INTERNET-DRAFT Tim Dierks Obsoletes (if approved): RFC 3268, 4346, 4366 Independent Intended status: Proposed Standard Eric Rescorla Network Resonance, Inc. <draft-ietf-tls-rfc4346-bis-05.txt> September 2007 (Expires March 2008) The Transport Layer Security (TLS) Protocol Version 1.2 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The IETF Trust (2007). 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 3 1.1 Requirements Terminology 5 1.2 Major Differences from TLS 1.1 5 Dierks & Rescorla Standards Track [Page 1]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 2. Goals 6 3. Goals of This Document 6 4. Presentation Language 7 4.1. Basic Block Size 7 4.2. Miscellaneous 7 4.3. Vectors 7 4.4. Numbers 8 4.5. Enumerateds 9 4.6. Constructed Types 10 4.6.1. Variants 10 4.7. Cryptographic Attributes 11 4.8. Constants 12 5. HMAC and the Pseudorandom Function 13 6. The TLS Record Protocol 14 6.1. Connection States 15 6.2. Record layer 17 6.2.1. Fragmentation 17 6.2.2. Record Compression and Decompression 19 6.2.3. Record Payload Protection 19 6.2.3.1. Null or Standard Stream Cipher 20 6.2.3.2. CBC Block Cipher 21 6.2.3.3. AEAD ciphers 22 6.3. Key Calculation 24 7. The TLS Handshaking Protocols 25 7.1. Change Cipher Spec Protocol 25 7.2. Alert Protocol 26 7.2.1. Closure Alerts 27 7.2.2. Error Alerts 28 7.3. Handshake Protocol Overview 31 7.4. Handshake Protocol 34 7.4.1. Hello Messages 35 7.4.1.1. Hello Request 35 7.4.1.2. Client Hello 36 7.4.1.3. Server Hello 39 7.4.1.4 Hello Extensions 41 7.4.1.4.1 Cert Hash Types 42 7.4.2. Server Certificate 42 7.4.3. Server Key Exchange Message 44 7.4.4. Certificate Request 46 7.4.5 Server hello done 48 7.4.6. Client Certificate 48 7.4.7. Client Key Exchange Message 49 7.4.7.1. RSA Encrypted Premaster Secret Message 49 7.4.7.2. Client Diffie-Hellman Public Value 52 7.4.8. Certificate verify 52 7.4.9. Finished 53 8. Cryptographic Computations 55 8.1. Computing the Master Secret 55 Dierks & Rescorla Standards Track [Page 2]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 8.1.1. RSA 55 8.1.2. Diffie-Hellman 55 9. Mandatory Cipher Suites 56 10. Application Data Protocol 56 11. Security Considerations 56 12. IANA Considerations 56 A. Protocol Constant Values 58 A.1. Record Layer 58 A.2. Change Cipher Specs Message 59 A.3. Alert Messages 59 A.4. Handshake Protocol 61 A.4.1. Hello Messages 61 A.4.2. Server Authentication and Key Exchange Messages 62 A.4.3. Client Authentication and Key Exchange Messages 64 A.4.4. Handshake Finalization Message 64 A.5. The CipherSuite 64 A.6. The Security Parameters 67 B. Glossary 68 C. CipherSuite Definitions 72 D. Implementation Notes 74 D.1 Random Number Generation and Seeding 74 D.2 Certificates and Authentication 74 D.3 CipherSuites 74 D.4 Implementation Pitfalls 74 E. Backward Compatibility 77 E.1 Compatibility with TLS 1.0/1.1 and SSL 3.0 77 E.2 Compatibility with SSL 2.0 78 E.3. Avoiding Man-in-the-Middle Version Rollback 80 F. Security Analysis 81 F.1. Handshake Protocol 81 F.1.1. Authentication and Key Exchange 81 F.1.1.1. Anonymous Key Exchange 81 F.1.1.2. RSA Key Exchange and Authentication 82 F.1.1.3. Diffie-Hellman Key Exchange with Authentication 82 F.1.2. Version Rollback Attacks 83 F.1.3. Detecting Attacks Against the Handshake Protocol 84 F.1.4. Resuming Sessions 84 F.2. Protecting Application Data 85 F.3. Explicit IVs 85 F.4. Security of Composite Cipher Modes 85 F.5 Denial of Service 86 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 Dierks & Rescorla Standards Track [Page 3]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Protocol. At the lowest level, layered on top of some reliable transport protocol (e.g., TCP[TCP]), is the TLS Record Protocol. The TLS Record Protocol provides connection security that has two basic properties: - The connection is private. Symmetric cryptography is used for data encryption (e.g., DES [DES], RC4 [SCH] etc.). The keys for this symmetric encryption are generated uniquely for each connection and are based on a secret negotiated by another protocol (such as the TLS Handshake Protocol). The Record Protocol can also be used without encryption. - The connection is reliable. Message transport includes a message integrity check using a keyed MAC. Secure hash functions (e.g., SHA, MD5, etc.) are used for MAC computations. The Record Protocol can operate without a MAC, but is generally only used in this mode while another protocol is using the Record Protocol as a transport for negotiating security parameters. The TLS Record Protocol is used for encapsulation of various higher- level protocols. One such encapsulated protocol, the TLS Handshake Protocol, allows the server and client to authenticate each other and to negotiate an encryption algorithm and cryptographic keys before the application protocol transmits or receives its first byte of data. The TLS Handshake Protocol provides connection security that has three basic properties: - The peer's identity can be authenticated using asymmetric, or public key, cryptography (e.g., RSA [RSA], DSS [DSS], etc.). This authentication can be made optional, but is generally required for at least one of the peers. - The negotiation of a shared secret is secure: the negotiated secret is unavailable to eavesdroppers, and for any authenticated connection the secret cannot be obtained, even by an attacker who can place himself in the middle of the connection. - The negotiation is reliable: no attacker can modify the negotiation communication without being detected by the parties to the communication. One advantage of TLS is that it is application protocol independent. Higher-level protocols can layer on top of the TLS Protocol 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. Dierks & Rescorla Standards Track [Page 4]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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 [RFC2119]. 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: - Merged in TLS Extensions definition and AES Cipher Suites from external documents [TLSEXT] and [TLSAES]. - Replacement of MD5/SHA-1 combination in the PRF. Addition of cipher-suite specified PRFs. - Replacement of MD5/SHA-1 combination in the digitally-signed element. - Allow the client to indicate which hash functions it supports for digital signature. - Allow the server to indicate which hash functions it supports for digital signature. - Addition of support for authenticated encryption with additional data modes. - Tightened up a number of requirements. - The usual clarifications and editorial work. - Added some guidance that DH groups should be checked. - Cleaned up description of Bleichenbacher/Klima attack defenses. - Tighter checking of EncryptedPreMasterSecret version numbers. - Stronger language about when alerts MUST be sent. - Added an Implementation Pitfalls sections - Harmonized the requirement to send an empty certificate list after certificate_request even when no certs are available. - Made the verify_data length depend on the cipher suite. Dierks & Rescorla Standards Track [Page 5]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 - TLS_RSA_WITH_AES_128_CBC_SHA is now the mandatory to implement cipher suite. 2. Goals The goals of TLS Protocol, in order of their priority, are as follows: 1. Cryptographic security: TLS should be used to establish a secure connection between two parties. 2. Interoperability: Independent programmers should be able to develop applications utilizing TLS that can successfully exchange cryptographic parameters without knowledge of one another's code. 3. Extensibility: TLS seeks to provide a framework into which new public key and bulk encryption methods can be incorporated as necessary. This will also accomplish two sub-goals: preventing the need to create a new protocol (and risking the introduction of possible new weaknesses) and avoiding the need to implement an entire new security library. 4. Relative efficiency: Cryptographic operations tend to be highly CPU intensive, particularly public key operations. For this reason, the TLS protocol has incorporated an optional session caching scheme to reduce the number of connections that need to be established from scratch. Additionally, care has been taken to reduce network activity. 3. Goals of This Document This document and the TLS protocol itself are based on the SSL 3.0 Protocol Specification as published by Netscape. The differences between this protocol and SSL 3.0 are not dramatic, but they are significant enough that 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 Dierks & Rescorla Standards Track [Page 6]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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 bytestream, a multi-byte item (a numeric in the example) is formed (using C notation) by: value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) | ... | byte[n-1]; This byte ordering for multi-byte values is the commonplace network byte order or big endian format. 4.2. Miscellaneous Comments begin with "/*" and end with "*/". Optional components are denoted by enclosing them in "[[ ]]" double brackets. Single-byte entities containing uninterpreted data are of type opaque. 4.3. Vectors A vector (single dimensioned array) is a stream of homogeneous data elements. The size of the vector may be specified at documentation time or left unspecified until runtime. In either case, the length declares the number of bytes, not the number of elements, in the vector. The syntax for specifying a new type, T', that is a fixed- length vector of type T is T T'[n]; Dierks & Rescorla Standards Track [Page 7]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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, sufficient to represent the value 400 (see Section 4.4). On the other hand, longer can represent up to 800 bytes of data, or 400 uint16 elements, and it may be empty. Its encoding will include a two-byte actual length field prepended to the vector. The length of an encoded vector must be an even multiple of the length of a single element (for example, a 17-byte vector of uint16 would be illegal). opaque mandatory<300..400>; /* length field is 2 bytes, cannot be empty */ uint16 longer<0..800>; /* zero to 400 16-bit unsigned integers */ 4.4. Numbers The basic numeric data type is an unsigned byte (uint8). All larger numeric data types are formed from fixed-length series of bytes concatenated as described in Section 4.1 and are also unsigned. The following numeric types are predefined. uint8 uint16[2]; uint8 uint24[3]; uint8 uint32[4]; uint8 uint64[8]; Dierks & Rescorla Standards Track [Page 8]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 All values, here and elsewhere in the specification, are stored in "network" or "big-endian" order; the uint32 represented by the hex bytes 01 02 03 04 is equivalent to the decimal value 16909060. 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; Enumerateds occupy as much space in the byte stream as would its maximal defined ordinal value. The following definition would cause one byte to be used to carry fields of type Color. enum { red(3), blue(5), white(7) } Color; One may optionally specify a value without its associated tag to force the width definition without defining a superfluous element. In the following example, Taste will consume two bytes in the data stream but can only assume the values 1, 2, or 4. enum { sweet(1), sour(2), bitter(4), (32000) } Taste; The names of the elements of an enumeration are scoped within the defined type. In the first example, a fully qualified reference to the second element of the enumeration would be Color.blue. Such qualification is not required if the target of the assignment is well specified. Color color = Color.blue; /* overspecified, legal */ Color color = blue; /* correct, type implicit */ For enumerateds that are never converted to external representation, the numerical information may be omitted. enum { low, medium, high } Amount; Dierks & Rescorla Standards Track [Page 9]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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. The body of the variant structure may be given a label for reference. The mechanism by which the variant is selected at runtime is not prescribed by the presentation language. struct { T1 f1; T2 f2; .... Tn fn; select (E) { case e1: Te1; case e2: Te2; .... case en: Ten; } [[fv]]; } [[Tv]]; For example: enum { apple, orange } VariantTag; struct { uint16 number; opaque string<0..10>; /* variable length */ } V1; Dierks & Rescorla Standards Track [Page 10]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 struct { uint32 number; opaque string[10]; /* fixed length */ } V2; struct { select (VariantTag) { /* value of selector is implicit */ case apple: V1; /* VariantBody, tag = apple */ case orange: V2; /* VariantBody, tag = orange */ } variant_body; /* optional label on variant */ } VariantRecord; Variant structures may be qualified (narrowed) by specifying a value for the selector prior to the type. For example, an orange VariantRecord is a narrowed type of a VariantRecord containing a variant_body of type V2. 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). In digital signing, one-way hash functions are used as input for a signing algorithm. A digitally-signed element is encoded as an opaque vector <0..2^16-1>, where the length is specified by the signing algorithm and key. In RSA signing, 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 and for digest algorithms without parameters (which include 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 which did not include a DigestInfo encoding. In DSS, 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 DSS signature is an opaque vector, as above, Dierks & Rescorla Standards Track [Page 11]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 the contents of which are the DER encoding of: Dss-Sig-Value ::= SEQUENCE { r INTEGER, s INTEGER } In stream cipher encryption, the plaintext is exclusive-ORed with an identical amount of output generated from a cryptographically secure keyed pseudorandom number generator. In block cipher encryption, every block of plaintext encrypts to a block of ciphertext. All block cipher encryption is done in CBC (Cipher Block Chaining) mode, and all items that are block-ciphered will be an exact multiple of the cipher block length. In AEAD encryption, the plaintext is simultaneously encrypted and integrity protected. The input may be of any length and the output is generally larger than the input in order to accomodate 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 hash[20]; } UserType; the contents of hash are used as input for the signing algorithm, and then the entire structure is encrypted with a stream cipher. The length of this structure, in bytes, would be equal to two bytes for field1 and field2, plus two bytes for the length of the signature, plus the length of the output of the signing algorithm. This is known because the algorithm and key used for the signing are known prior to encoding or decoding this structure. 4.8. Constants Dierks & Rescorla Standards Track [Page 12]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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 pseudo-random 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. 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: 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)) Dierks & Rescorla Standards Track [Page 13]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 P_hash can be iterated as many times as is necessary to produce the required quantity of data. For example, if P_SHA-1 is being used to create 64 bytes of data, it will have to be iterated 4 times (through A(4)), creating 80 bytes of output data; the last 16 bytes of the final iteration will then be discarded, leaving 64 bytes of output data. TLS's PRF is created by applying P_hash to the secret S 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 record protocol clients are described in this document: the handshake protocol, the alert protocol, the change cipher spec protocol, and the application data protocol. In order to allow extension of the TLS protocol, additional record types can be supported by the record protocol. New record type values are assigned by IANA as described in Section 12. 