TLSCiphertext.fragment structures. block-ciphered struct { opaque IV[CipherSpec.block_length]; opaque content[TLSCompressed.length]; opaque MAC[CipherSpec.hash_size]; uint8 padding[GenericBlockCipher.padding_length]; uint8 padding_length; } GenericBlockCipher; The MAC is generated as described in Section 6.2.3.1. IV Unlike previous versions of SSL and TLS, TLS 1.1 uses an explicit IV in order to prevent the attacks described by [CBCATT]. We recommend the following equivalently strong procedures. For clarity we use the following notation. IV The transmitted value of the IV field in the GenericBlockCipher structure. CBC residue The last ciphertext block of the previous record. mask The actual value that the cipher XORs with the plaintext prior to encryption of the first cipher block of the record. In prior versions of TLS, there was no IV field and the CBC residue and mask were one and the same. See Sections 6.1, 6.2.3.2, and 6.3, of [TLS1.0] for details of TLS 1.0 IV handling. Dierks & Rescorla Standards Track [Page 21]
RFC 4346 The TLS Protocol April 2006 One of the following two algorithms SHOULD be used to generate the per-record IV: (1) Generate a cryptographically strong random string R of length CipherSpec.block_length. Place R in the IV field. Set the mask to R. Thus, the first cipher block will be encrypted as E(R XOR Data). (2) Generate a cryptographically strong random number R of length CipherSpec.block_length and prepend it to the plaintext prior to encryption. In this case either: (a) The cipher may use a fixed mask such as zero. (b) The CBC residue from the previous record may be used as the mask. This preserves maximum code compatibility with TLS 1.0 and SSL 3. It also has the advantage that it does not require the ability to quickly reset the IV, which is known to be a problem on some systems. In either (2)(a) or (2)(b) the data (R || data) is fed into the encryption process. The first cipher block (containing E(mask XOR R) is placed in the IV field. The first block of content contains E(IV XOR data). The following alternative procedure MAY be used; however, it has not been demonstrated to be as cryptographically strong as the above procedures. The sender prepends a fixed block F to the plaintext (or, alternatively, a block generated with a weak PRNG). He then encrypts as in (2), above, using the CBC residue from the previous block as the mask for the prepended block. Note that in this case the mask for the first record transmitted by the application (the Finished) MUST be generated using a cryptographically strong PRNG. The decryption operation for all three alternatives is the same. The receiver decrypts the entire GenericBlockCipher structure and then discards the first cipher block, corresponding to the IV component. padding Padding that is added to force the length of the plaintext to be an integral multiple of the block cipher's block length. The padding MAY be any length up to 255 bytes, as long as it results in the TLSCiphertext.length being an integral multiple of the block length. Lengths longer than necessary might be desirable to frustrate attacks on a protocol that are based on analysis of the lengths of exchanged messages. Each uint8 in the padding data vector MUST be filled with the padding length value. The receiver Dierks & Rescorla Standards Track [Page 22]
RFC 4346 The TLS Protocol April 2006 MUST check this padding and SHOULD use the bad_record_mac alert to indicate padding errors. padding_length The padding length MUST be such that the total size of the GenericBlockCipher structure is a multiple of the cipher's block length. Legal values range from zero to 255, inclusive. This length specifies the length of the padding field exclusive of the padding_length field itself. The encrypted data length (TLSCiphertext.length) is one more than the sum of CipherSpec.block_length, TLSCompressed.length, CipherSpec.hash_size, and padding_length. Example: If the block length is 8 bytes, the content length (TLSCompressed.length) is 61 bytes, and the MAC length is 20 bytes, then the length before padding is 82 bytes (this does not include the IV, which may or may not be encrypted, as discussed above). Thus, the padding length modulo 8 must be equal to 6 in order to make the total length an even multiple of 8 bytes (the block length). The padding length can be 6, 14, 22, and so on, through 254. If the padding length were the minimum necessary, 6, the padding would be 6 bytes, each containing the value 6. Thus, the last 8 octets of the GenericBlockCipher before block encryption would be xx 06 06 06 06 06 06 06, where xx is the last octet of the MAC. Note: With block ciphers in CBC mode (Cipher Block Chaining), it is critical that the entire plaintext of the record be known before any ciphertext is transmitted. Otherwise, it is possible for the attacker to mount the attack described in [CBCATT]. Implementation Note: Canvel et al. [CBCTIME] have demonstrated a timing attack on CBC padding based on the time required to compute the MAC. In order to defend against this attack, implementations MUST ensure that record processing time is essentially the same whether or not the padding is correct. In general, the best way to do this is to compute the MAC even if the padding is incorrect, and only then reject the packet. For instance, if the pad appears to be incorrect, the implementation might assume a zero-length pad and then compute the MAC. This leaves a small timing channel, since MAC performance depends to some extent on the size of the data fragment, Dierks & Rescorla Standards Track [Page 23]
RFC 4346 The TLS Protocol April 2006 but it is not believed to be large enough to be exploitable, due to the large block size of existing MACs and the small size of the timing signal. 6.3. Key Calculation The Record Protocol requires an algorithm to generate keys, and MAC secrets from the security parameters provided by the handshake protocol. The master secret is hashed into a sequence of secure bytes, which are assigned to the MAC secrets and keys required by the current connection state (see Appendix A.6). CipherSpecs require a client write MAC secret, a server write MAC secret, a client write key, and a server write key, each of which is generated from the master secret in that order. Unused values are empty. When keys and MAC secrets are generated, the master secret is used as an entropy source. To generate the key material, compute key_block = PRF(SecurityParameters.master_secret, "key expansion", SecurityParameters.server_random + SecurityParameters.client_random); until enough output has been generated. Then the key_block is partitioned as follows: client_write_MAC_secret[SecurityParameters.hash_size] server_write_MAC_secret[SecurityParameters.hash_size] client_write_key[SecurityParameters.key_material_length] server_write_key[SecurityParameters.key_material_length] Implementation note: The currently defined cipher suite that requires the most material is AES_256_CBC_SHA, defined in [TLSAES]. It requires 2 x 32 byte keys, 2 x 20 byte MAC secrets, and 2 x 16 byte Initialization Vectors, for a total of 136 bytes of key material. 7. The TLS Handshaking Protocols TLS has three subprotocols that are used to allow peers to agree upon security parameters for the record layer, to authenticate themselves, to instantiate negotiated security parameters, and to report error conditions to each other. Dierks & Rescorla Standards Track [Page 24]
RFC 4346 The TLS Protocol April 2006 The Handshake Protocol is responsible for negotiating a session, which consists of the following items: session identifier An arbitrary byte sequence chosen by the server to identify an active or resumable session state. peer certificate X509v3 [X509] certificate of the peer. This element of the state may be null. compression method The algorithm used to compress data prior to encryption. cipher spec Specifies the bulk data encryption algorithm (such as null, DES, etc.) and a MAC algorithm (such as MD5 or SHA). It also defines cryptographic attributes such as the hash_size. (See Appendix A.6 for formal definition.) master secret 48-byte secret shared between the client and server. is resumable A flag indicating whether the session can be used to initiate new connections. These items are then used to create security parameters for use by the Record Layer when protecting application data. Many connections can be instantiated using the same session through the resumption feature of the TLS Handshake Protocol. 7.1. Change Cipher Spec Protocol The change cipher spec protocol exists to signal transitions in ciphering strategies. The protocol consists of a single message, which is encrypted and compressed under the current (not the pending) connection state. The message consists of a single byte of value 1. struct { enum { change_cipher_spec(1), (255) } type; } ChangeCipherSpec; The change cipher spec message is sent by both the client and the server to notify the receiving party that subsequent records will be protected under the newly negotiated CipherSpec and keys. Reception of this message causes the receiver to instruct the Record Layer to immediately copy the read pending state into the read current state. Dierks & Rescorla Standards Track [Page 25]
RFC 4346 The TLS Protocol April 2006 Immediately after sending this message, the sender MUST instruct the record layer to make the write pending state the write active state. (See Section 6.1.) The change cipher spec message is sent during the handshake after the security parameters have been agreed upon, but before the verifying finished message is sent (see Section 7.4.9). Note: If a rehandshake occurs while data is flowing on a connection, the communicating parties may continue to send data using the old CipherSpec. However, once the ChangeCipherSpec has been sent, the new CipherSpec MUST be used. The first side to send the ChangeCipherSpec does not know that the other side has finished computing the new keying material (e.g., if it has to perform a time consuming public key operation). Thus, a small window of time, during which the recipient must buffer the data, MAY exist. In practice, with modern machines this interval is likely to be fairly short. 7.2. Alert Protocol One of the content types supported by the TLS Record layer is the alert type. Alert messages convey the severity of the message and a description of the alert. Alert messages with a level of fatal result in the immediate termination of the connection. In this case, other connections corresponding to the session may continue, but the session identifier MUST be invalidated, preventing the failed session from being used to establish new connections. Like other messages, alert messages are encrypted and compressed, as specified by the current connection state. enum { warning(1), fatal(2), (255) } AlertLevel; enum { close_notify(0), unexpected_message(10), bad_record_mac(20), decryption_failed(21), record_overflow(22), decompression_failure(30), handshake_failure(40), no_certificate_RESERVED (41), bad_certificate(42), unsupported_certificate(43), certificate_revoked(44), certificate_expired(45), certificate_unknown(46), illegal_parameter(47), unknown_ca(48), Dierks & Rescorla Standards Track [Page 26]
RFC 4346 The TLS Protocol April 2006 access_denied(49), decode_error(50), decrypt_error(51), export_restriction_RESERVED(60), protocol_version(70), insufficient_security(71), internal_error(80), user_canceled(90), no_renegotiation(100), (255) } AlertDescription; struct { AlertLevel level; AlertDescription description; } Alert; 7.2.1. Closure Alerts The client and the server must share knowledge that the connection is ending in order to avoid a truncation attack. Either party may initiate the exchange of closing messages. close_notify This message notifies the recipient that the sender will not send any more messages on this connection. Note that as of TLS 1.1, failure to properly close a connection no longer requires that a session not be resumed. This is a change from TLS 1.0 to conform with widespread implementation practice. Either party may initiate a close by sending a close_notify alert. Any data received after a closure alert is ignored. Unless some other fatal alert has been transmitted, each party is required to send a close_notify alert before closing the write side of the connection. The other party MUST respond with a close_notify alert of its own and close down the connection immediately, discarding any pending writes. It is not required for the initiator of the close to wait for the responding close_notify alert before closing the read side of the connection. If the application protocol using TLS provides that any data may be carried over the underlying transport after the TLS connection is closed, the TLS implementation must receive the responding close_notify alert before indicating to the application layer that the TLS connection has ended. If the application protocol will not transfer any additional data, but will only close the underlying transport connection, then the implementation MAY choose to close the Dierks & Rescorla Standards Track [Page 27]
RFC 4346 The TLS Protocol April 2006 transport without waiting for the responding close_notify. No part of this standard should be taken to dictate the manner in which a usage profile for TLS manages its data transport, including when connections are opened or closed. Note: It is assumed that closing a connection reliably delivers pending data before destroying the transport. 7.2.2. Error Alerts Error handling in the TLS Handshake protocol is very simple. When an error is detected, the detecting party sends a message to the other party. Upon transmission or receipt of a fatal alert message, both parties immediately close the connection. Servers and clients MUST forget any session-identifiers, keys, and secrets associated with a failed connection. Thus, any connection terminated with a fatal alert MUST NOT be resumed. The following error alerts are defined: unexpected_message An inappropriate message was received. This alert is always fatal and should never be observed in communication between proper implementations. bad_record_mac This alert is returned if a record is received with an incorrect MAC. This alert also MUST be returned if an alert is sent because a TLSCiphertext decrypted in an invalid way: either it wasn't an even multiple of the block length, or its padding values, when checked, weren't correct. This message is always fatal. decryption_failed This alert MAY be returned if a TLSCiphertext decrypted in an invalid way: either it wasn't an even multiple of the block length, or its padding values, when checked, weren't correct. This message is always fatal. Note: Differentiating between bad_record_mac and decryption_failed alerts may permit certain attacks against CBC mode as used in TLS [CBCATT]. It is preferable to uniformly use the bad_record_mac alert to hide the specific type of the error. record_overflow A TLSCiphertext record was received that had a length more than 2^14+2048 bytes, or a record decrypted to a TLSCompressed record with more than 2^14+1024 bytes. This message is always fatal. Dierks & Rescorla Standards Track [Page 28]
