RFC 5246: The Transport Layer Security (TLS) Protocol Version 1.2
T. Dierks E. Rescorla
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Obsoleted by: 8446 PROPOSED STANDARD

Updated by: 5746, 5878, 6176, 7465, 7507, 7568, 7627,

7685, 7905, 7919, 8447, 9155 Errata Exist

Network Working Group T. Dierks Request for Comments: 5246 Independent Obsoletes: 3268, 4346, 4366 E. Rescorla Updates: 4492 RTFM, Inc. Category: Standards Track August 2008 The Transport Layer Security (TLS) Protocol Version 1.2 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. Abstract This document specifies Version 1.2 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. Table of Contents 1. Introduction ....................................................4 1.1. Requirements Terminology ...................................5 1.2. Major Differences from TLS 1.1 .............................5 2. Goals ...........................................................6 3. Goals of This Document ..........................................7 4. Presentation Language ...........................................7 4.1. Basic Block Size ...........................................7 4.2. Miscellaneous ..............................................8 4.3. Vectors ....................................................8 4.4. Numbers ....................................................9 4.5. Enumerateds ................................................9 4.6. Constructed Types .........................................10 4.6.1. Variants ...........................................10 4.7. Cryptographic Attributes ..................................12 4.8. Constants .................................................14 5. HMAC and the Pseudorandom Function .............................14 6. The TLS Record Protocol ........................................15 6.1. Connection States .........................................16 6.2. Record Layer ..............................................19 6.2.1. Fragmentation ......................................19 Dierks & Rescorla Standards Track [Page 1]

RFC 5246 TLS August 2008 6.2.2. Record Compression and Decompression ...............20 6.2.3. Record Payload Protection ..........................21 6.2.3.1. Null or Standard Stream Cipher ............22 6.2.3.2. CBC Block Cipher ..........................22 6.2.3.3. AEAD Ciphers ..............................24 6.3. Key Calculation ...........................................25 7. The TLS Handshaking Protocols ..................................26 7.1. Change Cipher Spec Protocol ...............................27 7.2. Alert Protocol ............................................28 7.2.1. Closure Alerts .....................................29 7.2.2. Error Alerts .......................................30 7.3. Handshake Protocol Overview ...............................33 7.4. Handshake Protocol ........................................37 7.4.1. Hello Messages .....................................38 7.4.1.1. Hello Request .............................38 7.4.1.2. Client Hello ..............................39 7.4.1.3. Server Hello ..............................42 7.4.1.4. Hello Extensions ..........................44 7.4.1.4.1. Signature Algorithms ...........45 7.4.2. Server Certificate .................................47 7.4.3. Server Key Exchange Message ........................50 7.4.4. Certificate Request ................................53 7.4.5. Server Hello Done ..................................55 7.4.6. Client Certificate .................................55 7.4.7. Client Key Exchange Message ........................57 7.4.7.1. RSA-Encrypted Premaster Secret Message ....58 7.4.7.2. Client Diffie-Hellman Public Value ........61 7.4.8. Certificate Verify .................................62 7.4.9. Finished ...........................................63 8. Cryptographic Computations .....................................64 8.1. Computing the Master Secret ...............................64 8.1.1. RSA ................................................65 8.1.2. Diffie-Hellman .....................................65 9. Mandatory Cipher Suites ........................................65 10. Application Data Protocol .....................................65 11. Security Considerations .......................................65 12. IANA Considerations ...........................................65 Appendix A. Protocol Data Structures and Constant Values ..........68 A.1. Record Layer ..............................................68 A.2. Change Cipher Specs Message ...............................69 A.3. Alert Messages ............................................69 A.4. Handshake Protocol ........................................70 A.4.1. Hello Messages .....................................71 A.4.2. Server Authentication and Key Exchange Messages ....72 A.4.3. Client Authentication and Key Exchange Messages ....74 A.4.4. Handshake Finalization Message .....................74 A.5. The Cipher Suite ..........................................75 A.6. The Security Parameters ...................................77 Dierks & Rescorla Standards Track [Page 2]

RFC 5246 TLS August 2008 A.7. Changes to RFC 4492 .......................................78 Appendix B. Glossary ..............................................78 Appendix C. Cipher Suite Definitions ..............................83 Appendix D. Implementation Notes ..................................85 D.1. Random Number Generation and Seeding ......................85 D.2. Certificates and Authentication ...........................85 D.3. Cipher Suites .............................................85 D.4. Implementation Pitfalls ...................................85 Appendix E. Backward Compatibility ................................87 E.1. Compatibility with TLS 1.0/1.1 and SSL 3.0 ................87 E.2. Compatibility with SSL 2.0 ................................88 E.3. Avoiding Man-in-the-Middle Version Rollback ...............90 Appendix F. Security Analysis .....................................91 F.1. Handshake Protocol ........................................91 F.1.1. Authentication and Key Exchange ....................91 F.1.1.1. Anonymous Key Exchange ....................91 F.1.1.2. RSA Key Exchange and Authentication .......92 F.1.1.3. Diffie-Hellman Key Exchange with Authentication ............................92 F.1.2. Version Rollback Attacks ...........................93 F.1.3. Detecting Attacks Against the Handshake Protocol ...94 F.1.4. Resuming Sessions ..................................94 F.2. Protecting Application Data ...............................94 F.3. Explicit IVs ..............................................95 F.4. Security of Composite Cipher Modes ........................95 F.5. Denial of Service .........................................96 F.6. Final Notes ...............................................96 Normative References ..............................................97 Informative References ............................................98 Working Group Information ........................................101 Contributors .....................................................101 Dierks & Rescorla Standards Track [Page 3]