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 a unexpected_message alert." Any protocol designed for use over TLS MUST be carefully designed to deal with all possible attacks against it. Note that because the type and length of a record are not protected by encryption, care SHOULD be taken to minimize the value of traffic analysis of these values. Dierks & Rescorla Standards Track [Page 14]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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 secret and the bulk encryption keys for the connection in both the read and the write directions. Logically, there are always four connection states outstanding: the current read and write states, and the pending read and write states. All records are processed under the current read and write states. The security parameters for the pending states can be set by the TLS Handshake Protocol, and the Change Cipher Spec can selectively make either of the pending states current, in which case the appropriate current state is disposed of and replaced with the pending state; the pending state is then reinitialized to an empty state. It is illegal to make a state that has not been initialized with security parameters a current state. The initial current state always specifies that no encryption, compression, or MAC will be used. The security parameters for a TLS Connection read and write state are set by providing the following values: connection end Whether this entity is considered the "client" or the "server" in this connection. bulk encryption algorithm An algorithm to be used for bulk encryption. This specification includes the key size of this algorithm, how much of that key is secret, whether it is a block, stream, or AEAD cipher, and the block size of the cipher (if appropriate). MAC algorithm An algorithm to be used for message authentication. This specification includes the size of the 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. Dierks & Rescorla Standards Track [Page 15]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 server random A 32-byte value provided by the server. These parameters are defined in the presentation language as: enum { server, client } ConnectionEnd; enum { null, rc4, rc2, des, 3des, des40, idea, aes } BulkCipherAlgorithm; enum { stream, block, aead } CipherType; enum { null, md5, sha, sha256, sha384, sha512} MACAlgorithm; /* The use of "sha" above is historical and denotes SHA-1 */ enum { null(0), (255) } CompressionMethod; /* The algorithms specified in CompressionMethod, BulkCipherAlgorithm, and MACAlgorithm may be added to. */ struct { ConnectionEnd entity; BulkCipherAlgorithm bulk_cipher_algorithm; CipherType cipher_type; uint8 enc_key_length; uint8 block_length; uint8 fixed_iv_length; uint8 record_iv_length; MACAlgorithm mac_algorithm; uint8 mac_length; uint8 mac_key_length; uint8 verify_data_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 four items: client write MAC secret server write MAC secret client write key server write key The client write parameters are used by the server when receiving and processing records and vice-versa. The algorithm used for generating Dierks & Rescorla Standards Track [Page 16]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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. MAC secret The MAC secret for this connection, as generated above. sequence number Each connection state contains a sequence number, which is maintained separately for read and write states. The sequence number MUST be set to zero whenever a connection state is made the active state. Sequence numbers are of type uint64 and may not exceed 2^64-1. Sequence numbers do not wrap. If a TLS implementation would need to wrap a sequence number, it must renegotiate instead. A sequence number is incremented after each record: specifically, the first record transmitted under a particular connection state MUST use sequence number 0. 6.2. Record layer The TLS Record Layer receives uninterpreted data from higher layers in non-empty blocks of arbitrary size. 6.2.1. Fragmentation The record layer fragments information blocks into TLSPlaintext records carrying data in chunks of 2^14 bytes or less. Client message boundaries are not preserved in the record layer (i.e., multiple client messages of the same ContentType MAY be coalesced into a single TLSPlaintext record, or a single message MAY be fragmented across several records). struct { uint8 major, minor; Dierks & Rescorla Standards Track [Page 17]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 } ProtocolVersion; enum { change_cipher_spec(20), alert(21), handshake(22), application_data(23), (255) } ContentType; struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[TLSPlaintext.length]; } TLSPlaintext; type The higher-level protocol used to process the enclosed fragment. version The version of the protocol being employed. This document describes TLS Version 1.2, 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 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 Change Cipher Spec 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. Dierks & Rescorla Standards Track [Page 18]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 6.2.2. Record Compression and Decompression All records are compressed using the compression algorithm defined in the current session state. There is always an active compression algorithm; however, initially it is defined as CompressionMethod.null. The compression algorithm translates a TLSPlaintext structure into a TLSCompressed structure. Compression functions are initialized with default state information whenever a connection state is made active. Compression must be lossless and may not increase the content length by more than 1024 bytes. If the decompression function encounters a TLSCompressed.fragment that would decompress to a length in excess of 2^14 bytes, it MUST report a fatal decompression failure error. struct { ContentType type; /* same as TLSPlaintext.type */ ProtocolVersion version;/* same as TLSPlaintext.version */ uint16 length; opaque fragment[TLSCompressed.length]; } TLSCompressed; length The length (in bytes) of the following TLSCompressed.fragment. The length 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) { Dierks & Rescorla Standards Track [Page 19]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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. 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_secret, seq_num + TLSCompressed.type + TLSCompressed.version + TLSCompressed.length + TLSCompressed.fragment); where "+" denotes concatenation. seq_num The sequence number for this record. hash The hashing algorithm specified by SecurityParameters.mac_algorithm. 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 Dierks & Rescorla Standards Track [Page 20]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 state from the end of one record is simply used on the subsequent packet. If the CipherSuite is TLS_NULL_WITH_NULL_NULL, encryption consists of the identity operation (i.e., the data is not encrypted, and the MAC size is zero, implying that no MAC is used). TLSCiphertext.length is TLSCompressed.length plus SecurityParameters.mac_length. 6.2.3.2. CBC Block Cipher For block ciphers (such as RC2, DES, or AES), the encryption and MAC functions convert TLSCompressed.fragment structures to and from block TLSCiphertext.fragment structures. 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 Dierks & Rescorla Standards Track [Page 21]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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 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 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. Dierks & Rescorla Standards Track [Page 22]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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]. These inputs are as follows. The key is either the client_write_key or the server_write_key. No MAC key is used. Each AEAD cipher suite has to 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 appropriate to use the partially implicit nonce technique described in Section 3.2.1 of [AEAD]; 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(key, IV, plaintext, additional_data) In order to decrypt and verify, the cipher takes as input the key, IV, the "additional_data", and the AEADEncrypted value. The output is Dierks & Rescorla Standards Track [Page 23]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 either the plaintext or an error indicating that the decryption failed. There is no separate integrity check. I.e., TLSCompressed.fragment = AEAD-Decrypt(write_key, IV, 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, and MAC secrets from the security parameters provided by the handshake protocol. The master secret is hashed into a sequence of secure bytes, which are assigned to the MAC secrets and keys required by the current connection state (see Appendix A.6). CipherSpecs require a client write MAC secret, a server write MAC secret, a client write key, and a server write key, each of which is generated from the master secret in that order. Unused values are empty. When keys and MAC secrets are generated, the master secret is used as an entropy source. To generate the key material, compute key_block = PRF(SecurityParameters.master_secret, "key expansion", SecurityParameters.server_random + SecurityParameters.client_random); until enough output has been generated. Then the key_block is partitioned as follows: client_write_MAC_secret[SecurityParameters.mac_key_length] server_write_MAC_secret[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] 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 Dierks & Rescorla Standards Track [Page 24]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 material is AES_256_CBC_SHA. It requires 2 x 32 byte keys and 2 x 20 byte MAC secrets, for a total 104 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: 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 bulk data encryption algorithm (such as null, DES, etc.) and a MAC algorithm (such as MD5 or SHA). It also defines cryptographic attributes such as the 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. Dierks & Rescorla Standards Track [Page 25]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 struct { enum { change_cipher_spec(1), (255) } type; } ChangeCipherSpec; The change cipher spec message is sent by both the client and the server to notify the receiving party that subsequent records will be protected under the newly negotiated CipherSpec and keys. Reception of this message causes the receiver to instruct the Record Layer to immediately copy the read pending state into the read current state. Immediately after sending this message, the sender MUST instruct the record layer to make the write pending state the write active state. (See Section 6.1.) The change cipher spec message is sent during the handshake after the security parameters have been agreed upon, but before the verifying finished message is sent. Note: If a rehandshake occurs while data is flowing on a connection, the communicating parties may continue to send data using the old CipherSpec. However, once the ChangeCipherSpec has been sent, the new CipherSpec MUST be used. The first side to send the ChangeCipherSpec does not know that the other side has finished computing the new keying material (e.g., if it has to perform a time consuming public key operation). Thus, a small window of time, during which the recipient must buffer the data, MAY exist. In practice, with modern machines this interval is likely to be fairly short. 7.2. Alert Protocol One of the content types supported by the TLS Record layer is the alert type. Alert messages convey the severity of the message and a description of the alert. Alert messages with a level of fatal result in the immediate termination of the connection. In this case, other connections corresponding to the session may continue, but the session identifier MUST be invalidated, preventing the failed session from being used to establish new connections. Like other messages, alert messages are encrypted and compressed, as specified by the current connection state. enum { warning(1), fatal(2), (255) } AlertLevel; enum { close_notify(0), unexpected_message(10), bad_record_mac(20), decryption_failed_RESERVED(21), record_overflow(22), decompression_failure(30), handshake_failure(40), no_certificate_RESERVED(41), Dierks & Rescorla Standards Track [Page 26]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 bad_certificate(42), unsupported_certificate(43), certificate_revoked(44), certificate_expired(45), certificate_unknown(46), illegal_parameter(47), unknown_ca(48), access_denied(49), decode_error(50), decrypt_error(51), export_restriction_RESERVED(60), protocol_version(70), insufficient_security(71), internal_error(80), user_canceled(90), no_renegotiation(100), 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. Dierks & Rescorla Standards Track [Page 27]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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 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. In cases where an implementation chooses to send an alert which MAY be a warning 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. The following error alerts are defined: unexpected_message An inappropriate message was received. This alert is always fatal and should never be observed in communication between proper implementations. bad_record_mac This alert is returned if a record is received with an incorrect Dierks & Rescorla Standards Track [Page 28]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 MAC. This alert also MUST be returned if an alert is sent because a TLSCiphertext decrypted in an invalid way: either it wasn't an even multiple of the block length, or its padding values, when checked, weren't correct. This message is always fatal. decryption_failed_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. decompression_failure The decompression function received improper input (e.g., data that would expand to excessive length). This message is always fatal. handshake_failure Reception of a handshake_failure alert message indicates that the sender was unable to negotiate an acceptable set of security parameters given the options available. This is a fatal error. no_certificate_RESERVED This alert was used in SSLv3 but not 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. 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 Dierks & Rescorla Standards Track [Page 29]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 other fields. This is always fatal. unknown_ca A valid certificate chain or partial chain was received, but the certificate was not accepted because the CA certificate could not be located or couldn't be matched with a known, trusted CA. This message is always fatal. access_denied A valid certificate was received, but when access control was applied, the sender decided not to proceed with negotiation. This message is always fatal. decode_error A message could not be decoded because some field was out of the specified range or the length of the message was incorrect. This message is always fatal. decrypt_error A handshake cryptographic operation failed, including being unable to correctly verify a signature, decrypt a key exchange, or validate a finished message. export_restriction_RESERVED This alert was used in 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. 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 Dierks & Rescorla Standards Track [Page 30]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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. For all errors where an alert level is not explicitly specified, the sending party MAY determine at its discretion whether this is a fatal error or not; if an alert with a level of warning is received, the receiving party MAY decide at its discretion whether to treat this as a fatal error or not. However, all messages that are transmitted with a level of fatal MUST be treated as fatal messages. New Alert values 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. 