HISTORIC
Internet Engineering Task Force [IETF] A. Freier Request for Comments: 6101 P. Karlton Category: Historic Netscape Communications ISSN: 2070-1721 P. Kocher Independent Consultant August 2011 The Secure Sockets Layer [SSL] Protocol Version 3.0 Abstract This document is published as a historical record of the SSL 3.0 protocol. The original Abstract follows. This document specifies version 3.0 of the Secure Sockets Layer [SSL 3.0] protocol, a security protocol that provides communications privacy over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. Foreword Although the SSL 3.0 protocol is a widely implemented protocol, a pioneer in secure communications protocols, and the basis for Transport Layer Security [TLS], it was never formally published by the IETF, except in several expired Internet-Drafts. This allowed no easy referencing to the protocol. We believe a stable reference to the original document should exist and for that reason, this document describes what is known as the last published version of the SSL 3.0 protocol, that is, the November 18, 1996, version of the protocol. There were no changes to the original document other than trivial editorial changes and the addition of a "Security Considerations" section. However, portions of the original document that no longer apply were not included. Such as the "Patent Statement" section, the "Reserved Ports Assignment" section, and the cipher-suite registrator note in the "The CipherSuite" section. The "US export rules" discussed in the document do not apply today but are kept intact to provide context for decisions taken in protocol design. The "Goals of This Document" section indicates the goals for adopters of SSL 3.0, not goals of the IETF. The authors and editors were retained as in the original document. The editor of this document is Nikos Mavrogiannopoulos []. The editor would like to thank Dan Harkins, Linda Dunbar, Sean Turner, and Geoffrey Keating for reviewing this document and providing helpful comments. Freier, et al. Historic [Page 1] RFC 6101 The SSL Protocol Version 3.0 August 2011 Status of This Memo This document is not an Internet Standards Track specification; it is published for the historical record. This document defines a Historic Document for the Internet community. This document is a product of the Internet Engineering Task Force [IETF]. It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group [IESG]. Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at //www.rfc-editor.org/info/rfc6101. Copyright Notice Copyright [c] 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents [//trustee.ietf.org/license-info] in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person[s] controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person[s] controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Freier, et al. Historic [Page 2] RFC 6101 The SSL Protocol Version 3.0 August 2011 Table of Contents 1. Introduction ....................................................5 2. Goals ...........................................................5 3. Goals of This Document ..........................................6 4. Presentation Language ...........................................6 4.1. Basic Block Size ...........................................7 4.2. Miscellaneous ..............................................7 4.3. Vectors ....................................................7 4.4. Numbers ....................................................8 4.5. Enumerateds ................................................8 4.6. Constructed Types ..........................................9 4.6.1. Variants ...........................................10 4.7. Cryptographic Attributes ..................................11 4.8. Constants .................................................12 5. SSL Protocol ...................................................12 5.1. Session and Connection States .............................12 5.2. Record Layer ..............................................14 5.2.1. Fragmentation ......................................14 5.2.2. Record Compression and Decompression ...............15 5.2.3. Record Payload Protection and the CipherSpec .......16 5.3. Change Cipher Spec Protocol ...............................18 5.4. Alert Protocol ............................................18 5.4.1. Closure Alerts .....................................19 5.4.2. Error Alerts .......................................20 5.5. Handshake Protocol Overview ...............................21 5.6. Handshake Protocol ........................................23 5.6.1. Hello messages .....................................24 5.6.2. Server Certificate .................................28 5.6.3. Server Key Exchange Message ........................28 5.6.4. Certificate Request ................................30 5.6.5. Server Hello Done ..................................31 5.6.6. Client Certificate .................................31 5.6.7. Client Key Exchange Message ........................31 5.6.8. Certificate Verify .................................34 5.6.9. Finished ...........................................35 5.7. Application Data Protocol .................................36 6. Cryptographic Computations .....................................36 6.1. Asymmetric Cryptographic Computations .....................36 6.1.1. RSA ................................................36 6.1.2. Diffie-Hellman .....................................37 6.1.3. FORTEZZA ...........................................37 6.2. Symmetric