RFC 5246 TLS August 2008 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., AES [AES], 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-1, 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], DSA [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. Dierks & Rescorla Standards Track [Page 4]

RFC 5246 TLS August 2008 One advantage of TLS is that it is application protocol independent. Higher-level protocols can layer on top of the TLS protocol 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. Requirements Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [REQ]. 1.2. Major Differences from TLS 1.1 This document is a revision of the TLS 1.1 [TLS1.1] protocol which contains improved flexibility, particularly for negotiation of cryptographic algorithms. The major changes are: - The MD5/SHA-1 combination in the pseudorandom function (PRF) has been replaced with cipher-suite-specified PRFs. All cipher suites in this document use P_SHA256. - The MD5/SHA-1 combination in the digitally-signed element has been replaced with a single hash. Signed elements now include a field that explicitly specifies the hash algorithm used. - Substantial cleanup to the client's and server's ability to specify which hash and signature algorithms they will accept. Note that this also relaxes some of the constraints on signature and hash algorithms from previous versions of TLS. - Addition of support for authenticated encryption with additional data modes. - TLS Extensions definition and AES Cipher Suites were merged in from external [TLSEXT] and [TLSAES]. - Tighter checking of EncryptedPreMasterSecret version numbers. - Tightened up a number of requirements. - Verify_data length now depends on the cipher suite (default is still 12). - Cleaned up description of Bleichenbacher/Klima attack defenses. Dierks & Rescorla Standards Track [Page 5]

RFC 5246 TLS August 2008 - Alerts MUST now be sent in many cases. - After a certificate_request, if no certificates are available, clients now MUST send an empty certificate list. - TLS_RSA_WITH_AES_128_CBC_SHA is now the mandatory to implement cipher suite. - Added HMAC-SHA256 cipher suites. - Removed IDEA and DES cipher suites. They are now deprecated and will be documented in a separate document. - Support for the SSLv2 backward-compatible hello is now a MAY, not a SHOULD, with sending it a SHOULD NOT. Support will probably become a SHOULD NOT in the future. - Added limited "fall-through" to the presentation language to allow multiple case arms to have the same encoding. - Added an Implementation Pitfalls sections - The usual clarifications and editorial work. 2. Goals The goals of the TLS protocol, in order of 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. 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. Dierks & Rescorla Standards Track [Page 6]

RFC 5246 TLS August 2008 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 the various versions of TLS and SSL 3.0 do not interoperate (although each protocol incorporates a mechanism by which an implementation can back down to 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. 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 byte stream, 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. Dierks & Rescorla Standards Track [Page 7]

RFC 5246 TLS August 2008 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 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, which is 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 Dierks & Rescorla Standards Track [Page 8]

RFC 5246 TLS August 2008 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 byte (big-endian) order; the uint32 represented by the hex bytes 01 02 03 04 is equivalent to the decimal value 16909060. Note that in some cases (e.g., DH parameters) it is necessary to represent integers as opaque vectors. In such cases, they are represented as unsigned integers (i.e., leading zero octets are not required even if the most significant bit is set). 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. enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te; An enumerated occupies 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; Dierks & Rescorla Standards Track [Page 9]

RFC 5246 TLS August 2008 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. 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. Case arms have limited fall-through: if two case arms follow in immediate succession with no fields in between, then they Dierks & Rescorla Standards Track [Page 10]

RFC 5246 TLS August 2008 both contain the same fields. Thus, in the example below, "orange" and "banana" both contain V2. Note that this is a new piece of syntax in TLS 1.2. 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 e3: case e4: Te3; .... case en: Ten; } [[fv]]; } [[Tv]]; For example: enum { apple, orange, banana } 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: case banana: V2; /* VariantBody, tag = orange or banana */ } variant_body; /* optional label on variant */ } VariantRecord; Dierks & Rescorla Standards Track [Page 11]

RFC 5246 TLS August 2008 4.7. Cryptographic Attributes The five cryptographic operations -- digital signing, stream cipher encryption, block cipher encryption, authenticated encryption with additional data (AEAD) encryption, and public key encryption -- are designated digitally-signed, stream-ciphered, block-ciphered, aead- 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). A digitally-signed element is encoded as a struct DigitallySigned: struct { SignatureAndHashAlgorithm algorithm; opaque signature<0..2^16-1>; } DigitallySigned; The algorithm field specifies the algorithm used (see Section 7.4.1.4.1 for the definition of this field). Note that the introduction of the algorithm field is a change from previous versions. The signature is a digital signature using those algorithms over the contents of the element. The contents themselves do not appear on the wire but are simply calculated. The length of the signature is specified by the signing algorithm and key. In RSA signing, the opaque vector contains the signature generated using the RSASSA-PKCS1-v1_5 signature scheme defined in [PKCS1]. As discussed in [PKCS1], the DigestInfo MUST be DER-encoded [X680] [X690]. For hash algorithms without parameters (which includes SHA-1), the DigestInfo.AlgorithmIdentifier.parameters field MUST be NULL, but implementations MUST accept both without parameters and with NULL parameters. Note that earlier versions of TLS used a different RSA signature scheme that did not include a DigestInfo encoding. In DSA, the 20 bytes of the SHA-1 hash are run directly through the Digital Signing Algorithm with no additional hashing. This produces two values, r and s. The DSA signature is an opaque vector, as above, the contents of which are the DER encoding of: Dss-Sig-Value ::= SEQUENCE { r INTEGER, s INTEGER } Dierks & Rescorla Standards Track [Page 12]