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 Dierks & Rescorla Standards Track [Page 31]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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 client hello message to which the server must respond with a server hello message, or else a fatal error will occur and the connection will fail. The client hello and server hello are used to establish security enhancement capabilities between client and server. The client hello and server hello establish the following attributes: Protocol Version, Session ID, Cipher Suite, and Compression Method. Additionally, two random values are generated and exchanged: ClientHello.random and ServerHello.random. The actual key exchange uses up to four messages: the server certificate, the server key exchange, the client certificate, and the client key exchange. New key exchange methods can be created by specifying a format for these messages and by defining the use of the messages to allow the client and server to agree upon a shared secret. This secret MUST be quite long; currently defined key exchange methods exchange secrets that range from 46 bytes upwards. Following the hello messages, the server will send its certificate, if it is to be authenticated. Additionally, a server key exchange Dierks & Rescorla Standards Track [Page 32]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 message may be sent, if it is required (e.g., if their server has no certificate, or if its certificate is for signing only). If the server is authenticated, it may request a certificate from the client, if that is appropriate to the cipher suite selected. Next, the server will send the server hello done message, indicating that the hello-message phase of the handshake is complete. The server will then wait for a client response. If the server has sent a certificate request message, the client MUST send the certificate message. The client key exchange message is now sent, and the content of that message will depend on the public key algorithm selected between the client hello and the server hello. If the client has sent a certificate with signing ability, a digitally-signed certificate verify message is sent to explicitly verify possession of the private key in the certificate. At this point, a change cipher spec message is sent by the client, and the client copies the pending Cipher Spec into the current Cipher Spec. The client then immediately sends the finished message under the new algorithms, keys, and secrets. In response, the server will send its own change cipher spec message, transfer the pending to the current Cipher Spec, and send its finished message under the new Cipher Spec. At this point, the handshake is complete, and the client and server may begin to exchange application layer data. (See flow chart below.) Application data MUST NOT be sent prior to the completion of the first handshake (before a cipher suite other than TLS_NULL_WITH_NULL_NULL is established). Client Server ClientHello --------> ServerHello Certificate* ServerKeyExchange* CertificateRequest* <-------- ServerHelloDone Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished --------> [ChangeCipherSpec] <-------- Finished Application Data <-------> Application Data Fig. 1. Message flow for a full handshake * Indicates optional or situation-dependent messages that are not always sent. Dierks & Rescorla Standards Track [Page 33]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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 change cipher spec messages and proceed directly to finished messages. Once the re-establishment is complete, the client and server MAY begin to exchange application layer data. (See flow chart below.) If a Session ID match is not found, the server generates a new session ID and the TLS client and server perform a full handshake. Client Server ClientHello --------> ServerHello [ChangeCipherSpec] <-------- Finished [ChangeCipherSpec] Finished --------> Application Data <-------> Application Data Fig. 2. Message flow for an abbreviated handshake The contents and significance of each message will be presented in detail in the following sections. 7.4. Handshake Protocol The TLS Handshake Protocol is one of the defined higher-level clients of the TLS Record Protocol. This protocol is used to negotiate the secure attributes of a session. Handshake messages are supplied to the TLS Record Layer, where they are encapsulated within one or more TLSPlaintext structures, which are processed and transmitted as specified by the current active session state. enum { hello_request(0), client_hello(1), server_hello(2), certificate(11), server_key_exchange (12), certificate_request(13), server_hello_done(14), Dierks & Rescorla Standards Track [Page 34]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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; 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 Hello Request message, which can be sent at any time, but which should be ignored by the client if it arrives in the middle of a handshake. New Handshake message 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 hello request message MAY be sent by the server at any time. Meaning of this message: Dierks & Rescorla Standards Track [Page 35]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Hello request is a simple notification that the client should begin the negotiation process anew by sending a client hello message when convenient. This message 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 hello request but does not receive a client hello in response, it may close the connection with a fatal alert. After sending a hello request, servers SHOULD NOT repeat the request until the subsequent handshake negotiation is complete. Structure of this message: struct { } HelloRequest; Note: This message MUST NOT be included in the message hashes that are maintained throughout the handshake and used in the finished messages and the certificate verify message. 7.4.1.2. Client Hello When this message will be sent: When a client first connects to a server it is required to send the client hello as its first message. The client can also send a client hello in response to a hello request or on its own initiative in order to renegotiate the security parameters in an existing connection. Structure of this message: The client hello message includes a random structure, which is used later in the protocol. struct { uint32 gmt_unix_time; opaque random_bytes[28]; } Random; gmt_unix_time Dierks & Rescorla Standards Track [Page 36]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 The current time and date in standard UNIX 32-bit format (seconds since the midnight starting Jan 1, 1970, GMT, ignoring leap seconds) according to the sender's internal clock. Clocks are not required to be set correctly by the basic TLS Protocol; higher- level or application protocols may define additional requirements. random_bytes 28 bytes generated by a secure random number generator. The client hello message includes a variable-length session identifier. If not empty, the value identifies a session between the same client and server whose security parameters the client wishes to reuse. The session identifier MAY be from an earlier connection, this connection, or from another currently active connection. The second option is useful if the client only wishes to update the random structures and derived values of a connection, and the third option makes it possible to establish several independent secure connections without repeating the full handshake protocol. These independent connections may occur sequentially or simultaneously; a SessionID becomes valid when the handshake negotiating it completes with the exchange of Finished messages and persists until it is removed due to aging or because a fatal error was encountered on a connection associated with the session. The actual contents of the SessionID are defined by the server. opaque SessionID<0..32>; Warning: Because the SessionID is transmitted without encryption or immediate MAC protection, servers MUST NOT place confidential information in session identifiers or let the contents of fake session identifiers cause any breach of security. (Note that the content of the handshake as a whole, including the SessionID, is protected by the Finished messages exchanged at the end of the handshake.) The CipherSuite list, passed from the client to the server in the client hello message, contains the combinations of cryptographic algorithms supported by the client in order of the client's preference (favorite choice first). Each CipherSuite defines a key exchange algorithm, a bulk encryption algorithm (including secret key length), 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. uint8 CipherSuite[2]; /* Cryptographic suite selector */ Dierks & Rescorla Standards Track [Page 37]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 The client hello includes a list of compression algorithms supported by the client, ordered according to the client's preference. enum { null(0), (255) } CompressionMethod; 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 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 should be empty if no session_id is available, or it 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. Dierks & Rescorla Standards Track [Page 38]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 compression_methods This is a list of the compression methods supported by the client, sorted by client preference. If the session_id field is not empty (implying a session resumption request) it MUST include the compression_method from that session. This vector MUST contain, and all implementations MUST support, CompressionMethod.null. Thus, a client and server will always be able to agree on a compression method. client_hello_extension_list Clients MAY request extended functionality from servers by sending data in the client_hello_extension_list. Here the new "client_hello_extension_list" field contains a list of extensions. 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 that supports the extensions mechanism MUST accept only client hello messages in either the original (TLS 1.0/TLS 1.1) ClientHello or the extended ClientHello format defined in this document, and (as for all other messages) 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 client hello message, the client waits for a server hello message. Any other handshake message returned by the server except for a hello request is treated as a fatal error. 7.4.1.3. Server Hello When this message will be sent: The server will send this message in response to a client hello message when it was able to find an acceptable set of algorithms. If it cannot find such a match, it will respond with a handshake failure alert. Structure of this message: struct { ProtocolVersion server_version; Random random; SessionID session_id; CipherSuite cipher_suite; CompressionMethod compression_method; select (extensions_present) { Dierks & Rescorla Standards Track [Page 39]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 case false: struct {}; case true: Extension extensions<0..2^16-1>; }; } ServerHello; 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. Client 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 Dierks & Rescorla Standards Track [Page 40]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 this field is the value from the resumed session state. server_hello_extension_list A list of extensions. Note that only extensions offered by the client can appear in the server's list. 7.4.1.4 Hello Extensions The extension format is: struct { ExtensionType extension_type; opaque extension_data<0..2^16-1>; } Extension; enum { signature_hash_types(TBD-BY-IANA), (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. 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 simply a refusal to support a particular feature. 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. Dierks & Rescorla Standards Track [Page 41]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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 Cert Hash Types The client MAY use the "signature_hash_types" to indicate to the server which hash functions may be used in digital signatures. The "extension_data" field of this extension contains: enum{ md5(0), sha1(1), sha256(2), sha384(3), sha512(4), (255) } HashType; struct { HashType types<1..255>; } SignatureHashTypes; These values indicate support for MD5 [MD5], SHA-1, SHA-256, SHA-384, and SHA-512 [SHA] respectively. The server MUST NOT send this extension. The values are indicated in descending order of preference. Clients SHOULD send this extension if they support any algorithm other than SHA-1. If this extension is not used, servers SHOULD assume that the client supports only SHA-1. 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. 7.4.2. Server Certificate When this message will be sent: The server MUST send a certificate whenever the agreed-upon key exchange method uses certificates for authentication (this includes all key exchange methods defined in this document except Dierks & Rescorla Standards Track [Page 42]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 DH_anon). This message will always immediately follow the server hello message. Meaning of this message: The certificate type MUST be appropriate for the selected cipher suite's key exchange algorithm, and is generally an X.509v3 certificate. It MUST contain a key that matches the key exchange method, as follows. Unless otherwise specified, the signing algorithm for the certificate MUST be the same as the algorithm for the certificate key. Unless otherwise specified, the public key MAY be of any length. Key Exchange Algorithm Certificate Key Type RSA RSA public key; the certificate MUST allow the key to be used for encryption. DHE_DSS DSS public key. DHE_RSA RSA public key that can be used for signing. DH_DSS Diffie-Hellman key. The algorithm used to sign the certificate MUST be DSS. DH_RSA Diffie-Hellman key. The algorithm used to sign the certificate MUST be RSA. All certificate profiles and key and cryptographic formats are defined by the IETF PKIX working group [PKIX]. When a key usage extension is present, the digitalSignature bit MUST be set for the key to be eligible for signing, as described above, and the keyEncipherment bit MUST be present to allow encryption, as described above. The keyAgreement bit must be set on Diffie-Hellman certificates. As CipherSuites that specify new key exchange methods are specified for the TLS Protocol, they will imply certificate format and the required encoded keying information. Structure of this message: opaque ASN.1Cert<1..2^24-1>; struct { ASN.1Cert certificate_list<0..2^24-1>; } Certificate; certificate_list Dierks & Rescorla Standards Track [Page 43]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 This is a sequence (chain) of X.509v3 certificates. The sender's certificate must come first in the list. Each following certificate must directly certify the one preceding it. Because certificate validation requires that root keys be distributed independently, the self-signed certificate that specifies the root certificate authority may optionally be omitted from the chain, under the assumption that the remote end must already possess it in order to validate it in any case. The same message type and structure will be used for the client's response to a certificate request message. Note that a client MAY send no certificates if it does not have an appropriate certificate to send in response to the server's authentication request. Note: PKCS #7 [PKCS7] is not used as the format for the certificate vector because PKCS #6 [PKCS6] extended certificates are not used. Also, PKCS #7 defines a SET rather than a SEQUENCE, making the task of parsing the list more difficult. 