RFC 4346 The TLS Protocol April 2006 decompression_failure The decompression function received improper input (e.g., data that would expand to excessive length). This message is always fatal. handshake_failure Reception of a handshake_failure alert message indicates that the sender was unable to negotiate an acceptable set of security parameters given the options available. This is a fatal error. no_certificate_RESERVED This alert was used in SSLv3 but not in TLS. It should not be sent by compliant implementations. bad_certificate A certificate was corrupt, contained signatures that did not verify correctly, etc. unsupported_certificate A certificate was of an unsupported type. certificate_revoked A certificate was revoked by its signer. certificate_expired A certificate has expired or is not currently valid. certificate_unknown Some other (unspecified) issue arose in processing the certificate, rendering it unacceptable. illegal_parameter A field in the handshake was out of range or inconsistent with other fields. This is always fatal. unknown_ca A valid certificate chain or partial chain was received, but the certificate was not accepted because the CA certificate could not be located or couldn't be matched with a known, trusted CA. This message is always fatal. access_denied A valid certificate was received, but when access control was applied, the sender decided not to proceed with negotiation. This message is always fatal. Dierks & Rescorla Standards Track [Page 29]
RFC 4346 The TLS Protocol April 2006 decode_error A message could not be decoded because some field was out of the specified range or the length of the message was incorrect. This message is always fatal. decrypt_error A handshake cryptographic operation failed, including being unable to correctly verify a signature, decrypt a key exchange, or validate a finished message. export_restriction_RESERVED This alert was used in TLS 1.0 but not TLS 1.1. protocol_version The protocol version the client has attempted to negotiate is recognized but not supported. (For example, old protocol versions might be avoided for security reasons). This message is always fatal. insufficient_security Returned instead of handshake_failure when a negotiation has failed specifically because the server requires ciphers more secure than those supported by the client. This message is always fatal. internal_error An internal error unrelated to the peer or the correctness of the protocol (such as a memory allocation failure) makes it impossible to continue. This message is always fatal. user_canceled This handshake is being canceled for some reason unrelated to a protocol failure. If the user cancels an operation after the handshake is complete, just closing the connection by sending a close_notify is more appropriate. This alert should be followed by a close_notify. This message is generally a warning. no_renegotiation Sent by the client in response to a hello request or by the server in response to a client hello after initial handshaking. Either of these would normally lead to renegotiation; when that is not appropriate, the recipient should respond with this alert. At that point, the original requester can decide whether to proceed with the connection. One case where this would be appropriate is where a server has spawned a process to satisfy a request; the process might receive security parameters (key length, authentication, etc.) at startup and it Dierks & Rescorla Standards Track [Page 30]
RFC 4346 The TLS Protocol April 2006 might be difficult to communicate changes to these parameters after that point. This message is always a warning. For all errors where an alert level is not explicitly specified, the sending party MAY determine at its discretion whether this is a fatal error or not; if an alert with a level of warning is received, the receiving party MAY decide at its discretion whether to treat this as a fatal error or not. However, all messages that are transmitted with a level of fatal MUST be treated as fatal messages. New alert values MUST be defined by RFC 2434 Standards Action. See Section 11 for IANA Considerations for alert values. 7.3. Handshake Protocol Overview The cryptographic parameters of the session state are produced by the TLS Handshake Protocol, which operates on top of the TLS Record Layer. When a TLS client and server first start communicating, they agree on a protocol version, select cryptographic algorithms, optionally authenticate each other, and use public-key encryption techniques to generate shared secrets. The TLS Handshake Protocol involves the following steps: - Exchange hello messages to agree on algorithms, exchange random values, and check for session resumption. - Exchange the necessary cryptographic parameters to allow the client and server to agree on a premaster secret. - Exchange certificates and cryptographic information to allow the client and server to authenticate themselves. - Generate a master secret from the premaster secret and exchanged random values. - Provide security parameters to the record layer. - Allow the client and server to verify that their peer has calculated the same security parameters and that the handshake occurred without tampering by an attacker. Note that higher layers should not be overly reliant on whether TLS always negotiates the strongest possible connection between two peers. There are a number of ways in which a man-in-the-middle attacker can attempt to make two entities drop down to the least secure method they support. The protocol has been designed to minimize this risk, but there are still attacks available. For Dierks & Rescorla Standards Track [Page 31]
RFC 4346 The TLS Protocol April 2006 example, an attacker could block access to the port a secure service runs on, or attempt to get the peers to negotiate an unauthenticated connection. The fundamental rule is that higher levels must be cognizant of what their security requirements are and never transmit information over a channel less secure than what they require. The TLS protocol is secure in that any cipher suite offers its promised level of security: if you negotiate 3DES with a 1024 bit RSA key exchange with a host whose certificate you have verified, you can expect to be that secure. However, one SHOULD never send data over a link encrypted with 40-bit security unless one feels that data is worth no more than the effort required to break that encryption. These goals are achieved by the handshake protocol, which can be summarized as follows: The client sends a client hello message to which the server must respond with a server hello message, or else a fatal error will occur and the connection will fail. The client hello and server hello are used to establish security enhancement capabilities between client and server. The client hello and server hello establish the following attributes: Protocol Version, Session ID, Cipher Suite, and Compression Method. Additionally, two random values are generated and exchanged: ClientHello.random and ServerHello.random. The actual key exchange uses up to four messages: the server certificate, the server key exchange, the client certificate, and the client key exchange. New key exchange methods can be created by specifying a format for these messages and by defining the use of the messages to allow the client and server to agree upon a shared secret. This secret MUST be quite long; currently defined key exchange methods exchange secrets that range from 48 to 128 bytes in length. Following the hello messages, the server will send its certificate, if it is to be authenticated. Additionally, a server key exchange message may be sent, if it is required (e.g., if the server has no certificate, or if its certificate is for signing only). If the server is authenticated, it may request a certificate from the client, if that is appropriate to the cipher suite selected. Next, the server will send the server hello done message, indicating that the hello-message phase of the handshake is complete. The server will then wait for a client response. If the server has sent a certificate request message, the client must send the certificate message. The client key exchange message is now sent, and the content of that message will depend on the public key algorithm selected between the client hello and the server hello. If the client has sent a certificate with signing ability, a digitally- Dierks & Rescorla Standards Track [Page 32]
RFC 4346 The TLS Protocol April 2006 signed certificate verify message is sent to explicitly verify the certificate. At this point, a change cipher spec message is sent by the client, and the client copies the pending Cipher Spec into the current Cipher Spec. The client then immediately sends the finished message under the new algorithms, keys, and secrets. In response, the server will send its own change cipher spec message, transfer the pending to the current Cipher Spec, and send its finished message under the new Cipher Spec. At this point, the handshake is complete, and the client and server may begin to exchange application layer data. (See flow chart below.) Application data MUST NOT be sent prior to the completion of the first handshake (before a cipher suite other TLS_NULL_WITH_NULL_NULL is established). Client Server ClientHello --------> ServerHello Certificate* ServerKeyExchange* CertificateRequest* <-------- ServerHelloDone Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished --------> [ChangeCipherSpec] <-------- Finished Application Data <-------> Application Data Fig. 1. Message flow for a full handshake * Indicates optional or situation-dependent messages that are not always sent. Note: To help avoid pipeline stalls, ChangeCipherSpec is an independent TLS Protocol content type, and is not actually a TLS handshake message. When the client and server decide to resume a previous session or duplicate an existing session (instead of negotiating new security parameters), the message flow is as follows: The client sends a ClientHello using the Session ID of the session to be resumed. The server then checks its session cache for a match. Dierks & Rescorla Standards Track [Page 33]
RFC 4346 The TLS Protocol April 2006 If a match is found, and the server is willing to re-establish the connection under the specified session state, it will send a ServerHello with the same Session ID value. At this point, both client and server MUST send change cipher spec messages and proceed directly to finished messages. Once the re-establishment is complete, the client and server MAY begin to exchange application layer data. (See flow chart below.) If a Session ID match is not found, the server generates a new session ID and the TLS client and server perform a full handshake. Client Server ClientHello --------> ServerHello [ChangeCipherSpec] <-------- Finished [ChangeCipherSpec] Finished --------> Application Data <-------> Application Data Fig. 2. Message flow for an abbreviated handshake The contents and significance of each message will be presented in detail in the following sections. 7.4. Handshake Protocol The TLS Handshake Protocol is one of the defined higher-level clients of the TLS Record Protocol. This protocol is used to negotiate the secure attributes of a session. Handshake messages are supplied to the TLS Record Layer, where they are encapsulated within one or more TLSPlaintext structures, which are processed and transmitted as specified by the current active session state. enum { hello_request(0), client_hello(1), server_hello(2), certificate(11), server_key_exchange (12), certificate_request(13), server_hello_done(14), certificate_verify(15), client_key_exchange(16), finished(20), (255) } HandshakeType; struct { HandshakeType msg_type; /* handshake type */ uint24 length; /* bytes in message */ select (HandshakeType) { case hello_request: HelloRequest; case client_hello: ClientHello; Dierks & Rescorla Standards Track [Page 34]
RFC 4346 The TLS Protocol April 2006 case server_hello: ServerHello; case certificate: Certificate; case server_key_exchange: ServerKeyExchange; case certificate_request: CertificateRequest; case server_hello_done: ServerHelloDone; case certificate_verify: CertificateVerify; case client_key_exchange: ClientKeyExchange; case finished: Finished; } body; } Handshake; The handshake protocol messages are presented below in the order they MUST be sent; sending handshake messages in an unexpected order results in a fatal error. Unneeded handshake messages can be omitted, however. Note one exception to the ordering: the Certificate message is used twice in the handshake (from server to client, then from client to server), but is described only in its first position. The one message that is not bound by these ordering rules is the Hello Request message, which can be sent at any time, but which should be ignored by the client if it arrives in the middle of a handshake. New Handshake message type values MUST be defined via RFC 2434 Standards Action. See Section 11 for IANA Considerations for these values. 7.4.1. Hello Messages The hello phase messages are used to exchange security enhancement capabilities between the client and server. When a new session begins, the Record Layer's connection state encryption, hash, and compression algorithms are initialized to null. The current connection state is used for renegotiation messages. 7.4.1.1. Hello request When this message will be sent: The hello request message MAY be sent by the server at any time. Meaning of this message: Hello request is a simple notification that the client should begin the negotiation process anew by sending a client hello message when convenient. This message will be ignored by the client if the client is currently negotiating a session. This message may be ignored by the client if it does not wish to renegotiate a session, or the client may, if it wishes, respond Dierks & Rescorla Standards Track [Page 35]