Cryptographic Calculations and the CipherSpec ...37 6.2.1. The Master Secret ..................................37 6.2.2. Converting the Master Secret into Keys and MAC Secrets ........................................37 7. Security Considerations ........................................39 8. Informative References .........................................40 Freier, et al. Historic [Page 3] RFC 6101 The SSL Protocol Version 3.0 August 2011 Appendix A. Protocol Constant Values ..............................42 A.1. Record Layer ...............................................42 A.2. Change Cipher Specs Message ................................43 A.3. Alert Messages .............................................43 A.4. Handshake Protocol .........................................44 A.4.1. Hello Messages .........................................44 A.4.2. Server Authentication and Key Exchange Messages ........45 A.5. Client Authentication and Key Exchange Messages ............46 A.5.1. Handshake Finalization Message .........................47 A.6. The CipherSuite ............................................47 A.7. The CipherSpec .............................................49 Appendix B. Glossary ..............................................50 Appendix C. CipherSuite Definitions ...............................53 Appendix D. Implementation Notes ..................................56 D.1. Temporary RSA Keys .........................................56 D.2. Random Number Generation and Seeding .......................56 D.3. Certificates and Authentication ............................57 D.4. CipherSuites ...............................................57 D.5. FORTEZZA ...................................................57 D.5.1. Notes on Use of FORTEZZA Hardware ......................57 D.5.2. FORTEZZA Cipher Suites .................................58 D.5.3. FORTEZZA Session Resumption ............................58 Appendix E. Version 2.0 Backward Compatibility ....................59 E.1. Version 2 Client Hello .....................................59 E.2. Avoiding Man-in-the-Middle Version Rollback ..............61 Appendix F. Security Analysis .....................................61 F.1. Handshake Protocol .........................................61 F.1.1. Authentication and Key Exchange ........................61 F.1.2. Version Rollback Attacks ...............................64 F.1.3. Detecting Attacks against the Handshake Protocol .......64 F.1.4. Resuming Sessions ......................................65 F.1.5. MD5 and SHA ............................................65 F.2. Protecting Application Data ................................65 F.3. Final Notes ................................................66 Appendix G. Acknowledgements ......................................66 G.1. Other Contributors .........................................66 G.2. Early Reviewers ............................................67 Freier, et al. Historic [Page 4] RFC 6101 The SSL Protocol Version 3.0 August 2011 1. Introduction The primary goal of the SSL protocol is to provide privacy and reliability between two communicating applications. The protocol is composed of two layers. At the lowest level, layered on top of some reliable transport protocol [e.g., TCP [RFC0793]], is the SSL record protocol. The SSL record protocol is used for encapsulation of various higher level protocols. One such encapsulated protocol, the SSL 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. One advantage of SSL is that it is application protocol independent. A higher level protocol can layer on top of the SSL protocol transparently. The SSL protocol provides connection security that has three basic properties: o The connection is private. Encryption is used after an initial handshake to define a secret key. Symmetric cryptography is used for data encryption [e.g., DES [DES], 3DES [3DES], RC4 [SCH]]. o The peer's identity can be authenticated using asymmetric, or public key, cryptography [e.g., RSA [RSA], DSS [DSS]]. o The connection is reliable. Message transport includes a message integrity check using a keyed Message Authentication Code [MAC] [RFC2104]. Secure hash functions [e.g., SHA, MD5] are used for MAC computations. 2. Goals The goals of SSL protocol version 3.0, in order of their priority, are: 1. Cryptographic security SSL should be used to establish a secure connection between two parties. 2. Interoperability Independent programmers should be able to develop applications utilizing SSL 3.0 that will then be able to successfully exchange cryptographic parameters without knowledge of one another's code. Freier, et al. Historic [Page 5] RFC 6101 The SSL Protocol Version 3.0 August 2011 Note: It is not the case that all instances of SSL [even in the same application domain] will be able to successfully connect. For instance, if the server supports a particular hardware token, and the client does not have access to such a token, then the connection will not succeed. 3. Extensibility SSL 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: to prevent the need to create a new protocol [and risking the introduction of possible new weaknesses] and to avoid 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 SSL 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 The SSL protocol version 3.0 specification is intended primarily for readers who will be implementing the protocol and those doing cryptographic analysis of it. The spec 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 interface definition, although it does cover select areas of policy as they are required for the maintenance of solid security. 