RFC 5246 TLS August 2008 Note: In current terminology, DSA refers to the Digital Signature Algorithm and DSS refers to the NIST standard. In the original SSL and TLS specs, "DSS" was used universally. This document uses "DSA" to refer to the algorithm, "DSS" to refer to the standard, and it uses "DSS" in the code point definitions for historical continuity. 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 AEAD encryption, the plaintext is simultaneously encrypted and integrity protected. The input may be of any length, and aead- ciphered output is generally larger than the input in order to accommodate the integrity check value. 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 encryption algorithm and key. RSA encryption is done using the RSAES-PKCS1-v1_5 encryption scheme defined in [PKCS1]. In the following example stream-ciphered struct { uint8 field1; uint8 field2; digitally-signed opaque { uint8 field3<0..255>; uint8 field4; }; } UserType; The contents of the inner struct (field3 and field4) are used as input for the signature/hash 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 signature and hash algorithm, plus two bytes for the length of the signature, plus the length of the output of the signing Dierks & Rescorla Standards Track [Page 13]

RFC 5246 TLS August 2008 algorithm. The length of the signature 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 The TLS record layer uses a keyed Message Authentication Code (MAC) to protect message integrity. The cipher suites defined in this document use a construction known as HMAC, described in [HMAC], which is based on a hash function. Other cipher suites MAY define their own MAC constructions, if needed. In addition, a construction is required to do expansion of secrets into blocks of data for the purposes of key generation or validation. This pseudorandom function (PRF) takes as input a secret, a seed, and an identifying label and produces an output of arbitrary length. In this section, we define one PRF, based on HMAC. This PRF with the SHA-256 hash function is used for all cipher suites defined in this document and in TLS documents published prior to this document when TLS 1.2 is negotiated. New cipher suites MUST explicitly specify a PRF and, in general, SHOULD use the TLS PRF with SHA-256 or a stronger standard hash function. 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: Dierks & Rescorla Standards Track [Page 14]

RFC 5246 TLS August 2008 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 necessary to produce the required quantity of data. For example, if P_SHA256 is being used to create 80 bytes of data, it will have to be iterated three times (through A(3)), creating 96 bytes of output data; the last 16 bytes of the final iteration will then be discarded, leaving 80 bytes of output data. TLS's PRF is created by applying P_hash to the secret as: PRF(secret, label, seed) = P_<hash>(secret, 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 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 protocols that use the record protocol 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 content types can be supported by the record protocol. New record content type values are assigned by IANA in the TLS Content Type Registry as described in Section 12. Dierks & Rescorla Standards Track [Page 15]

RFC 5246 TLS August 2008 Implementations MUST NOT send record types not defined in this document unless negotiated by some extension. If a TLS implementation receives an unexpected record type, it MUST send an unexpected_message alert. Any protocol designed for use over TLS must be carefully designed to deal with all possible attacks against it. As a practical matter, this means that the protocol designer must be aware of what security properties TLS does and does not provide and cannot safely rely on the latter. Note in particular that type and length of a record are not protected by encryption. If this information is itself sensitive, application designers may wish to take steps (padding, cover traffic) to minimize information leakage. 6.1. Connection States A TLS connection state is the operating environment of the TLS Record Protocol. It specifies a compression algorithm, an encryption algorithm, and a MAC algorithm. In addition, the parameters for these algorithms are known: the MAC key 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 ChangeCipherSpec 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. PRF algorithm An algorithm used to generate keys from the master secret (see Sections 5 and 6.3). Dierks & Rescorla Standards Track [Page 16]

RFC 5246 TLS August 2008 bulk encryption algorithm An algorithm to be used for bulk encryption. This specification includes the key size of this algorithm, whether it is a block, stream, or AEAD cipher, the block size of the cipher (if appropriate), and the lengths of explicit and implicit initialization vectors (or nonces). MAC algorithm An algorithm to be used for message authentication. This specification includes the size of the value returned by the MAC algorithm. compression algorithm An algorithm to be used for data compression. This specification must include all information the algorithm requires to do compression. master secret A 48-byte secret shared between the two peers in the connection. 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 { tls_prf_sha256 } PRFAlgorithm; enum { null, rc4, 3des, aes } BulkCipherAlgorithm; enum { stream, block, aead } CipherType; enum { null, hmac_md5, hmac_sha1, hmac_sha256, hmac_sha384, hmac_sha512} MACAlgorithm; enum { null(0), (255) } CompressionMethod; /* The algorithms specified in CompressionMethod, PRFAlgorithm, BulkCipherAlgorithm, and MACAlgorithm may be added to. */ Dierks & Rescorla Standards Track [Page 17]

RFC 5246 TLS August 2008 struct { ConnectionEnd entity; PRFAlgorithm prf_algorithm; BulkCipherAlgorithm bulk_cipher_algorithm; CipherType cipher_type; uint8 enc_key_length; uint8 block_length; uint8 fixed_iv_length; uint8 record_iv_length; MACAlgorithm mac_algorithm; uint8 mac_length; uint8 mac_key_length; 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 six items (some of which are not required by all ciphers, and are thus empty): client write MAC key server write MAC key client write encryption key server write encryption key client write IV server write IV 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. 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. Dierks & Rescorla Standards Track [Page 18]

RFC 5246 TLS August 2008 MAC key The MAC key 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). struct { uint8 major; uint8 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. Dierks & Rescorla Standards Track [Page 19]

RFC 5246 TLS August 2008 version The version of the protocol being employed. This document describes TLS Version 1.2, which uses the version { 3, 3 }. The version value 3.3 is historical, deriving from the use of {3, 1} for TLS 1.0. (See Appendix A.1.) Note that a client that supports multiple versions of TLS may not know what version will be employed before it receives the ServerHello. See Appendix E for discussion about what record layer version number should be employed for ClientHello. length The length (in bytes) of the following TLSPlaintext.fragment. The length MUST NOT exceed 2^14. fragment The application data. This data is transparent and treated as an independent block to be dealt with by the higher-level protocol specified by the type field. Implementations MUST NOT send zero-length fragments of Handshake, Alert, or ChangeCipherSpec content types. Zero-length fragments of Application data MAY be sent as they are potentially useful as a traffic analysis countermeasure. 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. 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. [RFC3749] describes compression algorithms for TLS. 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 MUST report a fatal decompression failure error. Dierks & Rescorla Standards Track [Page 20]