RFC 4346 The TLS Protocol April 2006 with a no_renegotiation alert. Since handshake messages are intended to have transmission precedence over application data, it is expected that the negotiation will begin before no more than a few records are received from the client. If the server sends a hello request but does not receive a client hello in response, it may close the connection with a fatal alert. After sending a hello request, servers SHOULD not repeat the request until the subsequent handshake negotiation is complete. Structure of this message: struct { } HelloRequest; Note: This message MUST NOT be included in the message hashes that are maintained throughout the handshake and used in the finished messages and the certificate verify message. 7.4.1.2. Client Hello When this message will be sent: When a client first connects to a server it is required to send the client hello as its first message. The client can also send a client hello in response to a hello request or on its own initiative in order to renegotiate the security parameters in an existing connection. Structure of this message: The client hello message includes a random structure, which is used later in the protocol. struct { uint32 gmt_unix_time; opaque random_bytes[28]; } Random; gmt_unix_time The current time and date in standard UNIX 32-bit format (seconds since the midnight starting Jan 1, 1970, GMT, ignoring leap seconds) according to the sender's internal clock. Clocks are not required to be set correctly by the basic TLS Protocol; higher-level or application protocols may define additional requirements. random_bytes 28 bytes generated by a secure random number generator. Dierks & Rescorla Standards Track [Page 36]
RFC 4346 The TLS Protocol April 2006 The client hello message includes a variable-length session identifier. If not empty, the value identifies a session between the same client and server whose security parameters the client wishes to reuse. The session identifier MAY be from an earlier connection, from this connection, or from another currently active connection. The second option is useful if the client only wishes to update the random structures and derived values of a connection, and the third option makes it possible to establish several independent secure connections without repeating the full handshake protocol. These independent connections may occur sequentially or simultaneously; a SessionID becomes valid when the handshake negotiating it completes with the exchange of Finished messages and persists until it is removed due to aging or because a fatal error was encountered on a connection associated with the session. The actual contents of the SessionID are defined by the server. opaque SessionID<0..32>; Warning: Because the SessionID is transmitted without encryption or immediate MAC protection, servers MUST not place confidential information in session identifiers or let the contents of fake session identifiers cause any breach of security. (Note that the content of the handshake as a whole, including the SessionID, is protected by the Finished messages exchanged at the end of the handshake.) The CipherSuite list, passed from the client to the server in the client hello message, contains the combinations of cryptographic algorithms supported by the client in order of the client's preference (favorite choice first). Each CipherSuite defines a key exchange algorithm, a bulk encryption algorithm (including secret key length), and a MAC algorithm. The server will select a cipher suite or, if no acceptable choices are presented, return a handshake failure alert and close the connection. uint8 CipherSuite[2]; /* Cryptographic suite selector */ The client hello includes a list of compression algorithms supported by the client, ordered according to the client's preference. Dierks & Rescorla Standards Track [Page 37]
RFC 4346 The TLS Protocol April 2006 enum { null(0), (255) } CompressionMethod; struct { ProtocolVersion client_version; Random random; SessionID session_id; CipherSuite cipher_suites<2..2^16-1>; CompressionMethod compression_methods<1..2^8-1>; } ClientHello; client_version The version of the TLS protocol by which the client wishes to communicate during this session. This SHOULD be the latest (highest valued) version supported by the client. For this version of the specification, the version will be 3.2. (See Appendix E for details about backward compatibility.) random A client-generated random structure. session_id The ID of a session the client wishes to use for this connection. This field should be empty if no session_id is available or if the client wishes to generate new security parameters. cipher_suites This is a list of the cryptographic options supported by the client, with the client's first preference first. If the session_id field is not empty (implying a session resumption request) this vector MUST include at least the cipher_suite from that session. Values are defined in Appendix A.5. compression_methods This is a list of the compression methods supported by the client, sorted by client preference. If the session_id field is not empty (implying a session resumption request) it MUST include the compression_method from that session. This vector MUST contain, and all implementations MUST support, CompressionMethod.null. Thus, a client and server will always be able to agree on a compression method. After sending the client hello message, the client waits for a server hello message. Any other handshake message returned by the server except for a hello request is treated as a fatal error. Forward compatibility note: In the interests of forward compatibility, it is permitted that a client hello message include extra data after the compression methods. This data MUST be included Dierks & Rescorla Standards Track [Page 38]
RFC 4346 The TLS Protocol April 2006 in the handshake hashes, but must otherwise be ignored. This is the only handshake message for which this is legal; for all other messages, the amount of data in the message MUST match the description of the message precisely. Note: For the intended use of trailing data in the ClientHello, see RFC 3546 [TLSEXT]. 7.4.1.3. Server Hello The server will send this message in response to a client hello message when it was able to find an acceptable set of algorithms. If it cannot find such a match, it will respond with a handshake failure alert. Structure of this message: struct { ProtocolVersion server_version; Random random; SessionID session_id; CipherSuite cipher_suite; CompressionMethod compression_method; } ServerHello; server_version This field will contain the lower of that suggested by the client in the client hello and the highest supported by the server. For this version of the specification, the version is 3.2. (See Appendix E for details about backward compatibility.) random This structure is generated by the server and MUST be independently generated from the ClientHello.random. Dierks & Rescorla Standards Track [Page 39]
RFC 4346 The TLS Protocol April 2006 session_id This is the identity of the session corresponding to this connection. If the ClientHello.session_id was non-empty, the server will look in its session cache for a match. If a match is found and the server is willing to establish the new connection using the specified session state, the server will respond with the same value as was supplied by the client. This indicates a resumed session and dictates that the parties must proceed directly to the finished messages. Otherwise this field will contain a different value identifying the new session. The server may return an empty session_id to indicate that the session will not be cached and therefore cannot be resumed. If a session is resumed, it must be resumed using the same cipher suite it was originally negotiated with. cipher_suite The single cipher suite selected by the server from the list in ClientHello.cipher_suites. For resumed sessions, this field is the value from the state of the session being resumed. compression_method The single compression algorithm selected by the server from the list in ClientHello.compression_methods. For resumed sessions this field is the value from the resumed session state. 7.4.2. Server Certificate When this message will be sent: The server MUST send a certificate whenever the agreed-upon key exchange method is not an anonymous one. This message will always immediately follow the server hello message. Meaning of this message: The certificate type MUST be appropriate for the selected cipher suite's key exchange algorithm, and is generally an X.509v3 certificate. It MUST contain a key that matches the key exchange method, as follows. Unless otherwise specified, the signing algorithm for the certificate MUST be the same as the algorithm for the certificate key. Unless otherwise specified, the public key MAY be of any length. Dierks & Rescorla Standards Track [Page 40]
RFC 4346 The TLS Protocol April 2006 Key Exchange Algorithm Certificate Key Type RSA RSA public key; the certificate MUST allow the key to be used for encryption. DHE_DSS DSS public key. DHE_RSA RSA public key that can be used for signing. DH_DSS Diffie-Hellman key. The algorithm used to sign the certificate MUST be DSS. DH_RSA Diffie-Hellman key. The algorithm used to sign the certificate MUST be RSA. All certificate profiles and key and cryptographic formats are defined by the IETF PKIX working group [PKIX]. When a key usage extension is present, the digitalSignature bit MUST be set for the key to be eligible for signing, as described above, and the keyEncipherment bit MUST be present to allow encryption, as described above. The keyAgreement bit must be set on Diffie-Hellman certificates. As CipherSuites that specify new key exchange methods are specified for the TLS Protocol, they will imply certificate format and the required encoded keying information. Structure of this message: opaque ASN.1Cert<1..2^24-1>; struct { ASN.1Cert certificate_list<0..2^24-1>; } Certificate; certificate_list This is a sequence (chain) of X.509v3 certificates. The sender's certificate must come first in the list. Each following certificate must directly certify the one preceding it. Because certificate validation requires that root keys be distributed independently, the self-signed certificate that specifies the root certificate authority may optionally be omitted from the chain, under the assumption that the remote end must already possess it in order to validate it in any case. The same message type and structure will be used for the client's response to a certificate request message. Note that a client MAY Dierks & Rescorla Standards Track [Page 41]
RFC 4346 The TLS Protocol April 2006 send no certificates if it does not have an appropriate certificate to send in response to the server's authentication request. Note: PKCS #7 [PKCS7] is not used as the format for the certificate vector because PKCS #6 [PKCS6] extended certificates are not used. Also, PKCS #7 defines a SET rather than a SEQUENCE, making the task of parsing the list more difficult. 7.4.3. Server Key Exchange Message When this message will be sent: This message will be sent immediately after the server certificate message (or the server hello message, if this is an anonymous negotiation). The server key exchange message is sent by the server only when the server certificate message (if sent) does not contain enough data to allow the client to exchange a premaster secret. This is true for the following key exchange methods: DHE_DSS DHE_RSA DH_anon It is not legal to send the server key exchange message for the following key exchange methods: RSA DH_DSS DH_RSA Meaning of this message: This message conveys cryptographic information to allow the client to communicate the premaster secret: either an RSA public key with which to encrypt the premaster secret, or a Diffie-Hellman public key with which the client can complete a key exchange (with the result being the premaster secret). As additional CipherSuites are defined for TLS that include new key exchange algorithms, the server key exchange message will be sent if and only if the certificate type associated with the key exchange algorithm does not provide enough information for the client to exchange a premaster secret. Dierks & Rescorla Standards Track [Page 42]
RFC 4346 The TLS Protocol April 2006 Structure of this message: enum { rsa, diffie_hellman } KeyExchangeAlgorithm; struct { opaque rsa_modulus<1..2^16-1>; opaque rsa_exponent<1..2^16-1>; } ServerRSAParams; rsa_modulus The modulus of the server's temporary RSA key. rsa_exponent The public exponent of the server's temporary RSA key. struct { opaque dh_p<1..2^16-1>; opaque dh_g<1..2^16-1>; opaque dh_Ys<1..2^16-1>; } ServerDHParams; /* Ephemeral DH parameters */ dh_p The prime modulus used for the Diffie-Hellman operation. dh_g The generator used for the Diffie-Hellman operation. dh_Ys The server's Diffie-Hellman public value (g^X mod p). struct { select (KeyExchangeAlgorithm) { case diffie_hellman: ServerDHParams params; Signature signed_params; case rsa: ServerRSAParams params; Signature signed_params; }; } ServerKeyExchange; Dierks & Rescorla Standards Track [Page 43]
RFC 4346 The TLS Protocol April 2006 struct { select (KeyExchangeAlgorithm) { case diffie_hellman: ServerDHParams params; case rsa: ServerRSAParams params; }; } ServerParams; params The server's key exchange parameters. signed_params For non-anonymous key exchanges, a hash of the corresponding params value, with the signature appropriate to that hash applied. md5_hash MD5(ClientHello.random + ServerHello.random + ServerParams); sha_hash SHA(ClientHello.random + ServerHello.random + ServerParams); enum { anonymous, rsa, dsa } SignatureAlgorithm; struct { select (SignatureAlgorithm) { case anonymous: struct { }; case rsa: digitally-signed struct { opaque md5_hash[16]; opaque sha_hash[20]; }; case dsa: digitally-signed struct { opaque sha_hash[20]; }; }; }; } Signature; 7.4.4. Certificate request When this message will be sent: A non-anonymous server can optionally request a certificate from the client, if it is appropriate for the selected cipher suite. Dierks & Rescorla Standards Track [Page 44]