7.4.3. Server Key Exchange Message When this message will be sent: This message will be sent immediately after the server certificate message (or the server hello message, if this is an anonymous negotiation). The server key exchange message is sent by the server only when the server certificate message (if sent) does not contain enough data to allow the client to exchange a premaster secret. This is true for the following key exchange methods: DHE_DSS DHE_RSA DH_anon It is not legal to send the server key exchange message for the following key exchange methods: RSA DH_DSS DH_RSA Meaning of this message: This message conveys cryptographic information to allow the client to communicate the premaster secret: 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. Dierks & Rescorla Standards Track [Page 44]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 As additional CipherSuites are defined for TLS that include new key exchange algorithms, the server key exchange message will be sent if and only if the certificate type associated with the key exchange algorithm does not provide enough information for the client to exchange a premaster secret. If the client has offered the SignatureHashTypes extension, the hash function MUST be one of those listed in that extension. Otherwise it MUST be assumed that only SHA-1 is supported. If the SignatureAlgorithm being used to sign the ServerKeyExchange message is DSA, the hash algorithm MUST be SHA-1. [TODO: This is incorrect. What it should say is that it must be specified in the SPKI of the cert. However, I don't believe this is actually defined. Rather, the DSA certs just say dsa. We need new certs to say dsaWithSHAXXX.] If the SignatureAlgorithm is RSA, then any hash function accepted by the client MAY be used. The selected hash function MUST be indicated in the digest_algorithm field of the signature structure. The hash algorithm is denoted Hash below. Hash.length is the length of the output of that algorithm. Structure of this message: enum { diffie_hellman, rsa} 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). struct { select (KeyExchangeAlgorithm) { case diffie_hellman: ServerDHParams params; Signature signed_params; }; Dierks & Rescorla Standards Track [Page 45]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 } ServerKeyExchange; struct { select (KeyExchangeAlgorithm) { case diffie_hellman: ServerDHParams params; }; } ServerParams; params The server's key exchange parameters. signed_params For non-anonymous key exchanges, a hash of the corresponding params value, with the signature appropriate to that hash applied. hash Hash(ClientHello.random + ServerHello.random + ServerParams) sha_hash SHA1(ClientHello.random + ServerHello.random + ServerParams) enum { anonymous, rsa, dsa } SignatureAlgorithm; struct { select (SignatureAlgorithm) { case anonymous: struct { }; case rsa: HashType digest_algorithm; // NEW digitally-signed struct { opaque hash[Hash.length]; }; case dsa: digitally-signed struct { opaque sha_hash[20]; }; }; }; } Signature; 7.4.4. Certificate Request When this message will be sent: A non-anonymous server can optionally request a certificate from the client, if appropriate for the selected cipher suite. This message, if sent, will immediately follow the Server Key Exchange Dierks & Rescorla Standards Track [Page 46]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 message (if it is sent; otherwise, the Server Certificate message). Structure of this message: enum { rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6), fortezza_dms_RESERVED(20), (255) } ClientCertificateType; opaque DistinguishedName<1..2^16-1>; struct { ClientCertificateType certificate_types<1..2^8-1>; HashType certificate_hash<1..2^8-1>; DistinguishedName certificate_authorities<0..2^16-1>; } CertificateRequest; certificate_types This field is a list of the types of certificates requested, sorted in order of the server's preference. certificate_types A list of the types of certificate types which the client may offer. rsa_sign a certificate containing an RSA key dss_sign a certificate containing a DSS key rsa_fixed_dh a certificate signed with RSA and containing a static DH key. dss_fixed_dh a certificate signed with DSS and containing a static DH key Certificate types rsa_sign and dss_sign SHOULD contain certificates signed with the same algorithm. However, this is not required. This is a holdover from TLS 1.0 and 1.1. certificate_hash A list of acceptable hash algorithms to be used in signatures in both the client certificate and the CertificateVerify. These algorithms are listed in descending order of preference. certificate_authorities A list of the distinguished names [X501] of acceptable Dierks & Rescorla Standards Track [Page 47]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 certificateauthorities, represented in DER-encoded format. These distinguished names may specify a desired distinguished name for a root CA or for a subordinate CA; thus, this message can be used both to describe known roots and a desired authorization space. If the certificate_authorities list is empty then the client MAY send any certificate of the appropriate ClientCertificateType, unless there is some external arrangement to the contrary. New ClientCertificateType values are assigned by IANA as described in Section 12. Note: Values listed as RESERVED may not be used. They were used in SSLv3. Note: It is a fatal handshake_failure alert for an anonymous server to request client authentication. 7.4.5 Server hello done When this message will be sent: The server hello done message is sent by the server to indicate the end of the server hello and associated messages. After sending this message, the server will wait for a client response. Meaning of this message: This message means that the server is done sending messages to support the key exchange, and the client can proceed with its phase of the key exchange. Upon receipt of the server hello done message, the client SHOULD verify that the server provided a valid certificate, if required and check that the server hello parameters are acceptable. Structure of this message: struct { } ServerHelloDone; 7.4.6. Client Certificate When this message will be sent: This is the first message the client can send after receiving a server hello done message. This message is only sent if the server requests a certificate. If no suitable certificate is available, the client MUST send a certificate message containing no certificates. That is, the certificate_list structure has a length of zero. If client authentication is required by the server for the handshake to continue, it may respond with a fatal handshake failure alert. Client certificates are sent using the Dierks & Rescorla Standards Track [Page 48]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Certificate structure defined in Section 7.4.2. Note: When using a static Diffie-Hellman based key exchange method (DH_DSS or DH_RSA), if client authentication is requested, the Diffie-Hellman group and generator encoded in the client's certificate MUST match the server specified Diffie-Hellman parameters if the client's parameters are to be used for the key exchange. 7.4.7. Client Key Exchange Message When this message will be sent: This message is always sent by the client. It MUST immediately follow the client certificate message, if it is sent. Otherwise it MUST be the first message sent by the client after it receives the server hello done message. Meaning of this message: With this message, the premaster secret is set, either though direct transmission of the RSA-encrypted secret, or by the transmission of Diffie-Hellman parameters that will allow each side to agree upon the same premaster secret. When the key exchange method is DH_RSA or DH_DSS, client certification has been requested, and the client was able to respond with a certificate that contained a Diffie-Hellman public key whose parameters (group and generator) matched those specified by the server in its certificate, this message MUST NOT contain any data. Structure of this message: The choice of messages depends on which key exchange method has been selected. See Section 7.4.3 for the KeyExchangeAlgorithm definition. struct { select (KeyExchangeAlgorithm) { case rsa: EncryptedPreMasterSecret; case diffie_hellman: ClientDiffieHellmanPublic; } exchange_keys; } ClientKeyExchange; 7.4.7.1. RSA Encrypted Premaster Secret Message Meaning of this message: If RSA is being used for key agreement and authentication, the client generates a 48-byte premaster secret, encrypts it using the public key from the server's certificate and sends the result Dierks & Rescorla Standards Track [Page 49]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 in an encrypted premaster secret message. This structure is a variant of the client key exchange message and is not a message in itself. Structure of this message: struct { ProtocolVersion client_version; opaque random[46]; } PreMasterSecret; client_version The latest (newest) version supported by the client. This is used to detect version roll-back attacks. random 46 securely-generated random bytes. struct { public-key-encrypted PreMasterSecret pre_master_secret; } EncryptedPreMasterSecret; pre_master_secret This random value is generated by the client and is used to generate the master secret, as specified in Section 8.1. Note: The version number in the PreMasterSecret is the version offered by the client in the ClientHello.client_version, not the version negotiated for the connection. This feature is designed to prevent rollback attacks. Unfortunately, some old implementations use the negotiated version instead and therefore checking the version number may lead to failure to interoperate with such incorrect client implementations. Client implementations MUST always send the correct version number in PreMasterSecret. If ClientHello.client_version is TLS 1.1 or higher, server implementations MUST check the version number as described in the note below. If the version number is earlier than 1.0, server implementations SHOULD check the version number, but MAY have a configuration option to disable the check. Note that if the check fails, the PreMasterSecret SHOULD be randomized as described below. Note: Attacks discovered by Bleichenbacher [BLEI] and Klima et al. [KPR03] can be used to attack a TLS server that reveals whether a particular message, when decrypted, is properly PKCS#1 formatted, contains a valid PreMasterSecret structure, or has the correct version number. Dierks & Rescorla Standards Track [Page 50]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 The best way to avoid these vulnerabilities is to treat incorrectly formatted messages in a manner indistinguishable from correctly formatted RSA blocks. In other words: 1. Generate a string R of 46 random bytes 2. Decrypt the message M 3. If the PKCS#1 padding is not correct, or the length of message M is not exactly 48 bytes: premaster secret = ClientHello.client_version || R else If ClientHello.client_version <= TLS 1.0, and version number check is explicitly disabled: premaster secret = M else: premaster secret = ClientHello.client_version || M[2..47] In any case, a TLS server MUST NOT generate an alert if processing an RSA-encrypted premaster secret message fails, or the version number is not as expected. Instead, it MUST continue the handshake with a randomly generated premaster secret. It may be useful to log the real cause of failure for troubleshooting purposes; however, care must be taken to avoid leaking the information to an attacker (though, e.g., timing, log files, or other channels.) The RSAES-OAEP encryption scheme defined in [PKCS1] is more secure against the Bleichenbacher attack. However, for maximal compatibility with earlier versions of TLS, this specification uses the RSAES- PKCS1-v1_5 scheme. No variants of the Bleichenbacher attack are known to exist provided that the above recommendations are followed. Implementation Note: Public-key-encrypted data is represented as an opaque vector <0..2^16-1> (see Section 4.7). Thus, the RSA-encrypted PreMasterSecret in a ClientKeyExchange is preceded by two length bytes. These bytes are redundant in the case of RSA because the EncryptedPreMasterSecret is the only data in the ClientKeyExchange and its length can therefore be unambiguously determined. The SSLv3 specification was not clear about the encoding of public-key- encrypted data, and therefore many SSLv3 implementations do not include the the length bytes, encoding the RSA encrypted data directly in the ClientKeyExchange message. This specification requires correct encoding of the EncryptedPreMasterSecret complete with length bytes. The resulting PDU is incompatible with many SSLv3 implementations. Implementors upgrading from SSLv3 MUST modify their implementations to generate and accept the correct encoding. Implementors who wish to be compatible with both SSLv3 and TLS should make their implementation's Dierks & Rescorla Standards Track [Page 51]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 behavior dependent on the protocol version. Implementation Note: It is now known that remote timing-based attacks on TLS are possible, at least when the client and server are on the same LAN. Accordingly, implementations that use static RSA keys MUST use RSA blinding or some other anti-timing technique, as described in [TIMING]. 7.4.7.2. Client Diffie-Hellman Public Value Meaning of this message: This structure conveys the client's Diffie-Hellman public value (Yc) if it was not already included in the client's certificate. The encoding used for Yc is determined by the enumerated PublicValueEncoding. This structure is a variant of the client key exchange message, and not a message in itself. Structure of this message: enum { implicit, explicit } PublicValueEncoding; implicit If the client certificate already contains a suitable Diffie- Hellman key, then Yc is implicit and does not need to be sent again. In this case, the client key exchange message will be sent, but it MUST be empty. explicit Yc needs to be sent. struct { select (PublicValueEncoding) { case implicit: struct { }; case explicit: opaque dh_Yc<1..2^16-1>; } dh_public; } ClientDiffieHellmanPublic; dh_Yc The client's Diffie-Hellman public value (Yc). 7.4.8. Certificate verify When this message will be sent: This message is used to provide explicit verification of a client certificate. This message is only sent following a client certificate that has signing capability (i.e. all certificates except those containing fixed Diffie-Hellman parameters). When sent, it MUST immediately follow the client key exchange message. Dierks & Rescorla Standards Track [Page 52]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Structure of this message: struct { Signature signature; } CertificateVerify; The Signature type is defined in 7.4.3. The hash function MUST be one of the algorithms offered in the CertificateRequest message. If the SignatureAlgorithm being used to sign the ServerKeyExchange message is DSA, the hash function used MUST be SHA-1. [TODO: This is incorrect. What it should say is that it must be specified in the SPKI of the cert. However, I don't believe this is actually defined. Rather, the DSA certs just say dsa. We need new certs to say dsaWithSHAXXX] If the SignatureAlgorithm is RSA, then any of the functions offered by the server may be used. The selected hash function MUST be indicated in the digest_algorithm field of the signature structure. The hash algorithm is denoted Hash below. CertificateVerify.signature.hash Hash(handshake_messages); CertificateVerify.signature.sha_hash SHA(handshake_messages); Here handshake_messages refers to all handshake messages sent or received starting at client hello up to but not including this message, including the type and length fields of the handshake messages. This is the concatenation of all the Handshake structures as defined in 7.4 exchanged thus far. 7.4.9. Finished When this message will be sent: A finished message is always sent immediately after a change cipher spec message to verify that the key exchange and authentication processes were successful. It is essential that a change cipher spec message be received between the other handshake messages and the Finished message. Meaning of this message: The finished message is the first protected with the just- negotiated algorithms, keys, and secrets. Recipients of finished Dierks & Rescorla Standards Track [Page 53]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 messages MUST verify that the contents are correct. Once a side has sent its Finished message and received and validated the Finished message from its peer, it may begin to send and receive application data over the connection. struct { opaque verify_data[SecurityParameters.verify_data_length]; } Finished; verify_data PRF(master_secret, finished_label, Hash(handshake_messages)) [0..SecurityParameters.verify_data_length-1]; finished_label For Finished messages sent by the client, the string "client finished". For Finished messages sent by the server, the string "server finished". Hash denotes the negotiated hash used for the PRF. If a new PRF is defined, then this hash MUST be specified. In previous versions of TLS, the verify_data was always 12 octets long. In the current version of TLS, it depends on the cipher suite. Any cipher suite which does not explicitly specify SecurityParameters.verify_data_length has a SecurityParameters.verify_data_length equal to 12. This includes all existing cipher suites. Note that this representation has the same encoding as with previous versions. Future cipher suites MAY specify other lengths but such length MUST be at least 12 bytes. handshake_messages All of the data from all messages in this handshake (not including any HelloRequest messages) up to but not including this message. This is only data visible at the handshake layer and does not include record layer headers. This is the concatenation of all the Handshake structures as defined in 7.4, exchanged thus far. It is a fatal error if a finished message is not preceded by a change cipher spec message at the appropriate point in the handshake. The value handshake_messages includes all handshake messages starting at client hello up to, but not including, this finished message. This may be different from handshake_messages in Section 7.4.8 because it would include the certificate verify message (if sent). Also, the Dierks & Rescorla Standards Track [Page 54]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 handshake_messages for the finished message sent by the client will be different from that for the finished message sent by the server, because the one that is sent second will include the prior one. Note: Change cipher spec messages, alerts, and any other record types are not handshake messages and are not included in the hash computations. Also, Hello Request messages are omitted from handshake hashes. 