4. Presentation Language This document deals with the formatting of data in an external representation. The following very basic and somewhat casually defined presentation syntax will be used. The syntax draws from several sources in its structure. Although it resembles the programming language "C" in its syntax and External Data Representation [XDR] [RFC1832] in both its syntax and intent, it Freier, et al. Historic [Page 6] RFC 6101 The SSL Protocol Version 3.0 August 2011 would be risky to draw too many parallels. The purpose of this presentation language is to document SSL only, not to have general application beyond that particular goal. 4.1. Basic Block Size The representation of all data items is explicitly specified. The basic data block size is one byte [i.e., 8 bits]. Multiple byte data items are concatenations of bytes, from left to right, from top to bottom. From the byte stream, a multi-byte item [a numeric in the example] is formed [using C notation] by: value = [byte[0] ServerHello Certificate* ServerKeyExchange* CertificateRequest* [ChangeCipherSpec] ServerHello [change cipher spec] Application Data Application Data The contents and significance of each message will be presented in detail in the following sections. 5.6. Handshake Protocol The SSL handshake protocol is one of the defined higher level clients of the SSL record protocol. This protocol is used to negotiate the secure attributes of a session. Handshake messages are supplied to the SSL record layer, where they are encapsulated within one or more SSLPlaintext structures, which are processed and transmitted as specified by the current active session state. enum { hello_request[0], client_hello[1], server_hello[2], certificate[11], server_key_exchange [12], certificate_request[13], server_hello_done[14], certificate_verify[15], client_key_exchange[16], finished[20], [255] } HandshakeType; struct { HandshakeType msg_type; /* handshake type */ uint24 length; /* bytes in message */ select [HandshakeType] { case hello_request: HelloRequest; case client_hello: ClientHello; case server_hello: ServerHello; case certificate: Certificate; case server_key_exchange: ServerKeyExchange; case certificate_request: CertificateRequest; case server_hello_done: ServerHelloDone; case certificate_verify: CertificateVerify; case client_key_exchange: ClientKeyExchange; case finished: Finished; } body; } Handshake; Freier, et al. Historic [Page 23] RFC 6101 The SSL Protocol Version 3.0 August 2011 The handshake protocol messages are presented in the order they must be sent; sending handshake messages in an unexpected order results in a fatal error. 5.6.1. Hello messages The hello phase messages are used to exchange security enhancement capabilities between the client and server. When a new session begins, the CipherSpec encryption, hash, and compression algorithms are initialized to null. The current CipherSpec is used for renegotiation messages. 5.6.1.1. Hello Request The hello request message may be sent by the server at any time, but will be ignored by the client if the handshake protocol is already underway. It is a simple notification that the client should begin the negotiation process anew by sending a client hello message when convenient. Note: Since handshake messages are intended to have transmission precedence over application data, it is expected that the negotiation begin in no more than one or two times the transmission time of a maximum-length application data message. After sending a hello request, servers should not repeat the request until the subsequent handshake negotiation is complete. A client that receives a hello request while in a handshake negotiation state should simply ignore the message. The structure of a hello request message is as follows: struct { } HelloRequest; 5.6.1.2. Client Hello 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. The client hello message includes a random structure, which is used later in the protocol. Freier, et al. Historic [Page 24] RFC 6101 The SSL Protocol Version 3.0 August 2011 struct { uint32 gmt_unix_time; opaque random_bytes[28]; } Random; gmt_unix_time: The current time and date in standard UNIX 32-bit format according to the sender's internal clock. Clocks are not required to be set correctly by the basic SSL 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 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, while the third option makes it possible to establish several simultaneous independent secure connections without repeating the full handshake protocol. The actual contents of the SessionID are defined by the server. opaque SessionID; Warning: Servers must not place confidential information in session identifiers or let the contents of fake session identifiers cause any breach of security. 