RFC 5246 TLS August 2008 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 MUST 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. struct { ContentType type; ProtocolVersion version; uint16 length; select (SecurityParameters.cipher_type) { case stream: GenericStreamCipher; case block: GenericBlockCipher; case aead: GenericAEADCipher; } 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 MUST NOT exceed 2^14 + 2048. Dierks & Rescorla Standards Track [Page 21]

RFC 5246 TLS August 2008 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[SecurityParameters.mac_length]; } GenericStreamCipher; The MAC is generated as: MAC(MAC_write_key, seq_num + TLSCompressed.type + TLSCompressed.version + TLSCompressed.length + TLSCompressed.fragment); where "+" denotes concatenation. seq_num The sequence number for this record. MAC The MAC algorithm specified by SecurityParameters.mac_algorithm. 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 cipher suite 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). For both null and stream ciphers, TLSCiphertext.length is TLSCompressed.length plus SecurityParameters.mac_length. 6.2.3.2. CBC Block Cipher For block ciphers (such as 3DES or AES), the encryption and MAC functions convert TLSCompressed.fragment structures to and from block TLSCiphertext.fragment structures. Dierks & Rescorla Standards Track [Page 22]

RFC 5246 TLS August 2008 struct { opaque IV[SecurityParameters.record_iv_length]; block-ciphered struct { opaque content[TLSCompressed.length]; opaque MAC[SecurityParameters.mac_length]; uint8 padding[GenericBlockCipher.padding_length]; uint8 padding_length; }; } GenericBlockCipher; The MAC is generated as described in Section 6.2.3.1. IV The Initialization Vector (IV) SHOULD be chosen at random, and MUST be unpredictable. Note that in versions of TLS prior to 1.1, there was no IV field, and the last ciphertext block of the previous record (the "CBC residue") was used as the IV. This was changed to prevent the attacks described in [CBCATT]. For block ciphers, the IV length is of length SecurityParameters.record_iv_length, which is equal to the SecurityParameters.block_size. 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 MUST check this padding and MUST 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 SecurityParameters.block_length, TLSCompressed.length, SecurityParameters.mac_length, 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 Dierks & Rescorla Standards Track [Page 23]

RFC 5246 TLS August 2008 IV. 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, 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. 6.2.3.3. AEAD Ciphers For AEAD [AEAD] ciphers (such as [CCM] or [GCM]), the AEAD function converts TLSCompressed.fragment structures to and from AEAD TLSCiphertext.fragment structures. struct { opaque nonce_explicit[SecurityParameters.record_iv_length]; aead-ciphered struct { opaque content[TLSCompressed.length]; }; } GenericAEADCipher; AEAD ciphers take as input a single key, a nonce, a plaintext, and "additional data" to be included in the authentication check, as described in Section 2.1 of [AEAD]. The key is either the client_write_key or the server_write_key. No MAC key is used. Each AEAD cipher suite MUST specify how the nonce supplied to the AEAD operation is constructed, and what is the length of the GenericAEADCipher.nonce_explicit part. In many cases, it is Dierks & Rescorla Standards Track [Page 24]

RFC 5246 TLS August 2008 appropriate to use the partially implicit nonce technique described in Section 3.2.1 of [AEAD]; with record_iv_length being the length of the explicit part. In this case, the implicit part SHOULD be derived from key_block as client_write_iv and server_write_iv (as described in Section 6.3), and the explicit part is included in GenericAEAEDCipher.nonce_explicit. The plaintext is the TLSCompressed.fragment. The additional authenticated data, which we denote as additional_data, is defined as follows: additional_data = seq_num + TLSCompressed.type + TLSCompressed.version + TLSCompressed.length; where "+" denotes concatenation. The aead_output consists of the ciphertext output by the AEAD encryption operation. The length will generally be larger than TLSCompressed.length, but by an amount that varies with the AEAD cipher. Since the ciphers might incorporate padding, the amount of overhead could vary with different TLSCompressed.length values. Each AEAD cipher MUST NOT produce an expansion of greater than 1024 bytes. Symbolically, AEADEncrypted = AEAD-Encrypt(write_key, nonce, plaintext, additional_data) In order to decrypt and verify, the cipher takes as input the key, nonce, the "additional_data", and the AEADEncrypted value. The output is either the plaintext or an error indicating that the decryption failed. There is no separate integrity check. That is: TLSCompressed.fragment = AEAD-Decrypt(write_key, nonce, AEADEncrypted, additional_data) If the decryption fails, a fatal bad_record_mac alert MUST be generated. 6.3. Key Calculation The Record Protocol requires an algorithm to generate keys required by the current connection state (see Appendix A.6) from the security parameters provided by the handshake protocol. Dierks & Rescorla Standards Track [Page 25]