8. Cryptographic Computations In order to begin connection protection, the TLS Record Protocol requires specification of a suite of algorithms, a master secret, and the client and server random values. The authentication, encryption, and MAC algorithms are determined by the cipher_suite selected by the server and revealed in the server hello message. The compression algorithm is negotiated in the hello messages, and the random values are exchanged in the hello messages. All that remains is to calculate the master secret. 8.1. Computing the Master Secret For all key exchange methods, the same algorithm is used to convert the pre_master_secret into the master_secret. The pre_master_secret should be deleted from memory once the master_secret has been computed. master_secret = PRF(pre_master_secret, "master secret", ClientHello.random + ServerHello.random) [0..47]; The master secret is always exactly 48 bytes in length. The length of the premaster secret will vary depending on key exchange method. 8.1.1. RSA When RSA is used for server authentication and key exchange, a 48-byte pre_master_secret is generated by the client, encrypted under the server's public key, and sent to the server. The server uses its private key to decrypt the pre_master_secret. Both parties then convert the pre_master_secret into the master_secret, as specified above. 8.1.2. Diffie-Hellman A conventional Diffie-Hellman computation is performed. The negotiated key (Z) is used as the pre_master_secret, and is converted into the master_secret, as specified above. Leading bytes of Z that Dierks & Rescorla Standards Track [Page 55]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 contain all zero bits are stripped before it is used as the pre_master_secret. Note: Diffie-Hellman parameters are specified by the server and may be either ephemeral or contained within the server's certificate. 9. Mandatory Cipher Suites In the absence of an application profile standard specifying otherwise, a TLS compliant application MUST implement the cipher suite TLS_RSA_WITH_AES_128_CBC_SHA. 10. Application Data Protocol Application data messages are carried by the Record Layer and are fragmented, compressed, and encrypted based on the current connection state. The messages are treated as transparent data to the record layer. 11. Security Considerations Security issues are discussed throughout this memo, especially in Appendices D, E, and F. 12. IANA Considerations This document uses several registries that were originally created in [RFC4346]. IANA is requested to update (has updated) these to reference this document. The registries and their allocation policies (unchanged from [TLS1.1]) are listed below. o TLS ClientCertificateType Identifiers Registry: Future values in the range 0-63 (decimal) inclusive are assigned via Standards Action [RFC2434]. Values in the range 64-223 (decimal) inclusive are assigned Specification Required [RFC2434]. Values from 224-255 (decimal) inclusive are reserved for Private Use [RFC2434]. o TLS Cipher Suite Registry: Future values with the first byte in the range 0-191 (decimal) inclusive are assigned via Standards Action [RFC2434]. Values with the first byte in the range 192-254 (decimal) are assigned via Specification Required [RFC2434]. Values with the first byte 255 (decimal) are reserved for Private Use [RFC2434]. o TLS ContentType Registry: Future values are allocated via Standards Action [RFC2434]. Dierks & Rescorla Standards Track [Page 56]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 o TLS Alert Registry: Future values are allocated via Standards Action [RFC2434]. o TLS HandshakeType Registry: Future values are allocated via Standards Action [RFC2434]. This document also uses a registry originally created in [RFC4366]. IANA is requested to update (has updated) it to reference this document. The registry and its allocation policy (unchanged from [RFC4366]) is listed below:. o TLS ExtensionType Registry: Future values are allocated via IETF Consensus [RFC2434] In addition, this document defines one new registry to be maintained by IANA: o TLS HashType Registry: The registry will be initially populated with the values described in Section 7.4.1.4.7. Future values in the range 0-63 (decimal) inclusive are assigned via Standards Action [RFC2434]. Values in the range 64-223 (decimal) inclusive are assigned via Specification Required [RFC2434]. Values from 224-255 (decimal) inclusive are reserved for Private Use [RFC2434]. This document defines one new TLS extension, cert_hash_type, which is to be (has been) allocated value TBD-BY-IANA in the TLS ExtensionType registry. This document also uses the TLS Compression Method Identifiers Registry, defined in [RFC3749]. IANA is requested to allocate value 0 for the "null" compression method. Dierks & Rescorla Standards Track [Page 57]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Appendix A. Protocol Constant Values This section describes protocol types and constants. A.1. Record Layer struct { uint8 major, minor; } ProtocolVersion; ProtocolVersion version = { 3, 3 }; /* TLS v1.2*/ enum { change_cipher_spec(20), alert(21), handshake(22), application_data(23), (255) } ContentType; struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[TLSPlaintext.length]; } TLSPlaintext; struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[TLSCompressed.length]; } TLSCompressed; struct { ContentType type; ProtocolVersion version; uint16 length; select (SecurityParameters.cipher_type) { case stream: GenericStreamCipher; case block: GenericBlockCipher; case aead: GenericAEADCipher; } fragment; } TLSCiphertext; stream-ciphered struct { opaque content[TLSCompressed.length]; opaque MAC[SecurityParameters.mac_length]; } GenericStreamCipher; struct { Dierks & Rescorla Standards Track [Page 58]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 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; aead-ciphered struct { opaque IV[SecurityParameters.iv_length]; opaque aead_output[AEADEncrypted.length]; } GenericAEADCipher; A.2. Change Cipher Specs Message struct { enum { change_cipher_spec(1), (255) } type; } ChangeCipherSpec; A.3. Alert Messages enum { warning(1), fatal(2), (255) } AlertLevel; enum { close_notify(0), unexpected_message(10), bad_record_mac(20), decryption_failed_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), export_restriction_RESERVED(60), protocol_version(70), insufficient_security(71), internal_error(80), user_canceled(90), Dierks & Rescorla Standards Track [Page 59]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 no_renegotiation(100), unsupported_extension(110), /* new */ (255) } AlertDescription; struct { AlertLevel level; AlertDescription description; } Alert; Dierks & Rescorla Standards Track [Page 60]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 A.4. Handshake Protocol enum { hello_request(0), client_hello(1), server_hello(2), certificate(11), server_key_exchange (12), certificate_request(13), server_hello_done(14), certificate_verify(15), client_key_exchange(16), finished(20) (255) } HandshakeType; struct { HandshakeType msg_type; uint24 length; select (HandshakeType) { case hello_request: HelloRequest; case client_hello: ClientHello; case server_hello: ServerHello; case certificate: Certificate; case server_key_exchange: ServerKeyExchange; case certificate_request: CertificateRequest; case server_hello_done: ServerHelloDone; case certificate_verify: CertificateVerify; case client_key_exchange: ClientKeyExchange; case finished: Finished; } body; } Handshake; A.4.1. Hello Messages struct { } HelloRequest; struct { uint32 gmt_unix_time; opaque random_bytes[28]; } Random; opaque SessionID<0..32>; uint8 CipherSuite[2]; enum { null(0), (255) } CompressionMethod; struct { ProtocolVersion client_version; Random random; SessionID session_id; CipherSuite cipher_suites<2..2^16-1>; Dierks & Rescorla Standards Track [Page 61]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 CompressionMethod compression_methods<1..2^8-1>; select (extensions_present) { case false: struct {}; case true: Extension extensions<0..2^16-1>; }; } ClientHello; 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; struct { ExtensionType extension_type; opaque extension_data<0..2^16-1>; } Extension; enum { signature_hash_types(TBD-BY-IANA), (65535) } ExtensionType; A.4.2. Server Authentication and Key Exchange Messages opaque ASN.1Cert<2^24-1>; struct { ASN.1Cert certificate_list<0..2^24-1>; } Certificate; enum { diffie_hellman } KeyExchangeAlgorithm; struct { opaque dh_p<1..2^16-1>; opaque dh_g<1..2^16-1>; opaque dh_Ys<1..2^16-1>; } ServerDHParams; Dierks & Rescorla Standards Track [Page 62]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 struct { select (KeyExchangeAlgorithm) { case diffie_hellman: ServerDHParams params; Signature signed_params; } } ServerKeyExchange; enum { anonymous, rsa, dsa } SignatureAlgorithm; struct { select (KeyExchangeAlgorithm) { case diffie_hellman: ServerDHParams params; }; } ServerParams; struct { select (SignatureAlgorithm) { case anonymous: struct { }; case rsa: HashType digest_algorithm; // NEW digitally-signed struct { opaque hash[Hash.length]; }; case dsa: digitally-signed struct { opaque sha_hash[20]; }; }; }; } Signature; enum { rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6), fortezza_dms_RESERVED(20), (255) } ClientCertificateType; opaque DistinguishedName<1..2^16-1>; struct { ClientCertificateType certificate_types<1..2^8-1>; DistinguishedName certificate_authorities<0..2^16-1>; } CertificateRequest; struct { } ServerHelloDone; Dierks & Rescorla Standards Track [Page 63]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 A.4.3. Client Authentication and Key Exchange Messages struct { select (KeyExchangeAlgorithm) { case rsa: EncryptedPreMasterSecret; case diffie_hellman: ClientDiffieHellmanPublic; } exchange_keys; } ClientKeyExchange; struct { ProtocolVersion client_version; opaque random[46]; } PreMasterSecret; struct { public-key-encrypted PreMasterSecret pre_master_secret; } EncryptedPreMasterSecret; enum { implicit, explicit } PublicValueEncoding; struct { select (PublicValueEncoding) { case implicit: struct {}; case explicit: opaque DH_Yc<1..2^16-1>; } dh_public; } ClientDiffieHellmanPublic; struct { Signature signature; } CertificateVerify; A.4.4. Handshake Finalization Message struct { opaque verify_data[SecurityParameters.verify_data_length]; } Finished; A.5. The CipherSuite The following values define the CipherSuite codes used in the client hello and server hello messages. A CipherSuite defines a cipher specification supported in TLS Version 1.2. TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a TLS connection during the first handshake on that channel, but MUST not be negotiated, as it provides no more protection than an Dierks & Rescorla Standards Track [Page 64]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 unsecured connection. CipherSuite TLS_NULL_WITH_NULL_NULL = { 0x00,0x00 }; The following CipherSuite definitions require that the server provide an RSA certificate that can be used for key exchange. The server may request either an RSA or a DSS signature-capable certificate in the certificate request message. CipherSuite TLS_RSA_WITH_NULL_MD5 = { 0x00,0x01 }; CipherSuite TLS_RSA_WITH_NULL_SHA = { 0x00,0x02 }; CipherSuite TLS_RSA_WITH_RC4_128_MD5 = { 0x00,0x04 }; CipherSuite TLS_RSA_WITH_RC4_128_SHA = { 0x00,0x05 }; CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA = { 0x00,0x07 }; CipherSuite TLS_RSA_WITH_DES_CBC_SHA = { 0x00,0x09 }; CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0A }; CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x2F }; CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x35 }; The following CipherSuite definitions are used for server- authenticated (and optionally client-authenticated) Diffie-Hellman. DH denotes cipher suites in which the server's certificate contains the Diffie-Hellman parameters signed by the certificate authority (CA). DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman parameters are signed by a DSS or RSA certificate, which has been signed by the CA. The signing algorithm used is specified after the DH or DHE parameter. The server can request an RSA or DSS signature- capable certificate from the client for client authentication or it may request a Diffie-Hellman certificate. Any Diffie-Hellman certificate provided by the client must use the parameters (group and generator) described by the server. CipherSuite TLS_DH_DSS_WITH_DES_CBC_SHA = { 0x00,0x0C }; CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0D }; CipherSuite TLS_DH_RSA_WITH_DES_CBC_SHA = { 0x00,0x0F }; CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x10 }; CipherSuite TLS_DHE_DSS_WITH_DES_CBC_SHA = { 0x00,0x12 }; CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x13 }; CipherSuite TLS_DHE_RSA_WITH_DES_CBC_SHA = { 0x00,0x15 }; CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x16 }; CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x30 }; CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x31 }; CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x32 }; CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x33 }; CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA = { 0x00, 0x36 }; CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x37 }; CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA = { 0x00, 0x38 }; CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x39 }; Dierks & Rescorla Standards Track [Page 65]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 The following cipher suites are used for completely anonymous Diffie- Hellman communications in which neither party is authenticated. Note that this mode is vulnerable to man-in-the-middle attacks. Using this mode therefore is of limited use: These ciphersuites MUST NOT be used by TLS 1.2 implementations unless the application layer has specifically requested to allow anonymous key exchange. (Anonymous key exchange may sometimes be acceptable, for example, to support opportunistic encryption when no set-up for authentication is in place, or when TLS is used as part of more complex security protocols that have other means to ensure authentication.) CipherSuite TLS_DH_anon_WITH_RC4_128_MD5 = { 0x00, 0x18 }; CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA = { 0x00, 0x1A }; CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x1B }; CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA = { 0x00, 0x34 }; CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA = { 0x00, 0x3A }; Note that using non-anonymous key exchange without actually verifying the key exchange is essentially equivalent to anonymous key exchange, and the same precautions apply. While non-anonymous key exchange will generally involve a higher computational and communicational cost than anonymous key exchange, it may be in the interest of interoperability not to disable non-anonymous key exchange when the application layer is allowing anonymous key exchange. When SSLv3 and TLS 1.0 were designed, the United States restricted the export of cryptographic software containing certain strong encryption algorithms. A series of cipher suites were designed to operate at reduced key lengths in order to comply with those regulations. Due to advances in computer performance, these algorithms are now unacceptably weak and export restrictions have since been loosened. TLS 1.2 implementations MUST NOT negotiate these cipher suites in TLS 1.2 mode. However, for backward compatibility they may be offered in the ClientHello for use with TLS 1.0 or SSLv3 only servers. TLS 1.2 clients MUST check that the server did not choose one of these cipher suites during the handshake. These ciphersuites are listed below for informational purposes and to reserve the numbers. CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x03 }; CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x06 }; CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x08 }; CipherSuite TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0B }; CipherSuite TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0E }; CipherSuite TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x11 }; CipherSuite TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x14 }; CipherSuite TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x17 }; CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x19 }; Dierks & Rescorla Standards Track [Page 66]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 New cipher suite values are assigned by IANA as described in Section 12. Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are reserved to avoid collision with Fortezza-based cipher suites in SSL 3. A.6. The Security Parameters These security parameters are determined by the TLS Handshake Protocol and provided as parameters to the TLS Record Layer in order to initialize a connection state. SecurityParameters includes: enum { null(0), (255) } CompressionMethod; enum { server, client } ConnectionEnd; enum { null, rc4, rc2, des, 3des, des40, aes, idea } BulkCipherAlgorithm; enum { stream, block, aead } CipherType; enum { null, md5, sha } MACAlgorithm; /* The algorithms specified in CompressionMethod, BulkCipherAlgorithm, and MACAlgorithm may be added to. */ struct { ConnectionEnd entity; 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; uint8 verify_data_length; CompressionMethod compression_algorithm; opaque master_secret[48]; opaque client_random[32]; opaque server_random[32]; } SecurityParameters; Dierks & Rescorla Standards Track [Page 67]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Appendix B. Glossary Advanced Encryption Standard (AES) AES is a widely used symmetric encryption algorithm. AES is a block cipher with a 128, 192, or 256 bit keys and a 16 byte block size. [AES] TLS currently only supports the 128 and 256 bit key sizes. application protocol An application protocol is a protocol that normally layers directly on top of the transport layer (e.g., TCP/IP). Examples include HTTP, TELNET, FTP, and SMTP. asymmetric cipher See public key cryptography. authenticated encryption with additional data (AEAD) A symmetric encryption algorithm that simultaneously provides confidentiality and message integrity. authentication Authentication is the ability of one entity to determine the identity of another entity. block cipher A block cipher is an algorithm that operates on plaintext in groups of bits, called blocks. 64 bits is a common block size. bulk cipher A symmetric encryption algorithm used to encrypt large quantities of data. cipher block chaining (CBC) CBC is a mode in which every plaintext block encrypted with a block cipher is first exclusive-ORed with the previous ciphertext block (or, in the case of the first block, with the initialization vector). For decryption, every block is first decrypted, then exclusive-ORed with the previous ciphertext block (or IV). certificate As part of the X.509 protocol (a.k.a. ISO Authentication framework), certificates are assigned by a trusted Certificate Authority and provide a strong binding between a party's identity or some other attributes and its public key. client The application entity that initiates a TLS connection to a Dierks & Rescorla Standards Track [Page 68]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 server. This may or may not imply that the client initiated the underlying transport connection. The primary operational difference between the server and client is that the server is generally authenticated, while the client is only optionally authenticated. client write key The key used to encrypt data written by the client. client write MAC secret The secret data used to authenticate data written by the client. connection A connection is a transport (in the OSI layering model definition) that provides a suitable type of service. For TLS, such connections are peer-to-peer relationships. The connections are transient. Every connection is associated with one session. Data Encryption Standard DES is a very widely used symmetric encryption algorithm. DES is a block cipher with a 56 bit key and an 8 byte block size. Note that in TLS, for key generation purposes, DES is treated as having an 8 byte key length (64 bits), but it still only provides 56 bits of protection. (The low bit of each key byte is presumed to be set to produce odd parity in that key byte.) DES can also be operated in a mode where three independent keys and three encryptions are used for each block of data; this uses 168 bits of key (24 bytes in the TLS key generation method) and provides the equivalent of 112 bits of security. [DES], [3DES] Digital Signature Standard (DSS) A standard for digital signing, including the Digital Signing Algorithm, approved by the National Institute of Standards and Technology, defined in NIST FIPS PUB 186, "Digital Signature Standard", published May, 1994 by the U.S. Dept. of Commerce. [DSS] digital signatures Digital signatures utilize public key cryptography and one-way hash functions to produce a signature of the data that can be authenticated, and is difficult to forge or repudiate. handshake An initial negotiation between client and server that establishes the parameters of their transactions. Initialization Vector (IV) When a block cipher is used in CBC mode, the initialization Dierks & Rescorla Standards Track [Page 69]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 vector is exclusive-ORed with the first plaintext block prior to encryption. IDEA A 64-bit block cipher designed by Xuejia Lai and James Massey. [IDEA] Message Authentication Code (MAC) A Message Authentication Code is a one-way hash computed from a message and some secret data. It is difficult to forge without knowing the secret data. Its purpose is to detect if the message has been altered. master secret Secure secret data used for generating encryption keys, MAC secrets, and IVs. MD5 MD5 is a secure hashing function that converts an arbitrarily long data stream into a digest of fixed size (16 bytes). [MD5] public key cryptography A class of cryptographic techniques employing two-key ciphers. Messages encrypted with the public key can only be decrypted with the associated private key. Conversely, messages signed with the private key can be verified with the public key. one-way hash function A one-way transformation that converts an arbitrary amount of data into a fixed-length hash. It is computationally hard to reverse the transformation or to find collisions. MD5 and SHA are examples of one-way hash functions. RC2 A block cipher developed by Ron Rivest, described in [RC2]. RC4 A stream cipher invented by Ron Rivest. A compatible cipher is described in [SCH]. RSA A very widely used public-key algorithm that can be used for either encryption or digital signing. [RSA] server The server is the application entity that responds to requests for connections from clients. See also under client. Dierks & Rescorla Standards Track [Page 70]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 session A TLS session is an association between a client and a server. Sessions are created by the handshake protocol. Sessions define a set of cryptographic security parameters that can be shared among multiple connections. Sessions are used to avoid the expensive negotiation of new security parameters for each connection. session identifier A session identifier is a value generated by a server that identifies a particular session. server write key The key used to encrypt data written by the server. server write MAC secret The secret data used to authenticate data written by the server. SHA The Secure Hash Algorithm is defined in FIPS PUB 180-2. It produces a 20-byte output. Note that all references to SHA actually use the modified SHA-1 algorithm. [SHA] SSL Netscape's Secure Socket Layer protocol [SSL3]. TLS is based on SSL Version 3.0 stream cipher An encryption algorithm that converts a key into a cryptographically strong keystream, which is then exclusive-ORed with the plaintext. symmetric cipher See bulk cipher. Transport Layer Security (TLS) This protocol; also, the Transport Layer Security working group of the Internet Engineering Task Force (IETF). See "Comments" at the end of this document. Dierks & Rescorla Standards Track [Page 71]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Appendix C. CipherSuite Definitions CipherSuite Key Cipher Hash Exchange TLS_NULL_WITH_NULL_NULL NULL NULL NULL TLS_RSA_WITH_NULL_MD5 RSA NULL MD5 TLS_RSA_WITH_NULL_SHA RSA NULL SHA TLS_RSA_WITH_RC4_128_MD5 RSA RC4_128 MD5 TLS_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA TLS_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA TLS_RSA_WITH_DES_CBC_SHA RSA DES_CBC SHA TLS_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA TLS_RSA_WITH_AES_128_CBC_SHA RSA AES_128_CBC SHA TLS_RSA_WITH_AES_256_CBC_SHA RSA AES_256_CBC SHA TLS_DH_DSS_WITH_DES_CBC_SHA DH_DSS DES_CBC SHA TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA TLS_DH_RSA_WITH_DES_CBC_SHA DH_RSA DES_CBC SHA TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA TLS_DHE_DSS_WITH_DES_CBC_SHA DHE_DSS DES_CBC SHA TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA TLS_DHE_RSA_WITH_DES_CBC_SHA DHE_RSA DES_CBC SHA TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA DHE_RSA 3DES_EDE_CBC SHA TLS_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5 TLS_DH_anon_WITH_DES_CBC_SHA DH_anon DES_CBC SHA TLS_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA TLS_DH_DSS_WITH_AES_128_CBC_SHA DH_DSS AES_128_CBC SHA TLS_DH_RSA_WITH_AES_128_CBC_SHA DH_RSA AES_128_CBC SHA TLS_DHE_DSS_WITH_AES_128_CBC_SHA DHE_DSS AES_128_CBC SHA TLS_DHE_RSA_WITH_AES_128_CBC_SHA DHE_RSA AES_128_CBC SHA TLS_DH_anon_WITH_AES_128_CBC_SHA DH_anon AES_128_CBC SHA TLS_DH_DSS_WITH_AES_256_CBC_SHA DH_DSS AES_256_CBC SHA TLS_DH_RSA_WITH_AES_256_CBC_SHA DH_RSA AES_256_CBC SHA TLS_DHE_DSS_WITH_AES_256_CBC_SHA DHE_DSS AES_256_CBC SHA TLS_DHE_RSA_WITH_AES_256_CBC_SHA DHE_RSA AES_256_CBC SHA TLS_DH_anon_WITH_AES_256_CBC_SHA DH_anon AES_256_CBC SHA Key Exchange Algorithm Description Key size limit DHE_DSS Ephemeral DH with DSS signatures None DHE_RSA Ephemeral DH with RSA signatures None DH_anon Anonymous DH, no signatures None DH_DSS DH with DSS-based certificates None DH_RSA DH with RSA-based certificates None NULL No key exchange N/A RSA RSA key exchange None Dierks & Rescorla Standards Track [Page 72]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Key Expanded IV Block Cipher Type Material Key Material Size Size NULL Stream 0 0 0 N/A IDEA_CBC Block 16 16 8 8 RC4_128 Stream 16 16 0 N/A DES_CBC Block 8 8 8 8 3DES_EDE_CBC Block 24 24 8 8 Type Indicates whether this is a stream cipher or a block cipher running in CBC mode. Key Material The number of bytes from the key_block that are used for generating the write keys. Expanded Key Material The number of bytes actually fed into the encryption algorithm. IV Size The amount of data needed to be generated for the initialization vector. Zero for stream ciphers; equal to the block size for block ciphers (this is equal to SecurityParameters.record_iv_length). Block Size The amount of data a block cipher enciphers in one chunk; a block cipher running in CBC mode can only encrypt an even multiple of its block size. Hash Hash Padding function Size Size NULL 0 0 MD5 16 48 SHA 20 40 Dierks & Rescorla Standards Track [Page 73]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Appendix D. Implementation Notes The TLS protocol cannot prevent many common security mistakes. This section provides several recommendations to assist implementors. D.1 Random Number Generation and Seeding TLS requires a cryptographically secure pseudorandom number generator (PRNG). Care must be taken in designing and seeding PRNGs. PRNGs based on secure hash operations, most notably SHA-1, are acceptable, but cannot provide more security than the size of the random number generator state. To estimate the amount of seed material being produced, add the number of bits of unpredictable information in each seed byte. For example, keystroke timing values taken from a PC compatible's 18.2 Hz timer provide 1 or 2 secure bits each, even though the total size of the counter value is 16 bits or more. Seeding a 128-bit PRNG would thus require approximately 100 such timer values. [RANDOM] provides guidance on the generation of random values. D.2 Certificates and Authentication Implementations are responsible for verifying the integrity of certificates and should generally support certificate revocation messages. Certificates should always be verified to ensure proper signing by a trusted Certificate Authority (CA). The selection and addition of trusted CAs should be done very carefully. Users should be able to view information about the certificate and root CA. D.3 CipherSuites TLS supports a range of key sizes and security levels, including some that provide no or minimal security. A proper implementation will probably not support many cipher suites. For instance, anonymous Diffie-Hellman is strongly discouraged because it cannot prevent man- in-the-middle attacks. Applications should also enforce minimum and maximum key sizes. For example, certificate chains containing 512-bit RSA keys or signatures are not appropriate for high-security applications. D.4 Implementation Pitfalls Implementation experience has shown that certain parts of earlier TLS specifications are not easy to understand, and have been a source of interoperability and security problems. Many of these areas have been clarified in this document, but this appendix contains a short list Dierks & Rescorla Standards Track [Page 74]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 of the most important things that require special attention from implementors. TLS protocol issues: o Do you correctly handle handshake messages that are fragmented to multiple TLS records (see Section 6.2.1)? Including corner cases like a ClientHello that is split to several small fragments? o Do you ignore the TLS record layer version number in all TLS records before ServerHello (see Appendix E.1)? o Do you handle TLS extensions in ClientHello correctly, including omitting the extensions field completely? o Do you support renegotiation, both client and server initiated? While renegotiation this is an optional feature, supporting it is highly recommended. o When the server has requested a client certificate, but no suitable certificate is available, do you correctly send an empty Certificate message, instead of omitting the whole message (see Section 7.4.6)? Cryptographic details: o In RSA-encrypted Premaster Secret, do you correctly send and verify the version number? When an error is encountered, do you continue the handshake to avoid the Bleichenbacher attack (see Section 7.4.7.1)? o What