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 [first choice first]. Each CipherSuite defines both a key exchange algorithm and a CipherSpec. The server will select a cipher suite or, if no acceptable choices are presented, return a handshake failure alert and close the connection. uint8 CipherSuite[2]; /* Cryptographic suite selector */ The client hello includes a list of compression algorithms supported by the client, ordered according to the client's preference. If the server supports none of those specified by the client, the session must fail. enum { null[0], [255] } CompressionMethod; Issue: Which compression methods to support is under investigation. Freier, et al. Historic [Page 25] RFC 6101 The SSL Protocol Version 3.0 August 2011 The structure of the client hello is as follows. struct { ProtocolVersion client_version; Random random; SessionID session_id; CipherSuite cipher_suites; CompressionMethod compression_methods; } ClientHello; client_version: The version of the SSL protocol by which the client wishes to communicate during this session. This should be the most recent [highest valued] version supported by the client. For this version of the specification, the version will be 3.0 [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 the client wishes to generate new security parameters. cipher_suites: This is a list of the cryptographic options supported by the client, sorted 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.6. 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], this vector must include at least the compression_method from that session. All implementations must support CompressionMethod.null. 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. Implementation note: Application data may not be sent before a finished message has been sent. Transmitted application data is known to be insecure until a valid finished message has been received. This absolute restriction is relaxed if there is a current, non-null encryption on this connection. Freier, et al. Historic [Page 26] RFC 6101 The SSL Protocol Version 3.0 August 2011 Forward compatibility note: In the interests of forward compatibility, it is permitted for a client hello message to include extra data after the compression methods. This data must be included in the handshake hashes, but must otherwise be ignored. 5.6.1.3. Server Hello The server processes the client hello message and responds with either a handshake_failure alert or server hello message. struct { ProtocolVersion server_version; Random random; SessionID session_id; CipherSuite cipher_suite; CompressionMethod compression_method; } ServerHello; server_version: This field will contain the lower of that suggested by the client in the client hello and the highest supported by the server. For this version of the specification, the version will be 3.0 [see Appendix E for details about backward compatibility]. random: This structure is generated by the server and must be different from [and independent of] 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. cipher_suite: The single cipher suite selected by the server from the list in ClientHello.cipher_suites. For resumed sessions, this field is the value from the state of the session being resumed. compression_method: The single compression algorithm selected by the server from the list in ClientHello.compression_methods. For resumed sessions, this field is the value from the resumed session state. Freier, et al. Historic [Page 27] RFC 6101 The SSL Protocol Version 3.0 August 2011 5.6.2. Server Certificate If the server is to be authenticated [which is generally the case], the server sends its certificate immediately following the server hello message. The certificate type must be appropriate for the selected cipher suite's key exchange algorithm, and is generally an X.509.v3 certificate [or a modified X.509 certificate in the case of FORTEZZA[tm] [FOR]]. The same message type will be used for the client's response to a certificate request message. opaque ASN.1Cert; struct { ASN.1Cert certificate_list; } Certificate; certificate_list: This is a sequence [chain] of X.509.v3 certificates, ordered with the sender's certificate first followed by any certificate authority certificates proceeding sequentially upward. 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. 5.6.3. Server Key Exchange Message The server key exchange message is sent by the server if it has no certificate, has a certificate only used for signing [e.g., DSS [DSS] certificates, signing-only RSA [RSA] certificates], or FORTEZZA KEA key exchange is used. This message is not used if the server certificate contains Diffie-Hellman [DH1] parameters. Note: According to current US export law, RSA moduli larger than 512 bits may not be used for key exchange in software exported from the US. With this message, larger RSA keys may be used as signature-only certificates to sign temporary shorter RSA keys for key exchange. enum { rsa, diffie_hellman, fortezza_kea } KeyExchangeAlgorithm; struct { opaque rsa_modulus; opaque rsa_exponent; } ServerRSAParams; Freier, et al. Historic [Page 28] RFC 6101 The SSL Protocol Version 3.0 August 2011 rsa_modulus: The modulus of the server's temporary RSA key. rsa_exponent: The public exponent of the server's temporary RSA key. struct { opaque dh_p; opaque dh_g; opaque dh_Ys; } 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 [gX mod p]. struct { opaque r_s [128]; } ServerFortezzaParams; r_s: Server random number for FORTEZZA KEA [Key Exchange Algorithm]. struct { select [KeyExchangeAlgorithm] { case diffie_hellman: ServerDHParams params; Signature signed_params; case rsa: ServerRSAParams params; Signature signed_params; case fortezza_kea: ServerFortezzaParams params; }; } ServerKeyExchange; params: The server's key exchange parameters. signed_params: A hash of the corresponding params value, with the signature appropriate to that hash applied. md5_hash: MD5[ClientHello.random + ServerHello.random + ServerParams]; Freier, et al. Historic [Page 29] RFC 6101 The SSL Protocol Version 3.0 August 2011 sha_hash: SHA[ClientHello.random + ServerHello.random + ServerParams]; enum { anonymous, rsa, dsa } SignatureAlgorithm; digitally-signed struct { select[SignatureAlgorithm] { case anonymous: struct { }; case rsa: opaque md5_hash[16]; opaque sha_hash[20]; case dsa: opaque sha_hash[20]; }; } Signature; 5.6.4. Certificate Request A non-anonymous server can optionally request a certificate from the client, if appropriate for the selected cipher suite. enum { rsa_sign[1], dss_sign[2], rsa_fixed_dh[3], dss_fixed_dh[4], rsa_ephemeral_dh[5], dss_ephemeral_dh[6], fortezza_kea[20], [255] } ClientCertificateType; opaque DistinguishedName; struct { ClientCertificateType certificate_types; DistinguishedName certificate_authorities; } CertificateRequest; certificate_types: This field is a list of the types of certificates requested, sorted in order of the server's preference. certificate_authorities: A list of the distinguished names of acceptable certificate authorities. Note: DistinguishedName is derived from [X509]. Note: It is a fatal handshake_failure alert for an anonymous server to request client identification. Freier, et al. Historic [Page 30] RFC 6101 The SSL Protocol Version 3.0 August 2011 5.6.5. Server Hello Done 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. struct { } ServerHelloDone; 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. 5.6.6. Client Certificate This is the first message the client can send after receiving a server hello done message. This message is only sent if the server requests a certificate. If no suitable certificate is available, the client should send a no_certificate alert instead. This alert is only a warning; however, the server may respond with a fatal handshake failure alert if client authentication is required. Client certificates are sent using the certificate defined in Section 5.6.2. Note: Client Diffie-Hellman certificates must match the server specified Diffie-Hellman parameters. 5.6.7. Client Key Exchange Message The choice of messages depends on which public key algorithm[s] has [have] been selected. See Section 5.6.3 for the KeyExchangeAlgorithm definition. struct { select [KeyExchangeAlgorithm] { case rsa: EncryptedPreMasterSecret; case diffie_hellman: ClientDiffieHellmanPublic; case fortezza_kea: FortezzaKeys; } exchange_keys; } ClientKeyExchange; The information to select the appropriate record structure is in the pending session state [see Section 5.1]. Freier, et al. Historic [Page 31] RFC 6101 The SSL Protocol Version 3.0 August 2011 5.6.7.1. RSA Encrypted Premaster Secret Message If RSA is being used for key agreement and authentication, the client generates a 48-byte premaster secret, encrypts it under the public key from the server's certificate or temporary RSA key from a server key exchange message, and sends the result in an encrypted premaster secret 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 6.1. 5.6.7.2. FORTEZZA Key Exchange Message Under FORTEZZA, the client derives a token encryption key [TEK] using the FORTEZZA Key Exchange Algorithm [KEA]. The client's KEA calculation uses the public key in the server's certificate along with private parameters in the client's token. The client sends public parameters needed for the server to generate the TEK, using its own private parameters. The client generates session keys, wraps them using the TEK, and sends the results to the server. The client generates IVs for the session keys and TEK and sends them also. The client generates a random 48-byte premaster secret, encrypts it using the TEK, and sends the result: Freier, et al. Historic [Page 32] RFC 6101 The SSL Protocol Version 3.0 August 2011 struct { opaque y_c; opaque r_c[128]; opaque y_signature[40]; opaque wrapped_client_write_key[12]; opaque wrapped_server_write_key[12]; opaque client_write_iv[24]; opaque server_write_iv[24]; opaque master_secret_iv[24]; block-ciphered opaque encrypted_pre_master_secret[48]; } FortezzaKeys; y_signature: y_signature is the signature of the KEA public key, signed with the client's DSS private key. y_c: The client's Yc value [public key] for the KEA calculation. If the client has sent a certificate, and its KEA public key is suitable, this value must be empty since the certificate already contains this value. If the client sent a certificate without a suitable public key, y_c is used and y_signature is the KEA public key signed with the client's DSS private key. For this value to be used, it must be between 64 and 128 bytes. r_c: The client's Rc value for the KEA calculation. wrapped_client_write_key: This is the client's write key, wrapped by the TEK. wrapped_server_write_key: This is the server's write key, wrapped by the TEK. client_write_iv: The IV for the client write key. server_write_iv: The IV for the server write key. master_secret_iv: This is the IV for the TEK used to encrypt the premaster secret. pre_master_secret: A random value, generated by the client and used to generate the master secret, as specified in Section 6.1. In the above structure, it is encrypted using the TEK. Freier, et al. Historic [Page 33] RFC 6101 The SSL Protocol Version 3.0 August 2011 5.6.7.3. Client Diffie-Hellman Public Value 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. enum { implicit, explicit } PublicValueEncoding; implicit: If the client certificate already contains the public value, then it is implicit and Yc does not need to be sent again. explicit: Yc needs to be sent. struct { select [PublicValueEncoding] { case implicit: struct { }; case explicit: opaque dh_Yc; } dh_public; } ClientDiffieHellmanPublic; dh_Yc: The client's Diffie-Hellman public value [Yc]. 