RFC 5246 TLS August 2008 The master secret is expanded into a sequence of secure bytes, which is then split to a client write MAC key, a server write MAC key, a client write encryption key, and a server write encryption key. Each of these is generated from the byte sequence in that order. Unused values are empty. Some AEAD ciphers may additionally require a client write IV and a server write IV (see Section 6.2.3.3). When keys and MAC keys 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_key[SecurityParameters.mac_key_length] server_write_MAC_key[SecurityParameters.mac_key_length] client_write_key[SecurityParameters.enc_key_length] server_write_key[SecurityParameters.enc_key_length] client_write_IV[SecurityParameters.fixed_iv_length] server_write_IV[SecurityParameters.fixed_iv_length] Currently, the client_write_IV and server_write_IV are only generated for implicit nonce techniques as described in Section 3.2.1 of [AEAD]. Implementation note: The currently defined cipher suite which requires the most material is AES_256_CBC_SHA256. It requires 2 x 32 byte keys and 2 x 32 byte MAC keys, for a total 128 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. The Handshake Protocol is responsible for negotiating a session, which consists of the following items: Dierks & Rescorla Standards Track [Page 26]

RFC 5246 TLS August 2008 session identifier An arbitrary byte sequence chosen by the server to identify an active or resumable session state. peer certificate X509v3 [PKIX] 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 pseudorandom function (PRF) used to generate keying material, the bulk data encryption algorithm (such as null, AES, etc.) and the MAC algorithm (such as HMAC-SHA1). It also defines cryptographic attributes such as the mac_length. (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 ChangeCipherSpec 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. Immediately after sending this message, the sender MUST instruct the record layer to make the write pending state the write active state. Dierks & Rescorla Standards Track [Page 27]

RFC 5246 TLS August 2008 (See Section 6.1.) The ChangeCipherSpec message is sent during the handshake after the security parameters have been agreed upon, but before the verifying Finished message is sent. 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 (warning or fatal) 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_RESERVED(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), Dierks & Rescorla Standards Track [Page 28]

RFC 5246 TLS August 2008 export_restriction_RESERVED(60), protocol_version(70), insufficient_security(71), internal_error(80), user_canceled(90), no_renegotiation(100), unsupported_extension(110), (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 transport without waiting for the responding close_notify. No part Dierks & Rescorla Standards Track [Page 29]

RFC 5246 TLS August 2008 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. Whenever an implementation encounters a condition which is defined as a fatal alert, it MUST send the appropriate alert prior to closing the connection. For all errors where an alert level is not explicitly specified, the sending party MAY determine at its discretion whether to treat this as a fatal error or not. If the implementation chooses to send an alert but intends to close the connection immediately afterwards, it MUST send that alert at the fatal alert level. If an alert with a level of warning is sent and received, generally the connection can continue normally. If the receiving party decides not to proceed with the connection (e.g., after having received a no_renegotiation alert that it is not willing to accept), it SHOULD send a fatal alert to terminate the connection. Given this, the sending party cannot, in general, know how the receiving party will behave. Therefore, warning alerts are not very useful when the sending party wants to continue the connection, and thus are sometimes omitted. For example, if a peer decides to accept an expired certificate (perhaps after confirming this with the user) and wants to continue the connection, it would not generally send a certificate_expired alert. 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. Dierks & Rescorla Standards Track [Page 30]

RFC 5246 TLS August 2008 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 and should never be observed in communication between proper implementations (except when messages were corrupted in the network). decryption_failed_RESERVED This alert was used in some earlier versions of TLS, and may have permitted certain attacks against the CBC mode [CBCATT]. It MUST NOT be sent by compliant implementations. 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 and should never be observed in communication between proper implementations (except when messages were corrupted in the network). decompression_failure The decompression function received improper input (e.g., data that would expand to excessive length). This message is always fatal and should never be observed in communication between proper implementations. 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 any version of TLS. It MUST 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. Dierks & Rescorla Standards Track [Page 31]

RFC 5246 TLS August 2008 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 message 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. 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 and should never be observed in communication between proper implementations (except when messages were corrupted in the network). decrypt_error A handshake cryptographic operation failed, including being unable to correctly verify a signature or validate a Finished message. This message is always fatal. export_restriction_RESERVED This alert was used in some earlier versions of TLS. It MUST NOT be sent by compliant implementations. 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. Dierks & Rescorla Standards Track [Page 32]

RFC 5246 TLS August 2008 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 might be difficult to communicate changes to these parameters after that point. This message is always a warning. unsupported_extension sent by clients that receive an extended server hello containing an extension that they did not put in the corresponding client hello. This message is always fatal. New Alert values are assigned by IANA as described in Section 12. 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. Dierks & Rescorla Standards Track [Page 33]

RFC 5246 TLS August 2008 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 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. These goals are achieved by the handshake protocol, which can be summarized as follows: The client sends a ClientHello message to which the server must respond with a ServerHello message, or else a fatal error will occur and the connection will fail. The ClientHello and ServerHello are used to establish security enhancement capabilities between client and server. The ClientHello and ServerHello 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. Dierks & Rescorla Standards Track [Page 34]

RFC 5246 TLS August 2008 The actual key exchange uses up to four messages: the server Certificate, the ServerKeyExchange, the client Certificate, and the ClientKeyExchange. 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 46 bytes upwards. Following the hello messages, the server will send its certificate in a Certificate message if it is to be authenticated. Additionally, a ServerKeyExchange 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 ServerHelloDone 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 CertificateRequest message, the client MUST send the Certificate message. The ClientKeyExchange message is now sent, and the content of that message will depend on the public key algorithm selected between the ClientHello and the ServerHello. If the client has sent a certificate with signing ability, a digitally-signed CertificateVerify message is sent to explicitly verify possession of the private key in the certificate. At this point, a ChangeCipherSpec 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 ChangeCipherSpec 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 than TLS_NULL_WITH_NULL_NULL is established). Dierks & Rescorla Standards Track [Page 35]

RFC 5246 TLS August 2008 Client Server ClientHello --------> ServerHello Certificate* ServerKeyExchange* CertificateRequest* <-------- ServerHelloDone Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished --------> [ChangeCipherSpec] <-------- Finished Application Data <-------> Application Data Figure 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. 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 ChangeCipherSpec 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. Dierks & Rescorla Standards Track [Page 36]