countermeasures do you use to prevent timing attacks against RSA decryption and signing operations (see Section 7.4.7.1)? o When verifying RSA signatures, do you accept both NULL and missing parameters (see Section 4.7)? Do you verify that the RSA padding doesn't have additional data after the hash value? [FI06] o When using Diffie-Hellman key exchange, do you correctly strip leading zero bytes from the negotiated key (see Section 8.1.2)? o Does your TLS client check that the Diffie-Hellman parameters sent by the server are acceptable (see Section F.1.1.3)? o How do you generate unpredictable IVs for CBC mode ciphers (see Section 6.2.3.2)? Dierks & Rescorla Standards Track [Page 75]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 o How do you address CBC mode timing attacks (Section 6.2.3.2)? o Do you use a strong and, most importantly, properly seeded random number generator (see Appendix D.1) for generating the premaster secret (for RSA key exchange), Diffie-Hellman private values, the DSA "k" parameter, and other security-critical values? Dierks & Rescorla Standards Track [Page 76]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Appendix E. Backward Compatibility E.1 Compatibility with TLS 1.0/1.1 and SSL 3.0 Since there are various versions of TLS (1.0, 1.1, 1.2, and any future versions) and SSL (2.0 and 3.0), means are needed to negotiate the specific protocol version to use. The TLS protocol provides a built-in mechanism for version negotiation so as not to bother other protocol components with the complexities of version selection. TLS versions 1.0, 1.1, and 1.2, and SSL 3.0 are very similar, and use compatible ClientHello messages; thus, supporting all of them is relatively easy. Similarly, servers can easily handle clients trying to use future versions of TLS as long as the ClientHello format remains compatible, and the client support the highest protocol version available in the server. A TLS 1.2 client who wishes to negotiate with such older servers will send a normal TLS 1.2 ClientHello, containing { 3, 3 } (TLS 1.2) in ClientHello.client_version. If the server does not support this version, it will respond with ServerHello containing an older version number. If the client agrees to use this version, the negotiation will proceed as appropriate for the negotiated protocol. If the version chosen by the server is not supported by the client (or not acceptable), the client MUST send a "protocol_version" alert message and close the connection. If a TLS server receives a ClientHello containing a version number greater than the highest version supported by the server, it MUST reply according to the highest version supported by the server. A TLS server can also receive a ClientHello containing version number smaller than the highest supported version. If the server wishes to negotiate with old clients, it will proceed as appropriate for the highest version supported by the server that is not greater than ClientHello.client_version. For example, if the server supports TLS 1.0, 1.1, and 1.2, and client_version is TLS 1.0, the server will proceed with a TLS 1.0 ServerHello. If server supports (or is willing to use) only versions greater than client_version, it MUST send a "protocol_version" alert message and close the connection. Whenever a client already knows the highest protocol known to a server (for example, when resuming a session), it SHOULD initiate the connection in that native protocol. Note: some server implementations are known to implement version negotiation incorrectly. For example, there are buggy TLS 1.0 servers Dierks & Rescorla Standards Track [Page 77]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 that simply close the connection when the client offers a version newer than TLS 1.0. Also, it is known that some servers will refuse connection if any TLS extensions are included in ClientHello. Interoperability with such buggy servers is a complex topic beyond the scope of this document, and may require multiple connection attempts by the client. Earlier versions of the TLS specification were not fully clear on what the record layer version number (TLSPlaintext.version) should contain when sending ClientHello (i.e., before it is known which version of the protocol will be employed). Thus, TLS servers compliant with this specification MUST accept any value {03,XX} as the record layer version number for ClientHello. TLS clients that wish to negotiate with older servers MAY send any value {03,XX} as the record layer version number. Typical values would be {03,00}, the lowest version number supported by the client, and the value of ClientHello.client_version. No single value will guarantee interoperability with all old servers, but this is a complex topic beyond the scope of this document. E.2 Compatibility with SSL 2.0 TLS 1.2 clients that wish to support SSL 2.0 servers MUST send version 2.0 CLIENT-HELLO messages defined in [SSL2]. The message MUST contain the same version number as would be used for ordinary ClientHello, and MUST encode the supported TLS ciphersuites in the CIPHER-SPECS-DATA field as described below. Warning: The ability to send version 2.0 CLIENT-HELLO messages will be phased out with all due haste, since the newer ClientHello format provides better mechanisms for moving to newer versions and negotiating extensions. TLS 1.2 clients SHOULD NOT support SSL 2.0. However, even TLS servers that do not support SSL 2.0 SHOULD accept version 2.0 CLIENT-HELLO messages. The message is presented below in sufficient detail for TLS server implementors; the true definition is still assumed to be [SSL2]. For negotiation purposes, 2.0 CLIENT-HELLO is interpreted the same way as a ClientHello with a "null" compression method and no extensions. Note that this message MUST be sent directly on the wire, not wrapped as a TLS record. For the purposes of calculating Finished and CertificateVerify, the msg_length field is not considered to be a part of the handshake message. uint8 V2CipherSpec[3]; Dierks & Rescorla Standards Track [Page 78]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 struct { uint16 msg_length; uint8 msg_type; Version version; uint16 cipher_spec_length; uint16 session_id_length; uint16 challenge_length; V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length]; opaque session_id[V2ClientHello.session_id_length]; opaque challenge[V2ClientHello.challenge_length; } V2ClientHello; msg_length The highest bit MUST be 1; the remaining bits contain the length of the following data in bytes. msg_type This field, in conjunction with the version field, identifies a version 2 client hello message. The value MUST be one (1). version Equal to ClientHello.client_version. cipher_spec_length This field is the total length of the field cipher_specs. It cannot be zero and MUST be a multiple of the V2CipherSpec length (3). session_id_length This field MUST have a value of zero for a client that claims to support TLS 1.2. challenge_length The length in bytes of the client's challenge to the server to authenticate itself. Historically, permissible values are between 16 and 32 bytes inclusive. When using the SSLv2 backward compatible handshake the client SHOULD use a 32 byte challenge. cipher_specs This is a list of all CipherSpecs the client is willing and able to use. In addition to the 2.0 cipher specs defined in [SSL2], this includes the TLS cipher suites normally sent in ClientHello.cipher_suites, each cipher suite prefixed by a zero byte. For example, TLS ciphersuite {0x00,0x0A} would be sent as {0x00,0x00,0x0A}. session_id This field MUST be empty. Dierks & Rescorla Standards Track [Page 79]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 challenge Corresponds to ClientHello.random. If the challenge length is less than 32, the TLS server will pad the data with leading (note: not trailing) zero bytes to make it 32 bytes long. Note: Requests to resume a TLS session MUST use a TLS client hello. E.3. Avoiding Man-in-the-Middle Version Rollback When TLS clients fall back to Version 2.0 compatibility mode, they MUST use special PKCS#1 block formatting. This is done so that TLS servers will reject Version 2.0 sessions with TLS-capable clients. When a client negotiates SSL 2.0 but also supports TLS, it MUST set the right-hand (least-significant) 8 random bytes of the PKCS padding (not including the terminal null of the padding) for the RSA encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY to 0x03 (the other padding bytes are random). When a TLS-capable server negotiates SSL 2.0 it SHOULD, after decrypting the ENCRYPTED-KEY-DATA field, check that these eight padding bytes are 0x03. If they are not, the server SHOULD generate a random value for SECRET-KEY-DATA, and continue the handshake (which will eventually fail since the keys will not match). Note that reporting the error situation to the client could make the server vulnerable to attacks described in [BLEI]. Dierks & Rescorla Standards Track [Page 80]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Appendix F. Security Analysis The TLS protocol is designed to establish a secure connection between a client and a server communicating over an insecure channel. This document makes several traditional assumptions, including that attackers have substantial computational resources and cannot obtain secret information from sources outside the protocol. Attackers are assumed to have the ability to capture, modify, delete, replay, and otherwise tamper with messages sent over the communication channel. This appendix outlines how TLS has been designed to resist a variety of attacks. F.1. Handshake Protocol The handshake protocol is responsible for selecting a CipherSpec and generating a Master Secret, which together comprise the primary cryptographic parameters associated with a secure session. The handshake protocol can also optionally authenticate parties who have certificates signed by a trusted certificate authority. F.1.1. Authentication and Key Exchange TLS supports three authentication modes: authentication of both parties, server authentication with an unauthenticated client, and total anonymity. Whenever the server is authenticated, the channel is secure against man-in-the-middle attacks, but completely anonymous sessions are inherently vulnerable to such attacks. Anonymous servers cannot authenticate clients. If the server is authenticated, its certificate message must provide a valid certificate chain leading to an acceptable certificate authority. Similarly, authenticated clients must supply an acceptable certificate to the server. Each party is responsible for verifying that the other's certificate is valid and has not expired or been revoked. The general goal of the key exchange process is to create a pre_master_secret known to the communicating parties and not to attackers. The pre_master_secret will be used to generate the master_secret (see Section 8.1). The master_secret is required to generate the finished messages, encryption keys, and MAC secrets (see Sections 7.4.9 and 6.3). By sending a correct finished message, parties thus prove that they know the correct pre_master_secret. F.1.1.1. Anonymous Key Exchange Completely anonymous sessions can be established using RSA or Diffie- Hellman for key exchange. With anonymous RSA, the client encrypts a pre_master_secret with the server's uncertified public key extracted from the server key exchange message. The result is sent in a client Dierks & Rescorla Standards Track [Page 81]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 key exchange message. Since eavesdroppers do not know the server's private key, it will be infeasible for them to decode the pre_master_secret. With Diffie-Hellman, the server's public parameters are contained in the server key exchange message and the client's are sent in the client key exchange message. Eavesdroppers who do not know the private values should not be able to find the Diffie-Hellman result (i.e. the pre_master_secret). Warning: Completely anonymous connections only provide protection against passive eavesdropping. Unless an independent tamper- proof channel is used to verify that the finished messages were not replaced by an attacker, server authentication is required in environments where active man-in-the-middle attacks are a concern. F.1.1.2. RSA Key Exchange and Authentication With RSA, key exchange and server authentication are combined. The public key is contained in the server's certificate. Note that compromise of the server's static RSA key results in a loss of confidentiality for all sessions protected under that static key. TLS users desiring Perfect Forward Secrecy should use DHE cipher suites. The damage done by exposure of a private key can be limited by changing one's private key (and certificate) frequently. After verifying the server's certificate, the client encrypts a pre_master_secret with the server's public key. By successfully decoding the pre_master_secret and producing a correct finished message, the server demonstrates that it knows the private key corresponding to the server certificate. When RSA is used for key exchange, clients are authenticated using the certificate verify message (see Section 7.4.9). The client signs a value derived from the master_secret and all preceding handshake messages. These handshake messages include the server certificate, which binds the signature to the server, and ServerHello.random, which binds the signature to the current handshake process. F.1.1.3. Diffie-Hellman Key Exchange with Authentication When Diffie-Hellman key exchange is used, the server can either supply a certificate containing fixed Diffie-Hellman parameters or use the server key exchange message to send a set of temporary Diffie-Hellman parameters signed with a DSS or RSA certificate. Temporary parameters are hashed with the hello.random values before signing to ensure that attackers do not replay old parameters. In Dierks & Rescorla Standards Track [Page 82]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 either case, the client can verify the certificate or signature to ensure that the parameters belong to the server. If the client has a certificate containing fixed Diffie-Hellman parameters, its certificate contains the information required to complete the key exchange. Note that in this case the client and server will generate the same Diffie-Hellman result (i.e., pre_master_secret) every time they communicate. To prevent the pre_master_secret from staying in memory any longer than necessary, it should be converted into the master_secret as soon as possible. Client Diffie-Hellman parameters must be compatible with those supplied by the server for the key exchange to work. If the client has a standard DSS or RSA certificate or is unauthenticated, it sends a set of temporary parameters to the server in the client key exchange message, then optionally uses a certificate verify message to authenticate itself. If the same DH keypair is to be used for multiple handshakes, either because the client or server has a certificate containing a fixed DH keypair or because the server is reusing DH keys, care must be taken to prevent small subgroup attacks. Implementations SHOULD follow the guidelines found in [SUBGROUP]. Small subgroup attacks are most easily avoided by using one of the DHE ciphersuites and generating a fresh DH private key (X) for each handshake. If a suitable base (such as 2) is chosen, g^X mod p can be computed very quickly, therefore the performance cost is minimized. Additionally, using a fresh key for each handshake provides Perfect Forward Secrecy. Implementations SHOULD generate a new X for each handshake when using DHE ciphersuites. Because TLS allows the server to provide arbitrary DH groups, the client SHOULD verify the correctness of the DH group. [TODO: provide a reference to some document describing how] and that it is of suitable size as defined by local policy. The client SHOULD also verify that the DH public exponent appears to be of adequate size. The server MAY choose to assist the client by providing a known group, such as those defined in [IKEALG] or [MODP]. These can be verified by simple comparison. F.1.2. Version Rollback Attacks Because TLS includes substantial improvements over SSL Version 2.0, attackers may try to make TLS-capable clients and servers fall back to