5.6.8. Certificate Verify This message is used to provide explicit verification of a client certificate. This message is only sent following any client certificate that has signing capability [i.e., all certificates except those containing fixed Diffie-Hellman parameters]. struct { Signature signature; } CertificateVerify; CertificateVerify.signature.md5_hash MD5[master_secret + pad_2 + MD5[handshake_messages + master_secret + pad_1]]; Certificate.signature.sha_hash SHA[master_secret + pad_2 + SHA[handshake_messages + master_secret + pad_1]]; pad_1: This is identical to the pad_1 defined in Section 5.2.3.1. pad_2: This is identical to the pad_2 defined in Section 5.2.3.1. Here, handshake_messages refers to all handshake messages starting at client hello up to but not including this message. Freier, et al. Historic [Page 34] RFC 6101 The SSL Protocol Version 3.0 August 2011 5.6.9. Finished A finished message is always sent immediately after a change cipher spec message to verify that the key exchange and authentication processes were successful. The finished message is the first protected with the just-negotiated algorithms, keys, and secrets. No acknowledgment of the finished message is required; parties may begin sending encrypted data immediately after sending the finished message. Recipients of finished messages must verify that the contents are correct. enum { client[0x434C4E54], server[0x53525652] } Sender; struct { opaque md5_hash[16]; opaque sha_hash[20]; } Finished; md5_hash: MD5[master_secret + pad2 + MD5[handshake_messages + Sender + master_secret + pad1]]; sha_hash: SHA[master_secret + pad2 + SHA[handshake_messages + Sender + master_secret + pad1]]; handshake_messages: All of the data from all handshake messages up to but not including this message. This is only data visible at the handshake layer and does not include record layer headers. It is a fatal error if a finished message is not preceeded by a change cipher spec message at the appropriate point in the handshake. The hash contained in finished messages sent by the server incorporate Sender.server; those sent by the client incorporate Sender.client. 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 5.6.8 because it would include the certificate verify message [if sent]. Note: Change cipher spec messages are not handshake messages and are not included in the hash computations. Freier, et al. Historic [Page 35] RFC 6101 The SSL Protocol Version 3.0 August 2011 5.7. 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. 6. Cryptographic Computations The key exchange, authentication, encryption, and MAC algorithms are determined by the cipher_suite selected by the server and revealed in the server hello message. 6.1. Asymmetric Cryptographic Computations The asymmetric algorithms are used in the handshake protocol to authenticate parties and to generate shared keys and secrets. For Diffie-Hellman, RSA, and FORTEZZA, 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 = MD5[pre_master_secret + SHA['A' + pre_master_secret + ClientHello.random + ServerHello.random]] + MD5[pre_master_secret + SHA['BB' + pre_master_secret + ClientHello.random + ServerHello.random]] + MD5[pre_master_secret + SHA['CCC' + pre_master_secret + ClientHello.random + ServerHello.random]]; 6.1.1. RSA When RSA is used for server authentication and key exchange, a 48- byte pre_master_secret is generated by the client, encrypted under the server's public key, and sent to the server. The server uses its private key to decrypt the pre_master_secret. Both parties then convert the pre_master_secret into the master_secret, as specified above. RSA digital signatures are performed using PKCS #1 [PKCS1] block type 1. RSA public key encryption is performed using PKCS #1 block type 2. Freier, et al. Historic [Page 36] RFC 6101 The SSL Protocol Version 3.0 August 2011 6.1.2. 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. Note: Diffie-Hellman parameters are specified by the server, and may be either ephemeral or contained within the server's certificate. 6.1.3. FORTEZZA A random 48-byte pre_master_secret is sent encrypted under the TEK and its IV. The server decrypts the pre_master_secret and converts it into a master_secret, as specified above. Bulk cipher keys and IVs for encryption are generated by the client's token and exchanged in the key exchange message; the master_secret is only used for MAC computations. 6.2. Symmetric Cryptographic Calculations and the CipherSpec The technique used to encrypt and verify the integrity of SSL records is specified by the currently active CipherSpec. A typical example would be to encrypt data using DES and generate authentication codes using MD5. The encryption and MAC algorithms are set to SSL_NULL_WITH_NULL_NULL at the beginning of the SSL handshake protocol, indicating that no message authentication or encryption is performed. The handshake protocol is used to negotiate a more secure CipherSpec and to generate cryptographic keys. 6.2.1. The Master Secret Before secure encryption or integrity verification can be performed on records, the client and server need to generate shared secret information known only to themselves. This value is a 48-byte quantity called the master secret. The master secret is used to generate keys and secrets for encryption and MAC computations. Some algorithms, such as FORTEZZA, may have their own procedure for generating encryption keys [the master secret is used only for MAC computations in FORTEZZA]. 