RFC 5246 TLS August 2008 Client Server ClientHello --------> ServerHello [ChangeCipherSpec] <-------- Finished [ChangeCipherSpec] Finished --------> Application Data <-------> Application Data Figure 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; 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; Dierks & Rescorla Standards Track [Page 37]

RFC 5246 TLS August 2008 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 described only in its first position. The one message that is not bound by these ordering rules is the HelloRequest 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 types are assigned by IANA as described in Section 12. 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 HelloRequest message MAY be sent by the server at any time. Meaning of this message: HelloRequest is a simple notification that the client should begin the negotiation process anew. In response, the client should send a ClientHello message when convenient. This message is not intended to establish which side is the client or server but merely to initiate a new negotiation. Servers SHOULD NOT send a HelloRequest immediately upon the client's initial connection. It is the client's job to send a ClientHello at that time. 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 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 HelloRequest but does not receive a ClientHello in response, it may close the connection with a fatal alert. Dierks & Rescorla Standards Track [Page 38]

RFC 5246 TLS August 2008 After sending a HelloRequest, servers SHOULD NOT repeat the request until the subsequent handshake negotiation is complete. Structure of this message: struct { } HelloRequest; 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 ClientHello as its first message. The client can also send a ClientHello in response to a HelloRequest or on its own initiative in order to renegotiate the security parameters in an existing connection. Structure of this message: The ClientHello 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, UTC, 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. Note that, for historical reasons, the data element is named using GMT, the predecessor of the current worldwide time base, UTC. random_bytes 28 bytes generated by a secure random number generator. The ClientHello 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, Dierks & Rescorla Standards Track [Page 39]

RFC 5246 TLS August 2008 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 cipher suite list, passed from the client to the server in the ClientHello message, contains the combinations of cryptographic algorithms supported by the client in order of the client's preference (favorite choice first). Each cipher suite defines a key exchange algorithm, a bulk encryption algorithm (including secret key length), a MAC algorithm, and a PRF. The server will select a cipher suite or, if no acceptable choices are presented, return a handshake failure alert and close the connection. If the list contains cipher suites the server does not recognize, support, or wish to use, the server MUST ignore those cipher suites, and process the remaining ones as usual. uint8 CipherSuite[2]; /* Cryptographic suite selector */ The ClientHello includes a list of compression algorithms supported by the client, ordered according to the client's preference. enum { null(0), (255) } CompressionMethod; Dierks & Rescorla Standards Track [Page 40]

RFC 5246 TLS August 2008 struct { ProtocolVersion client_version; Random random; SessionID session_id; CipherSuite cipher_suites<2..2^16-2>; CompressionMethod compression_methods<1..2^8-1>; select (extensions_present) { case false: struct {}; case true: Extension extensions<0..2^16-1>; }; } ClientHello; TLS allows extensions to follow the compression_methods field in an extensions block. The presence of extensions can be detected by determining whether there are bytes following the compression_methods at the end of the ClientHello. Note that this method of detecting optional data differs from the normal TLS method of having a variable-length field, but it is used for compatibility with TLS before extensions were defined. 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 3.3 (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 is 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 Dierks & Rescorla Standards Track [Page 41]

RFC 5246 TLS August 2008 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. extensions Clients MAY request extended functionality from servers by sending data in the extensions field. The actual "Extension" format is defined in Section 7.4.1.4. In the event that a client requests additional functionality using extensions, and this functionality is not supplied by the server, the client MAY abort the handshake. A server MUST accept ClientHello messages both with and without the extensions field, and (as for all other messages) it MUST check that the amount of data in the message precisely matches one of these formats; if not, then it MUST send a fatal "decode_error" alert. After sending the ClientHello message, the client waits for a ServerHello message. Any handshake message returned by the server, except for a HelloRequest, is treated as a fatal error. 7.4.1.3. Server Hello When this message will be sent: The server will send this message in response to a ClientHello 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; select (extensions_present) { case false: struct {}; case true: Extension extensions<0..2^16-1>; }; } ServerHello; Dierks & Rescorla Standards Track [Page 42]

RFC 5246 TLS August 2008 The presence of extensions can be detected by determining whether there are bytes following the compression_method field at the end of the 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 3.3. (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. 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. Note that there is no requirement that the server resume any session even if it had formerly provided a session_id. Clients MUST be prepared to do a full negotiation -- including negotiating new cipher suites -- during any handshake. 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. extensions A list of extensions. Note that only extensions offered by the client can appear in the server's list. Dierks & Rescorla Standards Track [Page 43]

RFC 5246 TLS August 2008 7.4.1.4. Hello Extensions The extension format is: struct { ExtensionType extension_type; opaque extension_data<0..2^16-1>; } Extension; enum { signature_algorithms(13), (65535) } ExtensionType; Here: - "extension_type" identifies the particular extension type. - "extension_data" contains information specific to the particular extension type. The initial set of extensions is defined in a companion document [TLSEXT]. The list of extension types is maintained by IANA as described in Section 12. An extension type MUST NOT appear in the ServerHello unless the same extension type appeared in the corresponding ClientHello. If a client receives an extension type in ServerHello that it did not request in the associated ClientHello, it MUST abort the handshake with an unsupported_extension fatal alert. Nonetheless, "server-oriented" extensions may be provided in the future within this framework. Such an extension (say, of type x) would require the client to first send an extension of type x in a ClientHello with empty extension_data to indicate that it supports the extension type. In this case, the client is offering the capability to understand the extension type, and the server is taking the client up on its offer. When multiple extensions of different types are present in the ClientHello or ServerHello messages, the extensions MAY appear in any order. There MUST NOT be more than one extension of the same type. Finally, note that extensions can be sent both when starting a new session and when requesting session resumption. Indeed, a client that requests session resumption does not in general know whether the server will accept this request, and therefore it SHOULD send the same extensions as it would send if it were not attempting resumption. Dierks & Rescorla Standards Track [Page 44]