RFC 4346 The TLS Protocol April 2006 This message, if sent, will immediately follow the Server Key Exchange message (if it is sent; otherwise, the Server Certificate message). Structure of this message: enum { rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6), fortezza_dms_RESERVED(20), (255) } ClientCertificateType; opaque DistinguishedName<1..2^16-1>; struct { ClientCertificateType certificate_types<1..2^8-1>; DistinguishedName certificate_authorities<0..2^16-1>; } CertificateRequest; certificate_types This field is a list of the types of certificates requested, sorted in order of the server's preference. certificate_authorities A list of the distinguished names of acceptable certificate authorities. These distinguished names may specify a desired distinguished name for a root CA or for a subordinate CA; thus, this message can be used to describe both known roots and a desired authorization space. If the certificate_authorities list is empty then the client MAY send any certificate of the appropriate ClientCertificateType, unless there is some external arrangement to the contrary. ClientCertificateType values are divided into three groups: 1. Values from 0 (zero) through 63 decimal (0x3F) inclusive are reserved for IETF Standards Track protocols. 2. Values from 64 decimal (0x40) through 223 decimal (0xDF) inclusive are reserved for assignment for non-Standards Track methods. 3. Values from 224 decimal (0xE0) through 255 decimal (0xFF) inclusive are reserved for private use. Dierks & Rescorla Standards Track [Page 45]
RFC 4346 The TLS Protocol April 2006 Additional information describing the role of IANA in the allocation of ClientCertificateType code points is described in Section 11. Note: Values listed as RESERVED may not be used. They were used in SSLv3. Note: DistinguishedName is derived from [X501]. DistinguishedNames are represented in DER-encoded format. Note: It is a fatal handshake_failure alert for an anonymous server to request client authentication. 7.4.5. Server Hello Done When this message will be sent: The server hello done message is sent by the server to indicate the end of the server hello and associated messages. After sending this message, the server will wait for a client response. Meaning of this message: This message means that the server is done sending messages to support the key exchange, and the client can proceed with its phase of the key exchange. Upon receipt of the server hello done message, the client SHOULD verify that the server provided a valid certificate, if required and check that the server hello parameters are acceptable. Structure of this message: struct { } ServerHelloDone; 7.4.6. Client certificate When this message will be sent: This is the first message the client can send after receiving a server hello done message. This message is only sent if the server requests a certificate. If no suitable certificate is available, the client SHOULD send a certificate message containing no certificates. That is, the certificate_list structure has a length of zero. If client authentication is required by the server for the handshake to continue, it may respond with a fatal handshake failure alert. Client certificates are sent using the Certificate structure defined in Section 7.4.2. Dierks & Rescorla Standards Track [Page 46]
RFC 4346 The TLS Protocol April 2006 Note: When using a static Diffie-Hellman based key exchange method (DH_DSS or DH_RSA), if client authentication is requested, the Diffie-Hellman group and generator encoded in the client's certificate MUST match the server specified Diffie-Hellman parameters if the client's parameters are to be used for the key exchange. 7.4.7. Client Key Exchange Message When this message will be sent: This message is always sent by the client. It MUST immediately follow the client certificate message, if it is sent. Otherwise it MUST be the first message sent by the client after it receives the server hello done message. Meaning of this message: With this message, the premaster secret is set, either though direct transmission of the RSA-encrypted secret or by the transmission of Diffie-Hellman parameters that will allow each side to agree upon the same premaster secret. When the key exchange method is DH_RSA or DH_DSS, client certification has been requested, and the client was able to respond with a certificate that contained a Diffie-Hellman public key whose parameters (group and generator) matched those specified by the server in its certificate, this message MUST not contain any data. Structure of this message: The choice of messages depends on which key exchange method has been selected. See Section 7.4.3 for the KeyExchangeAlgorithm definition. struct { select (KeyExchangeAlgorithm) { case rsa: EncryptedPreMasterSecret; case diffie_hellman: ClientDiffieHellmanPublic; } exchange_keys; } ClientKeyExchange; 7.4.7.1. RSA Encrypted Premaster Secret Message Meaning of this message: If RSA is being used for key agreement and authentication, the client generates a 48-byte premaster secret, encrypts it using the public key from the server's certificate or the temporary RSA key Dierks & Rescorla Standards Track [Page 47]
RFC 4346 The TLS Protocol April 2006 provided in a server key exchange message, and sends the result in an encrypted premaster secret message. This structure is a variant of the client key exchange message and is not a message in itself. Structure of this message: struct { ProtocolVersion client_version; opaque random[46]; } PreMasterSecret; client_version The latest (newest) version supported by the client. This is used to detect version roll-back attacks. Upon receiving the premaster secret, the server SHOULD check that this value matches the value transmitted by the client in the client hello message. random 46 securely-generated random bytes. struct { public-key-encrypted PreMasterSecret pre_master_secret; } EncryptedPreMasterSecret; pre_master_secret This random value is generated by the client and is used to generate the master secret, as specified in Section 8.1. Note: An attack discovered by Daniel Bleichenbacher [BLEI] can be used to attack a TLS server that is using PKCS#1 v 1.5 encoded RSA. The attack takes advantage of the fact that, by failing in different ways, a TLS server can be coerced into revealing whether a particular message, when decrypted, is properly PKCS#1 v1.5 formatted or not. The best way to avoid vulnerability to this attack is to treat incorrectly formatted messages in a manner indistinguishable from correctly formatted RSA blocks. Thus, when a server receives an incorrectly formatted RSA block, it should generate a random 48-byte value and proceed using it as the premaster secret. Thus, the server will act identically whether the received RSA block is correctly encoded or not. [PKCS1B] defines a newer version of PKCS#1 encoding that is more secure against the Bleichenbacher attack. However, for maximal compatibility with TLS 1.0, TLS 1.1 retains the original encoding. No variants of the Bleichenbacher attack Dierks & Rescorla Standards Track [Page 48]
RFC 4346 The TLS Protocol April 2006 are known to exist provided that the above recommendations are followed. Implementation Note: Public-key-encrypted data is represented as an opaque vector <0..2^16-1> (see Section 4.7). Thus, the RSA-encrypted PreMasterSecret in a ClientKeyExchange is preceded by two length bytes. These bytes are redundant in the case of RSA because the EncryptedPreMasterSecret is the only data in the ClientKeyExchange and its length can therefore be unambiguously determined. The SSLv3 specification was not clear about the encoding of public-key-encrypted data, and therefore many SSLv3 implementations do not include the length bytes, encoding the RSA encrypted data directly in the ClientKeyExchange message. This specification requires correct encoding of the EncryptedPreMasterSecret complete with length bytes. The resulting PDU is incompatible with many SSLv3 implementations. Implementors upgrading from SSLv3 must modify their implementations to generate and accept the correct encoding. Implementors who wish to be compatible with both SSLv3 and TLS should make their implementation's behavior dependent on the protocol version. Implementation Note: It is now known that remote timing-based attacks on SSL are possible, at least when the client and server are on the same LAN. Accordingly, implementations that use static RSA keys SHOULD use RSA blinding or some other anti-timing technique, as described in [TIMING]. Note: The version number in the PreMasterSecret MUST be the version offered by the client in the ClientHello, not the version negotiated for the connection. This feature is designed to prevent rollback attacks. Unfortunately, many implementations use the negotiated version instead, and therefore checking the version number may lead to failure to interoperate with such incorrect client implementations. Client implementations, MUST and Server implementations MAY, check the version number. In practice, since the TLS handshake MACs prevent downgrade and no good attacks are known on those MACs, ambiguity is not considered a serious security risk. Note that if servers choose to check the version number, they should randomize the Dierks & Rescorla Standards Track [Page 49]
RFC 4346 The TLS Protocol April 2006 PreMasterSecret in case of error, rather than generate an alert, in order to avoid variants on the Bleichenbacher attack. [KPR03] 7.4.7.2. Client Diffie-Hellman Public Value Meaning of this message: This structure conveys the client's Diffie-Hellman public value (Yc) if it was not already included in the client's certificate. The encoding used for Yc is determined by the enumerated PublicValueEncoding. This structure is a variant of the client key exchange message and not a message in itself. Structure of this message: enum { implicit, explicit } PublicValueEncoding; implicit If the client certificate already contains a suitable Diffie- Hellman key, then Yc is implicit and does not need to be sent again. In this case, the client key exchange message will be sent, but it MUST be empty. explicit Yc needs to be sent. struct { select (PublicValueEncoding) { case implicit: struct { }; case explicit: opaque dh_Yc<1..2^16-1>; } dh_public; } ClientDiffieHellmanPublic; dh_Yc The client's Diffie-Hellman public value (Yc). 7.4.8. Certificate verify When this message will be sent: This message is used to provide explicit verification of a client certificate. This message is only sent following a client certificate that has signing capability (i.e., all certificates except those containing fixed Diffie-Hellman parameters). When sent, it MUST immediately follow the client key exchange message. Dierks & Rescorla Standards Track [Page 50]
RFC 4346 The TLS Protocol April 2006 Structure of this message: struct { Signature signature; } CertificateVerify; The Signature type is defined in 7.4.3. CertificateVerify.signature.md5_hash MD5(handshake_messages); CertificateVerify.signature.sha_hash SHA(handshake_messages); Here handshake_messages refers to all handshake messages sent or received starting at client hello up to but not including this message, including the type and length fields of the handshake messages. This is the concatenation of all the Handshake structures, as defined in 7.4, exchanged thus far. 7.4.9. Finished When this message will be sent: A finished message is always sent immediately after a change cipher spec message to verify that the key exchange and authentication processes were successful. It is essential that a change cipher spec message be received between the other handshake messages and the Finished message. Meaning of this message: The finished message is the first protected with the just- negotiated algorithms, keys, and secrets. Recipients of finished messages MUST verify that the contents are correct. Once a side has sent its Finished message and received and validated the Finished message from its peer, it may begin to send and receive application data over the connection. struct { opaque verify_data[12]; } Finished; verify_data PRF(master_secret, finished_label, MD5(handshake_messages) + SHA-1(handshake_messages)) [0..11]; Dierks & Rescorla Standards Track [Page 51]
RFC 4346 The TLS Protocol April 2006 finished_label For Finished messages sent by the client, the string "client finished". For Finished messages sent by the server, the string "server finished". handshake_messages All of the data from all messages in this handshake (not including any HelloRequest messages) up to but not including this message. This is only data visible at the handshake layer and does not include record layer headers. This is the concatenation of all the Handshake structures, as defined in 7.4, exchanged thus far. It is a fatal error if a finished message is not preceded by a change cipher spec message at the appropriate point in the handshake. The value handshake_messages includes all handshake messages starting at client hello up to, but not including, this finished message. This may be different from handshake_messages in Section 7.4.8 because it would include the certificate verify message (if sent). Also, the handshake_messages for the finished message sent by the client will be different from that for the finished message sent by the server, because the one that is sent second will include the prior one. Note: Change cipher spec messages, alerts, and any other record types are not handshake messages and are not included in the hash computations. Also, Hello Request messages are omitted from handshake hashes. 8. Cryptographic Computations In order to begin connection protection, the TLS Record Protocol requires specification of a suite of algorithms, a master secret, and the client and server random values. The authentication, encryption, and MAC algorithms are determined by the cipher_suite selected by the server and revealed in the server hello message. The compression algorithm is negotiated in the hello messages, and the random values are exchanged in the hello messages. All that remains is to calculate the master secret. 8.1. Computing the Master Secret For all key exchange methods, the same algorithm is used to convert the pre_master_secret into the master_secret. The pre_master_secret should be deleted from memory once the master_secret has been computed. Dierks & Rescorla Standards Track [Page 52]