Version 2.0. This attack can occur if (and only if) two TLS- capable parties use an SSL 2.0 handshake. Dierks & Rescorla Standards Track [Page 83]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Although the solution using non-random PKCS #1 block type 2 message padding is inelegant, it provides a reasonably secure way for Version 3.0 servers to detect the attack. This solution is not secure against attackers who can brute force the key and substitute a new ENCRYPTED- KEY-DATA message containing the same key (but with normal padding) before the application specified wait threshold has expired. Altering the padding of the least significant 8 bytes of the PKCS padding does not impact security for the size of the signed hashes and RSA key lengths used in the protocol, since this is essentially equivalent to increasing the input block size by 8 bytes. F.1.3. Detecting Attacks Against the Handshake Protocol An attacker might try to influence the handshake exchange to make the parties select different encryption algorithms than they would normally chooses. For this attack, an attacker must actively change one or more handshake messages. If this occurs, the client and server will compute different values for the handshake message hashes. As a result, the parties will not accept each others' finished messages. Without the master_secret, the attacker cannot repair the finished messages, so the attack will be discovered. F.1.4. Resuming Sessions When a connection is established by resuming a session, new ClientHello.random and ServerHello.random values are hashed with the session's master_secret. Provided that the master_secret has not been compromised and that the secure hash operations used to produce the encryption keys and MAC secrets are secure, the connection should be secure and effectively independent from previous connections. Attackers cannot use known encryption keys or MAC secrets to compromise the master_secret without breaking the secure hash operations. Sessions cannot be resumed unless both the client and server agree. If either party suspects that the session may have been compromised, or that certificates may have expired or been revoked, it should force a full handshake. An upper limit of 24 hours is suggested for session ID lifetimes, since an attacker who obtains a master_secret may be able to impersonate the compromised party until the corresponding session ID is retired. Applications that may be run in relatively insecure environments should not write session IDs to stable storage. Dierks & Rescorla Standards Track [Page 84]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 F.2. Protecting Application Data The master_secret is hashed with the ClientHello.random and ServerHello.random to produce unique data encryption keys and MAC secrets for each connection. Outgoing data is protected with a MAC before transmission. To prevent message replay or modification attacks, the MAC is computed from the MAC secret, the sequence number, the message length, the message contents, and two fixed character strings. The message type field is necessary to ensure that messages intended for one TLS Record Layer client are not redirected to another. The sequence number ensures that attempts to delete or reorder messages will be detected. Since sequence numbers are 64 bits long, they should never overflow. Messages from one party cannot be inserted into the other's output, since they use independent MAC secrets. Similarly, the server-write and client-write keys are independent, so stream cipher keys are used only once. If an attacker does break an encryption key, all messages encrypted with it can be read. Similarly, compromise of a MAC key can make message modification attacks possible. Because MACs are also encrypted, message-alteration attacks generally require breaking the encryption algorithm as well as the MAC. Note: MAC secrets may be larger than encryption keys, so messages can remain tamper resistant even if encryption keys are broken. F.3. Explicit IVs [CBCATT] describes a chosen plaintext attack on TLS that depends on knowing the IV for a record. Previous versions of TLS [TLS1.0] used the CBC residue of the previous record as the IV and therefore enabled this attack. This version uses an explicit IV in order to protect against this attack. F.4. Security of Composite Cipher Modes TLS secures transmitted application data via the use of symmetric encryption and authentication functions defined in the negotiated ciphersuite. The objective is to protect both the integrity and confidentiality of the transmitted data from malicious actions by active attackers in the network. It turns out that the order in which encryption and authentication functions are applied to the data plays an important role for achieving this goal [ENCAUTH]. The most robust method, called encrypt-then-authenticate, first applies encryption to the data and then applies a MAC to the Dierks & Rescorla Standards Track [Page 85]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 ciphertext. This method ensures that the integrity and confidentiality goals are obtained with ANY pair of encryption and MAC functions, provided that the former is secure against chosen plaintext attacks and that the MAC is secure against chosen-message attacks. TLS uses another method, called authenticate-then-encrypt, in which first a MAC is computed on the plaintext and then the concatenation of plaintext and MAC is encrypted. This method has been proven secure for CERTAIN combinations of encryption functions and MAC functions, but it is not guaranteed to be secure in general. In particular, it has been shown that there exist perfectly secure encryption functions (secure even in the information-theoretic sense) that combined with any secure MAC function, fail to provide the confidentiality goal against an active attack. Therefore, new ciphersuites and operation modes adopted into TLS need to be analyzed under the authenticate-then-encrypt method to verify that they achieve the stated integrity and confidentiality goals. Currently, the security of the authenticate-then-encrypt method has been proven for some important cases. One is the case of stream ciphers in which a computationally unpredictable pad of the length of the message, plus the length of the MAC tag, is produced using a pseudo-random generator and this pad is xor-ed with the concatenation of plaintext and MAC tag. The other is the case of CBC mode using a secure block cipher. In this case, security can be shown if one applies one CBC encryption pass to the concatenation of plaintext and MAC and uses a new, independent, and unpredictable IV for each new pair of plaintext and MAC. In previous versions of SSL, CBC mode was used properly EXCEPT that it used a predictable IV in the form of the last block of the previous ciphertext. This made TLS open to chosen plaintext attacks. This version of the protocol is immune to those attacks. For exact details in the encryption modes proven secure, see [ENCAUTH]. F.5 Denial of Service TLS is susceptible to a number of denial of service (DoS) attacks. In particular, an attacker who initiates a large number of TCP connections can cause a server to consume large amounts of CPU doing RSA decryption. However, because TLS is generally used over TCP, it is difficult for the attacker to hide his point of origin if proper TCP SYN randomization is used [SEQNUM] by the TCP stack. Because TLS runs over TCP, it is also susceptible to a number of denial of service attacks on individual connections. In particular, attackers can forge RSTs, thereby terminating connections, or forge partial TLS records, thereby causing the connection to stall. These Dierks & Rescorla Standards Track [Page 86]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 attacks cannot in general be defended against by a TCP-using protocol. Implementors or users who are concerned with this class of attack should use IPsec AH [AH] or ESP [ESP]. Dierks & Rescorla Standards Track [Page 87]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Security Considerations Security issues are discussed throughout this memo, especially in Appendices D, E, and F. Changes in This Version [RFC Editor: Please delete this] - Added compression methods to the IANA considerations. - Misc. editorial changes/clarifications - Added an Implementation Pitfalls sections [Issue 26] - Harmonized the requirement to send an empty certificate list after certificate_request even when no certs are available. [Issue 48] - Made the verify_data length depend on the cipher suite [Issue 49] - TLS_RSA_WITH_AES_128_CBC_SHA is now the mandatory to implement cipher suite [Issue 56] Normative References [AES] National Institute of Standards and Technology, "Specification for the Advanced Encryption Standard (AES)" FIPS 197. November 26, 2001. [3DES] National Institute of Standards and Technology, "Recommendation for the Triple Data Encryption Algorithm (TDEA) Block Cipher", NIST Special Publication 800-67, May 2004. [DES] National Institute of Standards and Technology, "Data Encryption Standard (DES)", FIPS PUB 46-3, October 1999. [DSS] NIST FIPS PUB 186-2, "Digital Signature Standard," National Institute of Standards and Technology, U.S. Department of Commerce, 2000. PKI [HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. Dierks & Rescorla Standards Track [Page 88]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 [IDEA] X. Lai, "On the Design and Security of Block Ciphers," ETH Series in Information Processing, v. 1, Konstanz: Hartung- Gorre Verlag, 1992. [MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, April 1992. [PKCS1] J. Jonsson, B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003. [PKIX] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002. [RC2] Rivest, R., "A Description of the RC2(r) Encryption Algorithm", RFC 2268, March 1998. [SCH] B. Schneier. "Applied Cryptography: Protocols, Algorithms, and Source Code in C, 2nd ed.", Published by John Wiley & Sons, Inc. 1996. [SHA] NIST FIPS PUB 180-2, "Secure Hash Standard," National Institute of Standards and Technology, U.S. Department of Commerce., August 2001. [REQ] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 25, RFC 2434, October 1998. Informative References [AEAD] Mcgrew, D., "Authenticated Encryption", February 2007, draft-mcgrew-auth-enc-02.txt. [AH] Kent, S., and Atkinson, R., "IP Authentication Header", RFC 4302, December 2005. [BLEI] Bleichenbacher D., "Chosen Ciphertext Attacks against Protocols Based on RSA Encryption Standard PKCS #1" in Advances in Cryptology -- CRYPTO'98, LNCS vol. 1462, pages: 1-12, 1998. [CBCATT] Moeller, B., "Security of CBC Ciphersuites in SSL/TLS: Problems and Countermeasures", Dierks & Rescorla Standards Track [Page 89]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 http://www.openssl.org/~bodo/tls-cbc.txt. [CBCTIME] Canvel, B., Hiltgen, A., Vaudenay, S., and M. Vuagnoux, "Password Interception in a SSL/TLS Channel", Advances in Cryptology -- CRYPTO 2003, LNCS vol. 2729, 2003. [CCM] "NIST Special Publication 800-38C: The CCM Mode for Authentication and Confidentiality", http://csrc.nist.gov/publications/nistpubs/800-38C/SP800-38C.pdf [ENCAUTH] Krawczyk, H., "The Order of Encryption and Authentication for Protecting Communications (Or: How Secure is SSL?)", Crypto 2001. [ESP] Kent, S., and Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [FI06] Hal Finney, "Bleichenbacher's RSA signature forgery based on implementation error", ietf-openpgp@imc.org mailing list, 27 August 2006, http://www.imc.org/ietf-openpgp/mail- archive/msg14307.html. [GCM] "NIST Special Publication 800-38D DRAFT (June, 2007): Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC" [IKEALG] Schiller, J., "Cryptographic Algorithms for Use in the Internet Key Exchange Version 2 (IKEv2)", RFC 4307, December 2005. [KPR03] Klima, V., Pokorny, O., Rosa, T., "Attacking RSA-based Sessions in SSL/TLS", http://eprint.iacr.org/2003/052/, March 2003. [MODP] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)", RFC 3526, May 2003. [PKCS6] RSA Laboratories, "PKCS #6: RSA Extended Certificate Syntax Standard," version 1.5, November 1993. [PKCS7] RSA Laboratories, "PKCS #7: RSA Cryptographic Message Syntax Standard," version 1.5, November 1993. [RANDOM] Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. Dierks & Rescorla Standards Track [Page 90]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 [RFC3749] Hollenbeck, S., "Transport Layer Security Protocol Compression Methods", RFC 3749, May 2004. [RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., Wright, T., "Transport Layer Security (TLS) Extensions", RFC 4366, April 2006. [RSA] R. Rivest, A. Shamir, and L. M. Adleman, "A Method for Obtaining Digital Signatures and Public-Key Cryptosystems," Communications of the ACM, v. 21, n. 2, Feb 1978, pp. 120-126. [SEQNUM] Bellovin. S., "Defending Against Sequence Number Attacks", RFC 1948, May 1996. [SSL2] Hickman, Kipp, "The SSL Protocol", Netscape Communications Corp., Feb 9, 1995. [SSL3] A. Freier, P. Karlton, and P. Kocher, "The SSL 3.0 Protocol", Netscape Communications Corp., Nov 18, 1996. [SUBGROUP] Zuccherato, R., "Methods for Avoiding the "Small-Subgroup" Attacks on the Diffie-Hellman Key Agreement Method for S/MIME", RFC 2785, March 2000. [TCP] Postel, J., "Transmission Control Protocol," STD 7, RFC 793, September 1981. [TIMING] Boneh, D., Brumley, D., "Remote timing attacks are practical", USENIX Security Symposium 2003. [TLSAES] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS)", RFC 3268, June 2002. [TLSEXT], Eastlake, D.E., "Transport Layer Security (TLS) Extensions: Extension Definitions", July 2007, draft-ietf- tls-rfc4366-bis-00.txt. [TLS1.0] Dierks, T., and C. Allen, "The TLS Protocol, Version 1.0", RFC 2246, January 1999. [TLS1.1] Dierks, T., and E. Rescorla, "The TLS Protocol, Version 1.1", RFC 4346, April, 2006. [X501] ITU-T Recommendation X.501: Information Technology - Open Systems Interconnection - The Directory: Models, 1993. [XDR] Srinivansan, R., Sun Microsystems, "XDR: External Data Dierks & Rescorla Standards Track [Page 91]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Representation Standard", RFC 1832, August 1995. Credits Working Group Chairs Eric Rescorla EMail: ekr@networkresonance.com Pasi Eronen pasi.eronen@nokia.com Editors Tim Dierks Eric Rescorla Independent Network Resonance, Inc. EMail: tim@dierks.org EMail: ekr@networkresonance.com Other contributors Christopher Allen (co-editor of TLS 1.0) Alacrity Ventures ChristopherA@AlacrityManagement.com Martin Abadi University of California, Santa Cruz abadi@cs.ucsc.edu Steven M. Bellovin Columbia University smb@cs.columbia.edu Simon Blake-Wilson BCI EMail: sblakewilson@bcisse.com Ran Canetti IBM canetti@watson.ibm.com Pete Chown Skygate Technology Ltd pc@skygate.co.uk Dierks & Rescorla Standards Track [Page 92]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Taher Elgamal taher@securify.com Securify Anil Gangolli anil@busybuddha.org Kipp Hickman Alfred Hoenes David Hopwood Independent Consultant EMail: david.hopwood@blueyonder.co.uk Phil Karlton (co-author of SSLv3) Paul Kocher (co-author of SSLv3) Cryptography Research paul@cryptography.com Hugo Krawczyk Technion Israel Institute of Technology hugo@ee.technion.ac.il Jan Mikkelsen Transactionware EMail: janm@transactionware.com Magnus Nystrom RSA Security EMail: magnus@rsasecurity.com Robert Relyea Netscape Communications relyea@netscape.com Jim Roskind Netscape Communications jar@netscape.com Michael Sabin Dan Simon Microsoft, Inc. dansimon@microsoft.com Tom Weinstein Dierks & Rescorla Standards Track [Page 93]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Tim Wright Vodafone EMail: timothy.wright@vodafone.com Comments The discussion list for the IETF TLS working group is located at the e-mail address <tls@ietf.org>. Information on the group and information on how to subscribe to the list is at <https://www1.ietf.org/mailman/listinfo/tls> Archives of the list can be found at: <http://www.ietf.org/mail-archive/web/tls/current/index.html> Dierks & Rescorla Standards Track [Page 94]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Dierks & Rescorla Standards Track [Page 95]draft-ietf-tls-rfc4346-bis-05.txt TLS June 2007 Dierks & Rescorla Standards Track [Page 96]
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