RFC 5246 TLS August 2008 In general, the specification of each extension type needs to describe the effect of the extension both during full handshake and session resumption. Most current TLS extensions are relevant only when a session is initiated: when an older session is resumed, the server does not process these extensions in Client Hello, and does not include them in Server Hello. However, some extensions may specify different behavior during session resumption. There are subtle (and not so subtle) interactions that may occur in this protocol between new features and existing features which may result in a significant reduction in overall security. The following considerations should be taken into account when designing new extensions: - Some cases where a server does not agree to an extension are error conditions, and some are simply refusals to support particular features. In general, error alerts should be used for the former, and a field in the server extension response for the latter. - Extensions should, as far as possible, be designed to prevent any attack that forces use (or non-use) of a particular feature by manipulation of handshake messages. This principle should be followed regardless of whether the feature is believed to cause a security problem. Often the fact that the extension fields are included in the inputs to the Finished message hashes will be sufficient, but extreme care is needed when the extension changes the meaning of messages sent in the handshake phase. Designers and implementors should be aware of the fact that until the handshake has been authenticated, active attackers can modify messages and insert, remove, or replace extensions. - It would be technically possible to use extensions to change major aspects of the design of TLS; for example the design of cipher suite negotiation. This is not recommended; it would be more appropriate to define a new version of TLS -- particularly since the TLS handshake algorithms have specific protection against version rollback attacks based on the version number, and the possibility of version rollback should be a significant consideration in any major design change. 7.4.1.4.1. Signature Algorithms The client uses the "signature_algorithms" extension to indicate to the server which signature/hash algorithm pairs may be used in digital signatures. The "extension_data" field of this extension contains a "supported_signature_algorithms" value. Dierks & Rescorla Standards Track [Page 45]

RFC 5246 TLS August 2008 enum { none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5), sha512(6), (255) } HashAlgorithm; enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) } SignatureAlgorithm; struct { HashAlgorithm hash; SignatureAlgorithm signature; } SignatureAndHashAlgorithm; SignatureAndHashAlgorithm supported_signature_algorithms<2..2^16-2>; Each SignatureAndHashAlgorithm value lists a single hash/signature pair that the client is willing to verify. The values are indicated in descending order of preference. Note: Because not all signature algorithms and hash algorithms may be accepted by an implementation (e.g., DSA with SHA-1, but not SHA-256), algorithms here are listed in pairs. hash This field indicates the hash algorithm which may be used. The values indicate support for unhashed data, MD5 [MD5], SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512 [SHS], respectively. The "none" value is provided for future extensibility, in case of a signature algorithm which does not require hashing before signing. signature This field indicates the signature algorithm that may be used. The values indicate anonymous signatures, RSASSA-PKCS1-v1_5 [PKCS1] and DSA [DSS], and ECDSA [ECDSA], respectively. The "anonymous" value is meaningless in this context but used in Section 7.4.3. It MUST NOT appear in this extension. The semantics of this extension are somewhat complicated because the cipher suite indicates permissible signature algorithms but not hash algorithms. Sections 7.4.2 and 7.4.3 describe the appropriate rules. If the client supports only the default hash and signature algorithms (listed in this section), it MAY omit the signature_algorithms extension. If the client does not support the default algorithms, or supports other hash and signature algorithms (and it is willing to use them for verifying messages sent by the server, i.e., server certificates and server key exchange), it MUST send the Dierks & Rescorla Standards Track [Page 46]

RFC 5246 TLS August 2008 signature_algorithms extension, listing the algorithms it is willing to accept. If the client does not send the signature_algorithms extension, the server MUST do the following: - If the negotiated key exchange algorithm is one of (RSA, DHE_RSA, DH_RSA, RSA_PSK, ECDH_RSA, ECDHE_RSA), behave as if client had sent the value {sha1,rsa}. - If the negotiated key exchange algorithm is one of (DHE_DSS, DH_DSS), behave as if the client had sent the value {sha1,dsa}. - If the negotiated key exchange algorithm is one of (ECDH_ECDSA, ECDHE_ECDSA), behave as if the client had sent value {sha1,ecdsa}. Note: this is a change from TLS 1.1 where there are no explicit rules, but as a practical matter one can assume that the peer supports MD5 and SHA-1. Note: this extension is not meaningful for TLS versions prior to 1.2. Clients MUST NOT offer it if they are offering prior versions. However, even if clients do offer it, the rules specified in [TLSEXT] require servers to ignore extensions they do not understand. Servers MUST NOT send this extension. TLS servers MUST support receiving this extension. When performing session resumption, this extension is not included in Server Hello, and the server ignores the extension in Client Hello (if present). 7.4.2. Server Certificate When this message will be sent: The server MUST send a Certificate message whenever the agreed- upon key exchange method uses certificates for authentication (this includes all key exchange methods defined in this document except DH_anon). This message will always immediately follow the ServerHello message. Meaning of this message: This message conveys the server's certificate chain to the client. The certificate MUST be appropriate for the negotiated cipher suite's key exchange algorithm and any negotiated extensions. Dierks & Rescorla Standards Track [Page 47]

RFC 5246 TLS August 2008 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 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 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 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. The following rules apply to the certificates sent by the server: - The certificate type MUST be X.509v3, unless explicitly negotiated otherwise (e.g., [TLSPGP]). - The end entity certificate's public key (and associated restrictions) MUST be compatible with the selected key exchange algorithm. Key Exchange Alg. Certificate Key Type RSA RSA public key; the certificate MUST allow the RSA_PSK key to be used for encryption (the keyEncipherment bit MUST be set if the key usage extension is present). Note: RSA_PSK is defined in [TLSPSK]. Dierks & Rescorla Standards Track [Page 48]