RFC 4346 The TLS Protocol April 2006 master_secret = PRF(pre_master_secret, "master secret", ClientHello.random + ServerHello.random) [0..47]; The master secret is always exactly 48 bytes in length. The length of the premaster secret will vary depending on key exchange method. 8.1.1. RSA When RSA is used for server authentication and key exchange, a 48- byte pre_master_secret is generated by the client, encrypted under the server's public key, and sent to the server. The server uses its private key to decrypt the pre_master_secret. Both parties then convert the pre_master_secret into the master_secret, as specified above. RSA digital signatures are performed using PKCS #1 [PKCS1] block type 1. RSA public key encryption is performed using PKCS #1 block type 2. 8.1.2. Diffie-Hellman A conventional Diffie-Hellman computation is performed. The negotiated key (Z) is used as the pre_master_secret, and is converted into the master_secret, as specified above. Leading bytes of Z that contain all zero bits are stripped before it is used as the pre_master_secret. Note: Diffie-Hellman parameters are specified by the server and may be either ephemeral or contained within the server's certificate. 9. Mandatory Cipher Suites In the absence of an application profile standard specifying otherwise, a TLS compliant application MUST implement the cipher suite TLS_RSA_WITH_3DES_EDE_CBC_SHA. 10. Application Data Protocol Application data messages are carried by the Record Layer and are fragmented, compressed, and encrypted based on the current connection state. The messages are treated as transparent data to the record layer. 11. Security Considerations Security issues are discussed throughout this memo, especially in Appendices D, E, and F. Dierks & Rescorla Standards Track [Page 53]
RFC 4346 The TLS Protocol April 2006 12. IANA Considerations This document describes a number of new registries that have been created by IANA. We recommended that they be placed as individual registries items under a common TLS category. Section 7.4.3 describes a TLS ClientCertificateType Registry to be maintained by the IANA, defining a number of such code point identifiers. ClientCertificateType identifiers with values in the range 0-63 (decimal) inclusive are assigned via RFC 2434 Standards Action. Values from the range 64-223 (decimal) inclusive are assigned via [RFC2434] Specification Required. Identifier values from 224-255 (decimal) inclusive are reserved for RFC 2434 Private Use. The registry will initially be populated with the values in this document, Section 7.4.4. Section A.5 describes a TLS Cipher Suite Registry to be maintained by the IANA, and it defines a number of such cipher suite identifiers. Cipher suite values with the first byte in the range 0-191 (decimal) inclusive are assigned via RFC 2434 Standards Action. Values with the first byte in the range 192-254 (decimal) are assigned via RFC 2434 Specification Required. Values with the first byte 255 (decimal) are reserved for RFC 2434 Private Use. The registry will initially be populated with the values from Section A.5 of this document, [TLSAES], and from Section 3 of [TLSKRB]. Section 6 requires that all ContentType values be defined by RFC 2434 Standards Action. IANA has created a TLS ContentType registry, initially populated with values from Section 6.2.1 of this document. Future values MUST be allocated via Standards Action as described in [RFC2434]. Section 7.2.2 requires that all Alert values be defined by RFC 2434 Standards Action. IANA has created a TLS Alert registry, initially populated with values from Section 7.2 of this document and from Section 4 of [TLSEXT]. Future values MUST be allocated via Standards Action as described in [RFC2434]. Section 7.4 requires that all HandshakeType values be defined by RFC 2434 Standards Action. IANA has created a TLS HandshakeType registry, initially populated with values from Section 7.4 of this document and from Section 2.4 of [TLSEXT]. Future values MUST be allocated via Standards Action as described in [RFC2434]. Dierks & Rescorla Standards Track [Page 54]
RFC 4346 The TLS Protocol April 2006 Appendix A. Protocol Constant Values This section describes protocol types and constants. A.1. Record Layer struct { uint8 major, minor; } ProtocolVersion; ProtocolVersion version = { 3, 2 }; /* TLS v1.1 */ enum { change_cipher_spec(20), alert(21), handshake(22), application_data(23), (255) } ContentType; struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[TLSPlaintext.length]; } TLSPlaintext; struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[TLSCompressed.length]; } TLSCompressed; struct { ContentType type; ProtocolVersion version; uint16 length; select (CipherSpec.cipher_type) { case stream: GenericStreamCipher; case block: GenericBlockCipher; } fragment; } TLSCiphertext; stream-ciphered struct { opaque content[TLSCompressed.length]; opaque MAC[CipherSpec.hash_size]; } GenericStreamCipher; block-ciphered struct { opaque IV[CipherSpec.block_length]; Dierks & Rescorla Standards Track [Page 55]
RFC 4346 The TLS Protocol April 2006 opaque content[TLSCompressed.length]; opaque MAC[CipherSpec.hash_size]; uint8 padding[GenericBlockCipher.padding_length]; uint8 padding_length; } GenericBlockCipher; A.2. Change Cipher Specs Message struct { enum { change_cipher_spec(1), (255) } type; } ChangeCipherSpec; A.3. Alert Messages enum { warning(1), fatal(2), (255) } AlertLevel; enum { close_notify(0), unexpected_message(10), bad_record_mac(20), decryption_failed(21), record_overflow(22), decompression_failure(30), handshake_failure(40), no_certificate_RESERVED (41), bad_certificate(42), unsupported_certificate(43), certificate_revoked(44), certificate_expired(45), certificate_unknown(46), illegal_parameter(47), unknown_ca(48), access_denied(49), decode_error(50), decrypt_error(51), export_restriction_RESERVED(60), protocol_version(70), insufficient_security(71), internal_error(80), user_canceled(90), no_renegotiation(100), (255) } AlertDescription; struct { AlertLevel level; AlertDescription description; } Alert; Dierks & Rescorla Standards Track [Page 56]
RFC 4346 The TLS Protocol April 2006 A.4. Handshake Protocol enum { hello_request(0), client_hello(1), server_hello(2), certificate(11), server_key_exchange (12), certificate_request(13), server_hello_done(14), certificate_verify(15), client_key_exchange(16), finished(20), (255) } HandshakeType; struct { HandshakeType msg_type; uint24 length; select (HandshakeType) { case hello_request: HelloRequest; case client_hello: ClientHello; case server_hello: ServerHello; case certificate: Certificate; case server_key_exchange: ServerKeyExchange; case certificate_request: CertificateRequest; case server_hello_done: ServerHelloDone; case certificate_verify: CertificateVerify; case client_key_exchange: ClientKeyExchange; case finished: Finished; } body; } Handshake; A.4.1. Hello messages struct { } HelloRequest; struct { uint32 gmt_unix_time; opaque random_bytes[28]; } Random; opaque SessionID<0..32>; uint8 CipherSuite[2]; enum { null(0), (255) } CompressionMethod; struct { ProtocolVersion client_version; Random random; SessionID session_id; CipherSuite cipher_suites<2..2^16-1>; CompressionMethod compression_methods<1..2^8-1>; Dierks & Rescorla Standards Track [Page 57]
RFC 4346 The TLS Protocol April 2006 } ClientHello; struct { ProtocolVersion server_version; Random random; SessionID session_id; CipherSuite cipher_suite; CompressionMethod compression_method; } ServerHello; A.4.2. Server Authentication and Key Exchange Messages opaque ASN.1Cert<2^24-1>; struct { ASN.1Cert certificate_list<0..2^24-1>; } Certificate; enum { rsa, diffie_hellman } KeyExchangeAlgorithm; struct { opaque rsa_modulus<1..2^16-1>; opaque rsa_exponent<1..2^16-1>; } ServerRSAParams; struct { opaque dh_p<1..2^16-1>; opaque dh_g<1..2^16-1>; opaque dh_Ys<1..2^16-1>; } ServerDHParams; struct { select (KeyExchangeAlgorithm) { case diffie_hellman: ServerDHParams params; Signature signed_params; case rsa: ServerRSAParams params; Signature signed_params; }; } ServerKeyExchange; enum { anonymous, rsa, dsa } SignatureAlgorithm; struct { select (KeyExchangeAlgorithm) { case diffie_hellman: ServerDHParams params; Dierks & Rescorla Standards Track [Page 58]
RFC 4346 The TLS Protocol April 2006 case rsa: ServerRSAParams params; }; } ServerParams; struct { select (SignatureAlgorithm) { case anonymous: struct { }; case rsa: digitally-signed struct { opaque md5_hash[16]; opaque sha_hash[20]; }; case dsa: digitally-signed struct { opaque sha_hash[20]; }; }; }; } Signature; enum { rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6), fortezza_dms_RESERVED(20), (255) } ClientCertificateType; opaque DistinguishedName<1..2^16-1>; struct { ClientCertificateType certificate_types<1..2^8-1>; DistinguishedName certificate_authorities<0..2^16-1>; } CertificateRequest; struct { } ServerHelloDone; A.4.3. Client Authentication and Key Exchange Messages struct { select (KeyExchangeAlgorithm) { case rsa: EncryptedPreMasterSecret; case diffie_hellman: ClientDiffieHellmanPublic; } exchange_keys; } ClientKeyExchange; Dierks & Rescorla Standards Track [Page 59]
RFC 4346 The TLS Protocol April 2006 struct { ProtocolVersion client_version; opaque random[46]; } PreMasterSecret; struct { public-key-encrypted PreMasterSecret pre_master_secret; } EncryptedPreMasterSecret; enum { implicit, explicit } PublicValueEncoding; struct { select (PublicValueEncoding) { case implicit: struct {}; case explicit: opaque DH_Yc<1..2^16-1>; } dh_public; } ClientDiffieHellmanPublic; struct { Signature signature; } CertificateVerify; A.4.4. Handshake Finalization Message struct { opaque verify_data[12]; } Finished; A.5. The CipherSuite The following values define the CipherSuite codes used in the client hello and server hello messages. A CipherSuite defines a cipher specification supported in TLS Version 1.1. TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a TLS connection during the first handshake on that channel, but must not be negotiated, as it provides no more protection than an unsecured connection. CipherSuite TLS_NULL_WITH_NULL_NULL = { 0x00,0x00 }; The following CipherSuite definitions require that the server provide an RSA certificate that can be used for key exchange. The server may request either an RSA or a DSS signature-capable certificate in the certificate request message. Dierks & Rescorla Standards Track [Page 60]
RFC 4346 The TLS Protocol April 2006 CipherSuite TLS_RSA_WITH_NULL_MD5 = { 0x00,0x01 }; CipherSuite TLS_RSA_WITH_NULL_SHA = { 0x00,0x02 }; CipherSuite TLS_RSA_WITH_RC4_128_MD5 = { 0x00,0x04 }; CipherSuite TLS_RSA_WITH_RC4_128_SHA = { 0x00,0x05 }; CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA = { 0x00,0x07 }; CipherSuite TLS_RSA_WITH_DES_CBC_SHA = { 0x00,0x09 }; CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0A }; The following CipherSuite definitions are used for server- authenticated (and optionally client-authenticated) Diffie-Hellman. DH denotes cipher suites in which the server's certificate contains the Diffie-Hellman parameters signed by the certificate authority (CA). DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman parameters are signed by a DSS or RSA certificate that has been signed by the CA. The signing algorithm used is specified after the DH or DHE parameter. The server can request an RSA or DSS signature-capable certificate from the client for client authentication or it may request a Diffie-Hellman certificate. Any Diffie-Hellman certificate provided by the client must use the parameters (group and generator) described by the server. CipherSuite TLS_DH_DSS_WITH_DES_CBC_SHA = { 0x00,0x0C }; CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0D }; CipherSuite TLS_DH_RSA_WITH_DES_CBC_SHA = { 0x00,0x0F }; CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x10 }; CipherSuite TLS_DHE_DSS_WITH_DES_CBC_SHA = { 0x00,0x12 }; CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x13 }; CipherSuite TLS_DHE_RSA_WITH_DES_CBC_SHA = { 0x00,0x15 }; CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x16 }; The following cipher suites are used for completely anonymous Diffie-Hellman communications in which neither party is authenticated. Note that this mode is vulnerable to man-in-the- middle attacks and is therefore deprecated. CipherSuite TLS_DH_anon_WITH_RC4_128_MD5 = { 0x00,0x18 }; CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA = { 0x00,0x1A }; CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1B }; When SSLv3 and TLS 1.0 were designed, the United States restricted the export of cryptographic software containing certain strong encryption algorithms. A series of cipher suites were designed to operate at reduced key lengths in order to comply with those regulations. Due to advances in computer performance, these algorithms are now unacceptably weak, and export restrictions have since been loosened. TLS 1.1 implementations MUST NOT negotiate these cipher suites in TLS 1.1 mode. However, for backward compatibility they may be offered in the ClientHello for use with TLS Dierks & Rescorla Standards Track [Page 61]
RFC 4346 The TLS Protocol April 2006 1.0 or SSLv3-only servers. TLS 1.1 clients MUST check that the server did not choose one of these cipher suites during the handshake. These ciphersuites are listed below for informational purposes and to reserve the numbers. CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x03 }; CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x06 }; CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x08 }; CipherSuite TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0B }; CipherSuite TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0E }; CipherSuite TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x11 }; CipherSuite TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x14 }; CipherSuite TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x17 }; CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x19 }; The following cipher suites were defined in [TLSKRB] and are included here for completeness. See [TLSKRB] for details: CipherSuite TLS_KRB5_WITH_DES_CBC_SHA = { 0x00,0x1E }: CipherSuite TLS_KRB5_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1F }; CipherSuite TLS_KRB5_WITH_RC4_128_SHA = { 0x00,0x20 }; CipherSuite TLS_KRB5_WITH_IDEA_CBC_SHA = { 0x00,0x21 }; CipherSuite TLS_KRB5_WITH_DES_CBC_MD5 = { 0x00,0x22 }; CipherSuite TLS_KRB5_WITH_3DES_EDE_CBC_MD5 = { 0x00,0x23 }; CipherSuite TLS_KRB5_WITH_RC4_128_MD5 = { 0x00,0x24 }; CipherSuite TLS_KRB5_WITH_IDEA_CBC_MD5 = { 0x00,0x25 }; The following exportable cipher suites were defined in [TLSKRB] and are included here for completeness. TLS 1.1 implementations MUST NOT negotiate these cipher suites. CipherSuite TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA = { 0x00,0x26}; CipherSuite TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA = { 0x00,0x27}; CipherSuite TLS_KRB5_EXPORT_WITH_RC4_40_SHA = { 0x00,0x28}; CipherSuite TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5 = { 0x00,0x29}; CipherSuite TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x2A}; CipherSuite TLS_KRB5_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x2B}; The following cipher suites were defined in [TLSAES] and are included here for completeness. See [TLSAES] for details: CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x2F }; CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x30 }; CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x31 }; CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x32 }; CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x33 }; CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA = { 0x00, 0x34 }; Dierks & Rescorla Standards Track [Page 62]