RFC 5246 TLS August 2008 DHE_RSA RSA public key; the certificate MUST allow the ECDHE_RSA key to be used for signing (the digitalSignature bit MUST be set if the key usage extension is present) with the signature scheme and hash algorithm that will be employed in the server key exchange message. Note: ECDHE_RSA is defined in [TLSECC]. DHE_DSS DSA public key; the certificate MUST allow the key to be used for signing with the hash algorithm that will be employed in the server key exchange message. DH_DSS Diffie-Hellman public key; the keyAgreement bit DH_RSA MUST be set if the key usage extension is present. ECDH_ECDSA ECDH-capable public key; the public key MUST ECDH_RSA use a curve and point format supported by the client, as described in [TLSECC]. ECDHE_ECDSA ECDSA-capable public key; the certificate MUST allow the key to be used for signing with the hash algorithm that will be employed in the server key exchange message. The public key MUST use a curve and point format supported by the client, as described in [TLSECC]. - The "server_name" and "trusted_ca_keys" extensions [TLSEXT] are used to guide certificate selection. If the client provided a "signature_algorithms" extension, then all certificates provided by the server MUST be signed by a hash/signature algorithm pair that appears in that extension. Note that this implies that a certificate containing a key for one signature algorithm MAY be signed using a different signature algorithm (for instance, an RSA key signed with a DSA key). This is a departure from TLS 1.1, which required that the algorithms be the same. Note that this also implies that the DH_DSS, DH_RSA, ECDH_ECDSA, and ECDH_RSA key exchange algorithms do not restrict the algorithm used to sign the certificate. Fixed DH certificates MAY be signed with any hash/signature algorithm pair appearing in the extension. The names DH_DSS, DH_RSA, ECDH_ECDSA, and ECDH_RSA are historical. Dierks & Rescorla Standards Track [Page 49]

RFC 5246 TLS August 2008 If the server has multiple certificates, it chooses one of them based on the above-mentioned criteria (in addition to other criteria, such as transport layer endpoint, local configuration and preferences, etc.). If the server has a single certificate, it SHOULD attempt to validate that it meets these criteria. Note that there are certificates that use algorithms and/or algorithm combinations that cannot be currently used with TLS. For example, a certificate with RSASSA-PSS signature key (id-RSASSA-PSS OID in SubjectPublicKeyInfo) cannot be used because TLS defines no corresponding signature algorithm. As cipher suites that specify new key exchange methods are specified for the TLS protocol, they will imply the certificate format and the required encoded keying information. 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 ServerHello message, if this is an anonymous negotiation). The ServerKeyExchange 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 ServerKeyExchange message for the following key exchange methods: RSA DH_DSS DH_RSA Other key exchange algorithms, such as those defined in [TLSECC], MUST specify whether the ServerKeyExchange message is sent or not; and if the message is sent, its contents. Dierks & Rescorla Standards Track [Page 50]

RFC 5246 TLS August 2008 Meaning of this message: This message conveys cryptographic information to allow the client to communicate the premaster secret: a Diffie-Hellman public key with which the client can complete a key exchange (with the result being the premaster secret) or a public key for some other algorithm. Structure of this message: enum { dhe_dss, dhe_rsa, dh_anon, rsa, dh_dss, dh_rsa /* may be extended, e.g., for ECDH -- see [TLSECC] */ } KeyExchangeAlgorithm; 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). Dierks & Rescorla Standards Track [Page 51]

RFC 5246 TLS August 2008 struct { select (KeyExchangeAlgorithm) { case dh_anon: ServerDHParams params; case dhe_dss: case dhe_rsa: ServerDHParams params; digitally-signed struct { opaque client_random[32]; opaque server_random[32]; ServerDHParams params; } signed_params; case rsa: case dh_dss: case dh_rsa: struct {} ; /* message is omitted for rsa, dh_dss, and dh_rsa */ /* may be extended, e.g., for ECDH -- see [TLSECC] */ }; } ServerKeyExchange; params The server's key exchange parameters. signed_params For non-anonymous key exchanges, a signature over the server's key exchange parameters. If the client has offered the "signature_algorithms" extension, the signature algorithm and hash algorithm MUST be a pair listed in that extension. Note that there is a possibility for inconsistencies here. For instance, the client might offer DHE_DSS key exchange but omit any DSA pairs from its "signature_algorithms" extension. In order to negotiate correctly, the server MUST check any candidate cipher suites against the "signature_algorithms" extension before selecting them. This is somewhat inelegant but is a compromise designed to minimize changes to the original cipher suite design. In addition, the hash and signature algorithms MUST be compatible with the key in the server's end-entity certificate. RSA keys MAY be used with any permitted hash algorithm, subject to restrictions in the certificate, if any. Because DSA signatures do not contain any secure indication of hash algorithm, there is a risk of hash substitution if multiple hashes may be used with any key. Currently, DSA [DSS] may only be used with SHA-1. Future revisions of DSS [DSS-3] are expected to allow the use of other digest algorithms with DSA, as well as guidance as to which Dierks & Rescorla Standards Track [Page 52]

RFC 5246 TLS August 2008 digest algorithms should be used with each key size. In addition, future revisions of [PKIX] may specify mechanisms for certificates to indicate which digest algorithms are to be used with DSA. As additional cipher suites 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. 7.4.4. Certificate Request When this message will be sent: A non-anonymous server can optionally request a certificate from the client, if appropriate for the selected