RFC 4346 The TLS Protocol April 2006 CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x35 }; CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA = { 0x00, 0x36 }; CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x37 }; CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA = { 0x00, 0x38 }; CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x39 }; CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA = { 0x00, 0x3A }; The cipher suite space is divided into three regions: 1. Cipher suite values with first byte 0x00 (zero) through decimal 191 (0xBF) inclusive are reserved for the IETF Standards Track protocols. 2. Cipher suite values with first byte decimal 192 (0xC0) through decimal 254 (0xFE) inclusive are reserved for assignment for non-Standards Track methods. 3. Cipher suite values with first byte 0xFF are reserved for private use. Additional information describing the role of IANA in the allocation of cipher suite code points is described in Section 11. Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are reserved to avoid collision with Fortezza-based cipher suites in SSL 3. A.6. The Security Parameters These security parameters are determined by the TLS Handshake Protocol and provided as parameters to the TLS Record Layer in order to initialize a connection state. SecurityParameters includes: enum { null(0), (255) } CompressionMethod; enum { server, client } ConnectionEnd; enum { null, rc4, rc2, des, 3des, des40, aes, idea } BulkCipherAlgorithm; enum { stream, block } CipherType; enum { null, md5, sha } MACAlgorithm; /* The algorithms specified in CompressionMethod, BulkCipherAlgorithm, and MACAlgorithm may be added to. */ Dierks & Rescorla Standards Track [Page 63]
RFC 4346 The TLS Protocol April 2006 struct { ConnectionEnd entity; BulkCipherAlgorithm bulk_cipher_algorithm; CipherType cipher_type; uint8 key_size; uint8 key_material_length; MACAlgorithm mac_algorithm; uint8 hash_size; CompressionMethod compression_algorithm; opaque master_secret[48]; opaque client_random[32]; opaque server_random[32]; } SecurityParameters; Appendix B. Glossary Advanced Encryption Standard (AES) AES is a widely used symmetric encryption algorithm. AES is a block cipher with a 128, 192, or 256 bit keys and a 16 byte block size. [AES] TLS currently only supports the 128 and 256 bit key sizes. application protocol An application protocol is a protocol that normally layers directly on top of the transport layer (e.g., TCP/IP). Examples include HTTP, TELNET, FTP, and SMTP. asymmetric cipher See public key cryptography. authentication Authentication is the ability of one entity to determine the identity of another entity. block cipher A block cipher is an algorithm that operates on plaintext in groups of bits, called blocks. 64 bits is a common block size. bulk cipher A symmetric encryption algorithm used to encrypt large quantities of data. cipher block chaining (CBC) CBC is a mode in which every plaintext block encrypted with a block cipher is first exclusive-ORed with the previous ciphertext block (or, in the case of the first block, with the initialization vector). For decryption, every block is first decrypted, then exclusive-ORed with the previous ciphertext block (or IV). Dierks & Rescorla Standards Track [Page 64]
RFC 4346 The TLS Protocol April 2006 certificate As part of the X.509 protocol (a.k.a. ISO Authentication framework), certificates are assigned by a trusted Certificate Authority and provide a strong binding between a party's identity or some other attributes and its public key. client The application entity that initiates a TLS connection to a server. This may or may not imply that the client initiated the underlying transport connection. The primary operational difference between the server and client is that the server is generally authenticated, while the client is only optionally authenticated. client write key The key used to encrypt data written by the client. client write MAC secret The secret data used to authenticate data written by the client. connection A connection is a transport (in the OSI layering model definition) that provides a suitable type of service. For TLS, such connections are peer-to-peer relationships. The connections are transient. Every connection is associated with one session. Data Encryption Standard DES is a very widely used symmetric encryption algorithm. DES is a block cipher with a 56 bit key and an 8 byte block size. Note that in TLS, for key generation purposes, DES is treated as having an 8 byte key length (64 bits), but it still only provides 56 bits of protection. (The low bit of each key byte is presumed to be set to produce odd parity in that key byte.) DES can also be operated in a mode where three independent keys and three encryptions are used for each block of data; this uses 168 bits of key (24 bytes in the TLS key generation method) and provides the equivalent of 112 bits of security. [DES], [3DES] Digital Signature Standard (DSS) A standard for digital signing, including the Digital Signing Algorithm, approved by the National Institute of Standards and Technology, defined in NIST FIPS PUB 186, "Digital Signature Standard," published May 1994 by the U.S. Dept. of Commerce. [DSS] Dierks & Rescorla Standards Track [Page 65]
RFC 4346 The TLS Protocol April 2006 digital signatures Digital signatures utilize public key cryptography and one-way hash functions to produce a signature of the data that can be authenticated, and is difficult to forge or repudiate. handshake An initial negotiation between client and server that establishes the parameters of their transactions. Initialization Vector (IV) When a block cipher is used in CBC mode, the initialization vector is exclusive-ORed with the first plaintext block prior to encryption. IDEA A 64-bit block cipher designed by Xuejia Lai and James Massey. [IDEA] Message Authentication Code (MAC) A Message Authentication Code is a one-way hash computed from a message and some secret data. It is difficult to forge without knowing the secret data. Its purpose is to detect if the message has been altered. master secret Secure secret data used for generating encryption keys, MAC secrets, and IVs. MD5 MD5 is a secure hashing function that converts an arbitrarily long data stream into a digest of fixed size (16 bytes). [MD5] public key cryptography A class of cryptographic techniques employing two-key ciphers. Messages encrypted with the public key can only be decrypted with the associated private key. Conversely, messages signed with the private key can be verified with the public key. one-way hash function A one-way transformation that converts an arbitrary amount of data into a fixed-length hash. It is computationally hard to reverse the transformation or to find collisions. MD5 and SHA are examples of one-way hash functions. RC2 A block cipher developed by Ron Rivest at RSA Data Security, Inc. [RSADSI] described in [RC2]. Dierks & Rescorla Standards Track [Page 66]
RFC 4346 The TLS Protocol April 2006 RC4 A stream cipher invented by Ron Rivest. A compatible cipher is described in [SCH]. RSA A very widely used public-key algorithm that can be used for either encryption or digital signing. [RSA] server The server is the application entity that responds to requests for connections from clients. See also under client. session A TLS session is an association between a client and a server. Sessions are created by the handshake protocol. Sessions define a set of cryptographic security parameters that can be shared among multiple connections. Sessions are used to avoid the expensive negotiation of new security parameters for each connection. session identifier A session identifier is a value generated by a server that identifies a particular session. server write key The key used to encrypt data written by the server. server write MAC secret The secret data used to authenticate data written by the server. SHA The Secure Hash Algorithm is defined in FIPS PUB 180-2. It produces a 20-byte output. Note that all references to SHA actually use the modified SHA-1 algorithm. [SHA] SSL Netscape's Secure Socket Layer protocol [SSL3]. TLS is based on SSL Version 3.0 stream cipher An encryption algorithm that converts a key into a cryptographically strong keystream, which is then exclusive-ORed with the plaintext. symmetric cipher See bulk cipher. Dierks & Rescorla Standards Track [Page 67]
RFC 4346 The TLS Protocol April 2006 Transport Layer Security (TLS) This protocol; also, the Transport Layer Security working group of the Internet Engineering Task Force (IETF). See "Comments" at the end of this document. Appendix C. CipherSuite Definitions CipherSuite Key Exchange Cipher Hash TLS_NULL_WITH_NULL_NULL NULL NULL NULL TLS_RSA_WITH_NULL_MD5 RSA NULL MD5 TLS_RSA_WITH_NULL_SHA RSA NULL SHA TLS_RSA_WITH_RC4_128_MD5 RSA RC4_128 MD5 TLS_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA TLS_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA TLS_RSA_WITH_DES_CBC_SHA RSA DES_CBC SHA TLS_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA TLS_DH_DSS_WITH_DES_CBC_SHA DH_DSS DES_CBC SHA TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA TLS_DH_RSA_WITH_DES_CBC_SHA DH_RSA DES_CBC SHA TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA TLS_DHE_DSS_WITH_DES_CBC_SHA DHE_DSS DES_CBC SHA TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA TLS_DHE_RSA_WITH_DES_CBC_SHA DHE_RSA DES_CBC SHA TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA DHE_RSA 3DES_EDE_CBC SHA TLS_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5 TLS_DH_anon_WITH_DES_CBC_SHA DH_anon DES_CBC SHA TLS_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA Key Exchange Algorithm Description Key size limit DHE_DSS Ephemeral DH with DSS signatures None DHE_RSA Ephemeral DH with RSA signatures None DH_anon Anonymous DH, no signatures None DH_DSS DH with DSS-based certificates None DH_RSA DH with RSA-based certificates None RSA = none NULL No key exchange N/A RSA RSA key exchange None Dierks & Rescorla Standards Track [Page 68]
RFC 4346 The TLS Protocol April 2006 Key Expanded IV Block Cipher Type Material Key Material Size Size NULL Stream 0 0 0 N/A IDEA_CBC Block 16 16 8 8 RC2_CBC_40 Block 5 16 8 8 RC4_40 Stream 5 16 0 N/A RC4_128 Stream 16 16 0 N/A DES40_CBC Block 5 8 8 8 DES_CBC Block 8 8 8 8 3DES_EDE_CBC Block 24 24 8 8 Type Indicates whether this is a stream cipher or a block cipher running in CBC mode. Key Material The number of bytes from the key_block that are used for generating the write keys. Expanded Key Material The number of bytes actually fed into the encryption algorithm. IV Size The amount of data needed to be generated for the initialization vector. Zero for stream ciphers; equal to the block size for block ciphers. Block Size The amount of data a block cipher enciphers in one chunk; a block cipher running in CBC mode can only encrypt an even multiple of its block size. Hash Hash Padding function Size Size NULL 0 0 MD5 16 48 SHA 20 40 Appendix D. Implementation Notes The TLS protocol cannot prevent many common security mistakes. This section provides several recommendations to assist implementors. Dierks & Rescorla Standards Track [Page 69]
RFC 4346 The TLS Protocol April 2006 D.1. Random Number Generation and Seeding TLS requires a cryptographically secure pseudorandom number generator (PRNG). Care must be taken in designing and seeding PRNGs. PRNGs based on secure hash operations, most notably MD5 and/or SHA, are acceptable, but cannot provide more security than the size of the random number generator state. (For example, MD5-based PRNGs usually provide 128 bits of state.) To estimate the amount of seed material being produced, add the number of bits of unpredictable information in each seed byte. For example, keystroke timing values taken from a PC compatible's 18.2 Hz timer provide 1 or 2 secure bits each, even though the total size of the counter value is 16 bits or more. Seeding a 128-bit PRNG would thus require approximately 100 such timer values. [RANDOM] provides guidance on the generation of random values. D.2 Certificates and Authentication Implementations are responsible for verifying the integrity of certificates and should generally support certificate revocation messages. Certificates should always be verified to ensure proper signing by a trusted Certificate Authority (CA). The selection and addition of trusted CAs should be done very carefully. Users should be able to view information about the certificate and root CA. D.3 CipherSuites TLS supports a range of key sizes and security levels, including some that provide no or minimal security. A proper implementation will probably not support many cipher suites. For example, 40-bit encryption is easily broken, so implementations requiring strong security should not allow 40-bit keys. Similarly, anonymous Diffie- Hellman is strongly discouraged because it cannot prevent man-in- the-middle attacks. Applications should also enforce minimum and maximum key sizes. For example, certificate chains containing 512- bit RSA keys or signatures are not appropriate for high-security applications. Dierks & Rescorla Standards Track [Page 70]
RFC 4346 The TLS Protocol April 2006 Appendix E. Backward Compatibility with SSL For historical reasons and in order to avoid a profligate consumption of reserved port numbers, application protocols that are secured by TLS 1.1, TLS 1.0, SSL 3.0, and SSL 2.0 all frequently share the same connection port. For example, the https protocol (HTTP secured by SSL or TLS) uses port 443 regardless of which security protocol it is using. Thus, some mechanism must be determined to distinguish and negotiate among the various protocols. TLS versions 1.1 and 1.0, and SSL 3.0 are very similar; thus, supporting both is easy. TLS clients who wish to negotiate with such older servers SHOULD send client hello messages using the SSL 3.0 record format and client hello structure, sending {3, 2} for the version field to note that they support TLS 1.1. If the server supports only TLS 1.0 or SSL 3.0, it will respond with a downrev 3.0 server hello; if it supports TLS 1.1 it will respond with a TLS 1.1 server hello. The negotiation then proceeds as appropriate for the negotiated protocol. Similarly, a TLS 1.1 server that wishes to interoperate with TLS 1.0 or SSL 3.0 clients SHOULD accept SSL 3.0 client hello messages and respond with a SSL 3.0 server hello if an SSL 3.0 client hello with a version field of {3, 0} is received, denoting that this client does not support TLS. Similarly, if a SSL 3.0 or TLS 1.0 hello with a version field of {3, 1} is received, the server SHOULD respond with a TLS 1.0 hello with a version field of {3, 1}. Whenever a client already knows the highest protocol known to a server (for example, when resuming a session), it SHOULD initiate the connection in that native protocol. TLS 1.1 clients that support SSL Version 2.0 servers MUST send SSL Version 2.0 client hello messages [SSL2]. TLS servers SHOULD accept either client hello format if they wish to support SSL 2.0 clients on the same connection port. The only deviations from the Version 2.0 specification are the ability to specify a version with a value of three and the support for more ciphering types in the CipherSpec. Warning: The ability to send Version 2.0 client hello messages will be phased out with all due haste. Implementors SHOULD make every effort to move forward as quickly as possible. Version 3.0 provides better mechanisms for moving to newer versions. Dierks & Rescorla Standards Track [Page 71]
RFC 4346 The TLS Protocol April 2006 The following cipher specifications are carryovers from SSL Version 2.0. These are assumed to use RSA for key exchange and authentication. V2CipherSpec TLS_RC4_128_WITH_MD5 = { 0x01,0x00,0x80 }; V2CipherSpec TLS_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 }; V2CipherSpec TLS_RC2_CBC_128_CBC_WITH_MD5 = { 0x03,0x00,0x80 }; V2CipherSpec TLS_RC2_CBC_128_CBC_EXPORT40_WITH_MD5 = { 0x04,0x00,0x80 }; V2CipherSpec TLS_IDEA_128_CBC_WITH_MD5 = { 0x05,0x00,0x80 }; V2CipherSpec TLS_DES_64_CBC_WITH_MD5 = { 0x06,0x00,0x40 }; V2CipherSpec TLS_DES_192_EDE3_CBC_WITH_MD5 = { 0x07,0x00,0xC0 }; Cipher specifications native to TLS can be included in Version 2.0 client hello messages using the syntax below. Any V2CipherSpec element with its first byte equal to zero will be ignored by Version 2.0 servers. Clients sending any of the above V2CipherSpecs SHOULD also include the TLS equivalent (see Appendix A.5): V2CipherSpec (see TLS name) = { 0x00, CipherSuite }; Note: TLS 1.1 clients may generate the SSLv2 EXPORT cipher suites in handshakes for backward compatibility but MUST NOT negotiate them in TLS 1.1 mode. E.1. Version 2 Client Hello The Version 2.0 client hello message is presented below using this document's presentation model. The true definition is still assumed to be the SSL Version 2.0 specification. Note that this message MUST be sent directly on the wire, not wrapped as an SSLv3 record uint8 V2CipherSpec[3]; struct { uint16 msg_length; uint8 msg_type; Version version; uint16 cipher_spec_length; uint16 session_id_length; uint16 challenge_length; V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length]; opaque session_id[V2ClientHello.session_id_length]; opaque challenge[V2ClientHello.challenge_length; } V2ClientHello; Dierks & Rescorla Standards Track [Page 72]
RFC 4346 The TLS Protocol April 2006 msg_length This field is the length of the following data in bytes. The high bit MUST be 1 and is not part of the length. msg_type This field, in conjunction with the version field, identifies a version 2 client hello message. The value SHOULD be one (1). version The highest version of the protocol supported by the client (equals ProtocolVersion.version; see Appendix A.1). cipher_spec_length This field is the total length of the field cipher_specs. It cannot be zero and MUST be a multiple of the V2CipherSpec length (3). session_id_length This field MUST have a value of zero. challenge_length The length in bytes of the client's challenge to the server to authenticate itself. When using the SSLv2 backward compatible handshake the client MUST use a 32-byte challenge. cipher_specs This is a list of all CipherSpecs the client is willing and able to use. There MUST be at least one CipherSpec acceptable to the server. session_id This field MUST be empty. challenge The client challenge to the server for the server to identify itself is a (nearly) arbitrary-length random. The TLS server will right-justify the challenge data to become the ClientHello.random data (padded with leading zeroes, if necessary), as specified in this protocol specification. If the length of the challenge is greater than 32 bytes, only the last 32 bytes are used. It is legitimate (but not necessary) for a V3 server to reject a V2 ClientHello that has fewer than 16 bytes of challenge data. Note: Requests to resume a TLS session MUST use a TLS client hello. Dierks & Rescorla Standards Track [Page 73]
RFC 4346 The TLS Protocol April 2006 E.2. Avoiding Man-in-the-Middle Version Rollback When TLS clients fall back to Version 2.0 compatibility mode, they SHOULD use special PKCS #1 block formatting. This is done so that TLS servers will reject Version 2.0 sessions with TLS-capable clients. When TLS clients are in Version 2.0 compatibility mode, they set the right-hand (least significant) 8 random bytes of the PKCS padding (not including the terminal null of the padding) for the RSA encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY to 0x03 (the other padding bytes are random). After decrypting the ENCRYPTED-KEY-DATA field, servers that support TLS SHOULD issue an error if these eight padding bytes are 0x03. Version 2.0 servers receiving blocks padded in this manner will proceed normally. Appendix F. Security Analysis The TLS protocol is designed to establish a secure connection between a client and a server communicating over an insecure channel. This document makes several traditional assumptions, including that attackers have substantial computational resources and cannot obtain secret information from sources outside the protocol. Attackers are assumed to have the ability to capture, modify, delete, replay, and otherwise tamper with messages sent over the communication channel. This appendix outlines how TLS has been designed to resist a variety of attacks. F.1. Handshake Protocol The handshake protocol is responsible for selecting a CipherSpec and generating a Master Secret, which together comprise the primary cryptographic parameters associated with a secure session. The handshake protocol can also optionally authenticate parties who have certificates signed by a trusted certificate authority. F.1.1. Authentication and Key Exchange TLS supports three authentication modes: authentication of both parties, server authentication with an unauthenticated client, and total anonymity. Whenever the server is authenticated, the channel is secure against man-in-the-middle attacks, but completely anonymous sessions are inherently vulnerable to such attacks. Anonymous servers cannot authenticate clients. If the server is authenticated, its certificate message must provide a valid certificate chain leading to an acceptable certificate authority. Similarly, authenticated clients must supply an acceptable certificate to the Dierks & Rescorla Standards Track [Page 74]
RFC 4346 The TLS Protocol April 2006 server. Each party is responsible for verifying that the other's certificate is valid and has not expired or been revoked. The general goal of the key exchange process is to create a pre_master_secret known to the communicating parties and not to attackers. The pre_master_secret will be used to generate the master_secret (see Section 8.1). The master_secret is required to generate the finished messages, encryption keys, and MAC secrets (see Sections 7.4.8, 7.4.9, and 6.3). By sending a correct finished message, parties thus prove that they know the correct pre_master_secret. F.1.1.1. Anonymous Key Exchange Completely anonymous sessions can be established using RSA or Diffie- Hellman for key exchange. With anonymous RSA, the client encrypts a pre_master_secret with the server's uncertified public key extracted from the server key exchange message. The result is sent in a client key exchange message. Since eavesdroppers do not know the server's private key, it will be infeasible for them to decode the pre_master_secret. Note: No anonymous RSA Cipher Suites are defined in this document. With Diffie-Hellman, the server's public parameters are contained in the server key exchange message and the client's are sent in the client key exchange message. Eavesdroppers who do not know the private values should not be able to find the Diffie-Hellman result (i.e., the pre_master_secret). Warning: Completely anonymous connections only provide protection against passive eavesdropping. Unless an independent tamper-proof channel is used to verify that the finished messages were not replaced by an attacker, server authentication is required in environments where active man-in-the-middle attacks are a concern. F.1.1.2. RSA Key Exchange and Authentication With RSA, key exchange and server authentication are combined. The public key either may be contained in the server's certificate or may be a temporary RSA key sent in a server key exchange message. When temporary RSA keys are used, they are signed by the server's RSA certificate. The signature includes the current ClientHello.random, so old signatures and temporary keys cannot be replayed. Servers may use a single temporary RSA key for multiple negotiation sessions. Note: The temporary RSA key option is useful if servers need large Dierks & Rescorla Standards Track [Page 75]
RFC 4346 The TLS Protocol April 2006 certificates but must comply with government-imposed size limits on keys used for key exchange. Note that if ephemeral RSA is not used, compromise of the server's static RSA key results in a loss of confidentiality for all sessions protected under that static key. TLS users desiring Perfect Forward Secrecy should use DHE cipher suites. The damage done by exposure of a private key can be limited by changing one's private key (and certificate) frequently. After verifying the server's certificate, the client encrypts a pre_master_secret with the server's public key. By successfully decoding the pre_master_secret and producing a correct finished message, the server demonstrates that it knows the private key corresponding to the server certificate. When RSA is used for key exchange, clients are authenticated using the certificate verify message (see Section 7.4.8). The client signs a value derived from the master_secret and all preceding handshake messages. These handshake messages include the server certificate, which binds the signature to the server, and ServerHello.random, which binds the signature to the current handshake process. F.1.1.3. Diffie-Hellman Key Exchange with Authentication When Diffie-Hellman key exchange is used, the server can either supply a certificate containing fixed Diffie-Hellman parameters or use the server key exchange message to send a set of temporary Diffie-Hellman parameters signed with a DSS or RSA certificate. Temporary parameters are hashed with the hello.random values before signing to ensure that attackers do not replay old parameters. In either case, the client can verify the certificate or signature to ensure that the parameters belong to the server. If the client has a certificate containing fixed Diffie-Hellman parameters, its certificate contains the information required to complete the key exchange. Note that in this case the client and server will generate the same Diffie-Hellman result (i.e., pre_master_secret) every time they communicate. To prevent the pre_master_secret from staying in memory any longer than necessary, it should be converted into the master_secret as soon as possible. Client Diffie-Hellman parameters must be compatible with those supplied by the server for the key exchange to work. If the client has a standard DSS or RSA certificate or is unauthenticated, it sends a set of temporary parameters to the server in the client key exchange message, then optionally uses a certificate verify message to authenticate itself. Dierks & Rescorla Standards Track [Page 76]
RFC 4346 The TLS Protocol April 2006 If the same DH keypair is to be used for multiple handshakes, either because the client or server has a certificate containing a fixed DH keypair or because the server is reusing DH keys, care must be taken to prevent small subgroup attacks. Implementations SHOULD follow the guidelines found in [SUBGROUP]. Small subgroup attacks are most easily avoided by using one of the DHE ciphersuites and generating a fresh DH private key (X) for each handshake. If a suitable base (such as 2) is chosen, g^X mod p can be computed very quickly, therefore the performance cost is minimized. Additionally, using a fresh key for each handshake provides Perfect Forward Secrecy. Implementations SHOULD generate a new X for each handshake when using DHE ciphersuites. F.1.2. Version Rollback Attacks Because TLS includes substantial improvements over SSL Version 2.0, attackers may try to make TLS-capable clients and servers fall back to Version 2.0. This attack can occur if (and only if) two TLS- capable parties use an SSL 2.0 handshake. Although the solution using non-random PKCS #1 block type 2 message padding is inelegant, it provides a reasonably secure way for Version 3.0 servers to detect the attack. This solution is not secure against attackers who can brute force the key and substitute a new ENCRYPTED-KEY-DATA message containing the same key (but with normal padding) before the application specified wait threshold has expired. Parties concerned about attacks of this scale should not use 40-bit encryption keys. Altering the padding of the least-significant 8 bytes of the PKCS padding does not impact security for the size of the signed hashes and RSA key lengths used in the protocol, since this is essentially equivalent to increasing the input block size by 8 bytes. F.1.3. Detecting Attacks against the Handshake Protocol An attacker might try to influence the handshake exchange to make the parties select different encryption algorithms than they would normally chooses. For this attack, an attacker must actively change one or more handshake messages. If this occurs, the client and server will compute different values for the handshake message hashes. As a result, the parties will not accept each others' finished messages. Without the master_secret, the attacker cannot repair the finished messages, so the attack will be discovered. Dierks & Rescorla Standards Track [Page 77]
RFC 4346 The TLS Protocol April 2006 F.1.4. Resuming Sessions When a connection is established by resuming a session, new ClientHello.random and ServerHello.random values are hashed with the session's master_secret. Provided that the master_secret has not been compromised and that the secure hash operations used to produce the encryption keys and MAC secrets are secure, the connection should be secure and effectively independent from previous connections. Attackers cannot use known encryption keys or MAC secrets to compromise the master_secret without breaking the secure hash operations (which use both SHA and MD5). Sessions cannot be resumed unless both the client and server agree. If either party suspects that the session may have been compromised, or that certificates may have expired or been revoked, it should force a full handshake. An upper limit of 24 hours is suggested for session ID lifetimes, since an attacker who obtains a master_secret may be able to impersonate the compromised party until the corresponding session ID is retired. Applications that may be run in relatively insecure environments should not write session IDs to stable storage. F.1.5. MD5 and SHA TLS uses hash functions very conservatively. Where possible, both MD5 and SHA are used in tandem to ensure that non-catastrophic flaws in one algorithm will not break the overall protocol. F.2. Protecting Application Data The master_secret is hashed with the ClientHello.random and ServerHello.random to produce unique data encryption keys and MAC secrets for each connection. Outgoing data is protected with a MAC before transmission. To prevent message replay or modification attacks, the MAC is computed from the MAC secret, the sequence number, the message length, the message contents, and two fixed character strings. The message type field is necessary to ensure that messages intended for one TLS Record Layer client are not redirected to another. The sequence number ensures that attempts to delete or reorder messages will be detected. Since sequence numbers are 64 bits long, they should never overflow. Messages from one party cannot be inserted into the other's output, since they use independent MAC secrets. Similarly, the server-write and client-write keys are independent, so stream cipher keys are used only once. Dierks & Rescorla Standards Track [Page 78]
RFC 4346 The TLS Protocol April 2006 If an attacker does break an encryption key, all messages encrypted with it can be read. Similarly, compromise of a MAC key can make message modification attacks possible. Because MACs are also encrypted, message-alteration attacks generally require breaking the encryption algorithm as well as the MAC. Note: MAC secrets may be larger than encryption keys, so messages can remain tamper resistant even if encryption keys are broken. F.3. Explicit IVs [CBCATT] describes a chosen plaintext attack on TLS that depends on knowing the IV for a record. Previous versions of TLS [TLS1.0] used the CBC residue of the previous record as the IV and therefore enabled this attack. This version uses an explicit IV in order to protect against this attack. F.4. Security of Composite Cipher Modes TLS secures transmitted application data via the use of symmetric encryption and authentication functions defined in the negotiated ciphersuite. The objective is to protect both the integrity and confidentiality of the transmitted data from malicious actions by active attackers in the network. It turns out that the order in which encryption and authentication functions are applied to the data plays an important role for achieving this goal [ENCAUTH]. The most robust method, called encrypt-then-authenticate, first applies encryption to the data and then applies a MAC to the ciphertext. This method ensures that the integrity and confidentiality goals are obtained with ANY pair of encryption and MAC functions, provided that the former is secure against chosen plaintext attacks and that the MAC is secure against chosen-message attacks. TLS uses another method, called authenticate-then-encrypt, in which first a MAC is computed on the plaintext and then the concatenation of plaintext and MAC is encrypted. This method has been proven secure for CERTAIN combinations of encryption functions and MAC functions, but it is not guaranteed to be secure in general. In particular, it has been shown that there exist perfectly secure encryption functions (secure even in the information-theoretic sense) that combined with any secure MAC function, fail to provide the confidentiality goal against an active attack. Therefore, new ciphersuites and operation modes adopted into TLS need to be analyzed under the authenticate-then-encrypt method to verify that they achieve the stated integrity and confidentiality goals. Dierks & Rescorla Standards Track [Page 79]
RFC 4346 The TLS Protocol April 2006 Currently, the security of the authenticate-then-encrypt method has been proven for some important cases. One is the case of stream ciphers in which a computationally unpredictable pad of the length of the message, plus the length of the MAC tag, is produced using a pseudo-random generator and this pad is xor-ed with the concatenation of plaintext and MAC tag. The other is the case of CBC mode using a secure block cipher. In this case, security can be shown if one applies one CBC encryption pass to the concatenation of plaintext and MAC and uses a new, independent, and unpredictable IV for each new pair of plaintext and MAC. In previous versions of SSL, CBC mode was used properly EXCEPT that it used a predictable IV in the form of the last block of the previous ciphertext. This made TLS open to chosen plaintext attacks. This version of the protocol is immune to those attacks. For exact details in the encryption modes proven secure, see [ENCAUTH]. F.5. Denial of Service TLS is susceptible to a number of denial of service (DoS) attacks. In particular, an attacker who initiates a large number of TCP connections can cause a server to consume large amounts of CPU doing RSA decryption. However, because TLS is generally used over TCP, it is difficult for the attacker to hide his point of origin if proper TCP SYN randomization is used [SEQNUM] by the TCP stack. Because TLS runs over TCP, it is also susceptible to a number of denial of service attacks on individual connections. In particular, attackers can forge RSTs, thereby terminating connections, or forge partial TLS records, thereby causing the connection to stall. These attacks cannot in general be defended against by a TCP-using protocol. Implementors or users who are concerned with this class of attack should use IPsec AH [AH-ESP] or ESP [AH-ESP]. F.6. Final Notes For TLS to be able to provide a secure connection, both the client and server systems, keys, and applications must be secure. In addition, the implementation must be free of security errors. The system is only as strong as the weakest key exchange and authentication algorithm supported, and only trustworthy cryptographic functions should be used. Short public keys, 40-bit bulk encryption keys, and anonymous servers should be used with great caution. Implementations and users must be careful when deciding which certificates and certificate authorities are acceptable; a dishonest certificate authority can do tremendous damage. Dierks & Rescorla Standards Track [Page 80]
RFC 4346 The TLS Protocol April 2006 Normative References [AES] National Institute of Standards and Technology, "Specification for the Advanced Encryption Standard (AES)" FIPS 197. November 26, 2001. [3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES," IEEE Spectrum, v. 16, n. 7, July 1979, pp. 40-41. [DES] ANSI X3.106, "American National Standard for Information Systems-Data Link Encryption," American National Standards Institute, 1983. [DSS] NIST FIPS PUB 186-2, "Digital Signature Standard," National Institute of Standards and Technology, U.S. Department of Commerce, 2000. [HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [IDEA] X. Lai, "On the Design and Security of Block Ciphers," ETH Series in Information Processing, v. 1, Konstanz: Hartung-Gorre Verlag, 1992. [MD5] Rivest, R., "The MD5 Message-Digest Algorithm ", RFC 1321, April 1992. [PKCS1A] B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 1.5", RFC 2313, March 1998. [PKCS1B] J. Jonsson, B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003. [PKIX] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002. [RC2] Rivest, R., "A Description of the RC2(r) Encryption Algorithm", RFC 2268, March 1998. [SCH] B. Schneier. "Applied Cryptography: Protocols, Algorithms, and Source Code in C, 2ed", Published by John Wiley & Sons, Inc. 1996. Dierks & Rescorla Standards Track [Page 81]
RFC 4346 The TLS Protocol April 2006 [SHA] NIST FIPS PUB 180-2, "Secure Hash Standard," National Institute of Standards and Technology, U.S. Department of Commerce., August 2001. [REQ] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [TLSAES] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS)", RFC 3268, June 2002. [TLSEXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 3546, June 2003. [TLSKRB] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher Suites to Transport Layer Security (TLS)", RFC 2712, October 1999. Informative References [AH-ESP] Kent, S., "IP Authentication Header", RFC 4302, December 2005. Eastlake 3rd, D., "Cryptographic Algorithm Implementation Requirements for Encapsulating Security Payload (ESP) and Authentication Header (AH)", RFC 4305, December 2005. [BLEI] Bleichenbacher D., "Chosen Ciphertext Attacks against Protocols Based on RSA Encryption Standard PKCS #1" in Advances in Cryptology -- CRYPTO'98, LNCS vol. 1462, pages: 1-12, 1998. [CBCATT] Moeller, B., "Security of CBC Ciphersuites in SSL/TLS: Problems and Countermeasures", http://www.openssl.org/~bodo/tls-cbc.txt. [CBCTIME] Canvel, B., "Password Interception in a SSL/TLS Channel", http://lasecwww.epfl.ch/memo_ssl.shtml, 2003. [ENCAUTH] Krawczyk, H., "The Order of Encryption and Authentication for Protecting Communications (Or: How Secure is SSL?)", Crypto 2001. Dierks & Rescorla Standards Track [Page 82]
RFC 4346 The TLS Protocol April 2006 [KPR03] Klima, V., Pokorny, O., Rosa, T., "Attacking RSA-based Sessions in SSL/TLS", http://eprint.iacr.org/2003/052/, March 2003. [PKCS6] RSA Laboratories, "PKCS #6: RSA Extended Certificate Syntax Standard," version 1.5, November 1993. [PKCS7] RSA Laboratories, "PKCS #7: RSA Cryptographic Message Syntax Standard," version 1.5, November 1993. [RANDOM] Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [RSA] R. Rivest, A. Shamir, and L. M. Adleman, "A Method for Obtaining Digital Signatures and Public-Key Cryptosystems," Communications of the ACM, v. 21, n. 2, Feb 1978, pp. 120-126. [SEQNUM] Bellovin, S., "Defending Against Sequence Number Attacks", RFC 1948, May 1996. [SSL2] Hickman, Kipp, "The SSL Protocol", Netscape Communications Corp., Feb 9, 1995. [SSL3] A. Frier, P. Karlton, and P. Kocher, "The SSL 3.0 Protocol", Netscape Communications Corp., Nov 18, 1996. [SUBGROUP] Zuccherato, R., "Methods for Avoiding the "Small-Subgroup" Attacks on the Diffie-Hellman Key Agreement Method for S/MIME", RFC 2785, March 2000. [TCP] Hellstrom, G. and P. Jones, "RTP Payload for Text Conversation", RFC 4103, June 2005. [TIMING] Boneh, D., Brumley, D., "Remote timing attacks are practical", USENIX Security Symposium 2003. [TLS1.0] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [X501] ITU-T Recommendation X.501: Information Technology - Open Systems Interconnection - The Directory: Models, 1993. [X509] ITU-T Recommendation X.509 (1997 E): Information Technology - Open Systems Interconnection - "The Directory - Authentication Framework". 1988. Dierks & Rescorla Standards Track [Page 83]
RFC 4346 The TLS Protocol April 2006 [XDR] Srinivasan, R., "XDR: External Data Representation Standard", RFC 1832, August 1995. Authors' Addresses Working Group Chairs Win Treese EMail: treese@acm.org Eric Rescorla EMail: ekr@rtfm.com Editors Tim Dierks Independent EMail: tim@dierks.org Eric Rescorla RTFM, Inc. EMail: ekr@rtfm.com Other Contributors Christopher Allen (co-editor of TLS 1.0) Alacrity Ventures EMail: ChristopherA@AlacrityManagement.com Martin Abadi University of California, Santa Cruz EMail: abadi@cs.ucsc.edu Ran Canetti IBM EMail: canetti@watson.ibm.com Dierks & Rescorla Standards Track [Page 84]
RFC 4346 The TLS Protocol April 2006 Taher Elgamal Securify EMail: taher@securify.com Anil Gangolli EMail: anil@busybuddha.org Kipp Hickman Phil Karlton (co-author of SSLv3) Paul Kocher (co-author of SSLv3) Cryptography Research EMail: paul@cryptography.com Hugo Krawczyk Technion Israel Institute of Technology EMail: hugo@ee.technion.ac.il Robert Relyea Netscape Communications EMail: relyea@netscape.com Jim Roskind Netscape Communications EMail: jar@netscape.com Michael Sabin Dan Simon Microsoft, Inc. EMail: dansimon@microsoft.com Tom Weinstein Dierks & Rescorla Standards Track [Page 85]
RFC 4346 The TLS Protocol April 2006 Comments The discussion list for the IETF TLS working group is located at the e-mail address <ietf-tls@lists.consensus.com>. Information on the group and information on how to subscribe to the list is at <http://lists.consensus.com/>. Archives of the list can be found at: <http://www.imc.org/ietf-tls/mail-archive/> Dierks & Rescorla Standards Track [Page 86]
RFC 4346 The TLS Protocol April 2006 Full Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgement Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Dierks & Rescorla Standards Track [Page 87]