6.2.2. Converting the Master Secret into Keys and MAC Secrets The master secret is hashed into a sequence of secure bytes, which are assigned to the MAC secrets, keys, and non-export IVs required by the current CipherSpec [see Appendix A.7]. CipherSpecs require a client write MAC secret, a server write MAC secret, a client write key, a server write key, a client write IV, and a server write IV, which are generated from the master secret in that order. Unused Freier, et al. Historic [Page 37] RFC 6101 The SSL Protocol Version 3.0 August 2011 values, such as FORTEZZA keys communicated in the KeyExchange message, are empty. The following inputs are available to the key definition process: opaque MasterSecret[48] ClientHello.random ServerHello.random When generating keys and MAC secrets, the master secret is used as an entropy source, and the random values provide unencrypted salt material and IVs for exportable ciphers. To generate the key material, compute key_block = MD5[master_secret + SHA[`A' + master_secret + ServerHello.random + ClientHello.random]] + MD5[master_secret + SHA[`BB' + master_secret + ServerHello.random + ClientHello.random]] + MD5[master_secret + SHA[`CCC' + master_secret + ServerHello.random + ClientHello.random]] + [...]; until enough output has been generated. Then, the key_block is partitioned as follows. client_write_MAC_secret[CipherSpec.hash_size] server_write_MAC_secret[CipherSpec.hash_size] client_write_key[CipherSpec.key_material] server_write_key[CipherSpec.key_material] client_write_IV[CipherSpec.IV_size] /* non-export ciphers */ server_write_IV[CipherSpec.IV_size] /* non-export ciphers */ Any extra key_block material is discarded. Exportable encryption algorithms [for which CipherSpec.is_exportable is true] require additional processing as follows to derive their final write keys: final_client_write_key = MD5[client_write_key + ClientHello.random + ServerHello.random]; final_server_write_key = MD5[server_write_key + ServerHello.random + ClientHello.random]; Freier, et al. Historic [Page 38] RFC 6101 The SSL Protocol Version 3.0 August 2011 Exportable encryption algorithms derive their IVs from the random messages: client_write_IV = MD5[ClientHello.random + ServerHello.random]; server_write_IV = MD5[ServerHello.random + ClientHello.random]; MD5 outputs are trimmed to the appropriate size by discarding the least-significant bytes. 6.2.2.1. Export Key Generation Example SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5 requires five random bytes for each of the two encryption keys and 16 bytes for each of the MAC keys, for a total of 42 bytes of key material. MD5 produces 16 bytes of output per call, so three calls to MD5 are required. The MD5 outputs are concatenated into a 48-byte key_block with the first MD5 call providing bytes zero through 15, the second providing bytes 16 through 31, etc. The key_block is partitioned, and the write keys are salted because this is an exportable encryption algorithm. client_write_MAC_secret = key_block[0..15] server_write_MAC_secret = key_block[16..31] client_write_key = key_block[32..36] server_write_key = key_block[37..41] final_client_write_key = MD5[client_write_key + ClientHello.random + ServerHello.random][0..15]; final_server_write_key = MD5[server_write_key + ServerHello.random + ClientHello.random][0..15]; client_write_IV = MD5[ClientHello.random + ServerHello.random][0..7]; server_write_IV = MD5[ServerHello.random + ClientHello.random][0..7]; 7. Security Considerations See Appendix F. Freier, et al. Historic [Page 39] RFC 6101 The SSL Protocol Version 3.0 August 2011 8. Informative References [DH1] Diffie, W. and M. Hellman, "New Directions in Cryptography", IEEE Transactions on Information Theory V. IT-22, n. 6, pp. 74-84, June 1977. [SSL-2] Hickman, K., "The SSL Protocol", February 1995. [3DES] Tuchman, W., "Hellman Presents No Shortcut Solutions To DES", IEEE Spectrum, v. 16, n. 7, pp 40-41, July 1979. [DES] ANSI X3.106, "American National Standard for Information Systems-Data Link Encryption", American National Standards Institute, 1983. [DSS] NIST FIPS PUB 186, "Digital Signature Standard", National Institute of Standards and Technology U.S. Department of Commerce, May 1994. [FOR] NSA X22, "FORTEZZA: Application Implementers Guide", Document # PD4002103-1.01, April 1995. [RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC 959, October 1985. [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC1945] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996. [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol Specification", STD 8, RFC 854, May 1983. [RFC1832] Srinivasan, R., "XDR: External Data Representation Standard", RFC 1832, August 1995. Freier, et al. Historic [Page 40] RFC 6101 The SSL Protocol Version 3.0 August 2011 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [IDEA] Lai, X., "On the Design and Security of Block Ciphers", ETH Series in Information Processing, v. 1, Konstanz: Hartung-Gorre Verlag, 1992. [PKCS1] RSA Laboratories, "PKCS #1: RSA Encryption Standard version 1.5", November 1993. [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. [RSA] Rivest, R., Shamir, A., and L. Adleman, "A Method for Obtaining Digital Signatures and Public-Key Cryptosystems", Communications of the ACM v. 21, n. 2 pp. 120-126., February 1978. [SCH] Schneier, B., "Applied Cryptography: Protocols, Algorithms, and Source Code in C", John Wiley & Sons, 1994. [SHA] NIST FIPS PUB 180-1, "Secure Hash Standard", May 1994. National Institute of Standards and Technology, U.S. Department of Commerce, DRAFT [X509] CCITT, "The Directory - Authentication Framework", Recommendation X.509 , 1988. [RSADSI] RSA Data Security, Inc., "Unpublished works". Freier, et al. Historic [Page 41] RFC 6101 The SSL Protocol Version 3.0 August 2011 Appendix A. Protocol Constant Values This section describes protocol types and constants. A.1. Record Layer struct { uint8 major, minor; } ProtocolVersion; ProtocolVersion version = { 3,0 }; enum { change_cipher_spec[20], alert[21], handshake[22], application_data[23], [255] } ContentType; struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[SSLPlaintext.length]; } SSLPlaintext; struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[SSLCompressed.length]; } SSLCompressed; struct { ContentType type; ProtocolVersion version; uint16 length; select [CipherSpec.cipher_type] { case stream: GenericStreamCipher; case block: GenericBlockCipher; } fragment; } SSLCiphertext; stream-ciphered struct { opaque content[SSLCompressed.length]; opaque MAC[CipherSpec.hash_size]; } GenericStreamCipher; block-ciphered struct { opaque content[SSLCompressed.length]; Freier, et al. Historic [Page 42] RFC 6101 The SSL Protocol Version 3.0 August 2011 opaque MAC[CipherSpec.hash_size]; uint8 padding[GenericBlockCipher.padding_length]; uint8 padding_length; } GenericBlockCipher; A.2. Change Cipher Specs Message struct { enum { change_cipher_spec[1], [255] } type; } ChangeCipherSpec; A.3. Alert Messages enum { warning[1], fatal[2], [255] } AlertLevel; enum { close_notify[0], unexpected_message[10], bad_record_mac[20], decompression_failure[30], handshake_failure[40], no_certificate[41], bad_certificate[42], unsupported_certificate[43], certificate_revoked[44], certificate_expired[45], certificate_unknown[46], illegal_parameter [47], [255] } AlertDescription; struct { AlertLevel level; AlertDescription description; } Alert; Freier, et al. Historic [Page 43] RFC 6101 The SSL Protocol Version 3.0 August 2011 A.4. Handshake Protocol enum { hello_request[0], client_hello[1], server_hello[2], certificate[11], server_key_exchange [12], certificate_request[13], server_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_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; uint8 CipherSuite[2]; enum { null[0], [255] } CompressionMethod; struct { ProtocolVersion client_version; Random random; SessionID session_id; CipherSuite cipher_suites; CompressionMethod compression_methods; Freier, et al. Historic [Page 44] RFC 6101 The SSL Protocol Version 3.0 August 2011 } ClientHello; struct { ProtocolVersion server_version; Random random; SessionID session_id; CipherSuite cipher_suite; CompressionMethod compression_method; } ServerHello; A.4.2. Server Authentication and Key Exchange Messages opaque ASN.1Cert; struct { ASN.1Cert certificate_list; } Certificate; enum { rsa, diffie_hellman, fortezza_kea } KeyExchangeAlgorithm; struct { opaque RSA_modulus; opaque RSA_exponent; } ServerRSAParams; struct { opaque DH_p; opaque DH_g; opaque DH_Ys; } ServerDHParams; struct { opaque r_s [128] } ServerFortezzaParams struct { select [KeyExchangeAlgorithm] { case diffie_hellman: ServerDHParams params; Signature signed_params; case rsa: ServerRSAParams params; Signature signed_params; case fortezza_kea: ServerFortezzaParams params; }; } ServerKeyExchange; Freier, et al. Historic [Page 45] RFC 6101 The SSL Protocol Version 3.0 August 2011 enum { anonymous, rsa, dsa } SignatureAlgorithm; digitally-signed struct { select[SignatureAlgorithm] { case anonymous: struct { }; case rsa: opaque md5_hash[16]; opaque sha_hash[20]; case dsa: opaque sha_hash[20]; }; } Signature; enum { RSA_sign[1], DSS_sign[2], RSA_fixed_DH[3], DSS_fixed_DH[4], RSA_ephemeral_DH[5], DSS_ephemeral_DH[6], FORTEZZA_MISSI[20], [255] } CertificateType; opaque DistinguishedName; struct { CertificateType certificate_types; DistinguishedName certificate_authorities; } CertificateRequest; struct { } ServerHelloDone; A.5. Client Authentication and Key Exchange Messages struct { select [KeyExchangeAlgorithm] { case rsa: EncryptedPreMasterSecret; case diffie_hellman: DiffieHellmanClientPublicValue; case fortezza_kea: FortezzaKeys; } exchange_keys; } ClientKeyExchange; struct { ProtocolVersion client_version; opaque random[46]; } PreMasterSecret; struct { public-key-encrypted PreMasterSecret pre_master_secret; } EncryptedPreMasterSecret; Freier, et al. Historic [Page 46] RFC 6101 The SSL Protocol Version 3.0 August 2011 struct { opaque y_c; opaque r_c[128]; opaque y_signature[40]; opaque wrapped_client_write_key[12]; opaque wrapped_server_write_key[12]; opaque client_write_iv[24]; opaque server_write_iv[24]; opaque master_secret_iv[24]; opaque encrypted_preMasterSecret[48]; } FortezzaKeys; enum { implicit, explicit } PublicValueEncoding; struct { select [PublicValueEncoding] { case implicit: struct {}; case explicit: opaque DH_Yc; } dh_public; } ClientDiffieHellmanPublic; struct { Signature signature; } CertificateVerify; A.5.1. Handshake Finalization Message struct { opaque md5_hash[16]; opaque sha_hash[20]; } Finished; A.6. The CipherSuite The following values define the CipherSuite codes used in the client hello and server hello messages. A CipherSuite defines a cipher specifications supported in SSL version 3.0. CipherSuite SSL_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. Freier, et al. Historic [Page 47] RFC 6101 The SSL Protocol Version 3.0 August 2011 CipherSuite SSL_RSA_WITH_NULL_MD5 = { 0x00,0x01 }; CipherSuite SSL_RSA_WITH_NULL_SHA = { 0x00,0x02 }; CipherSuite SSL_RSA_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x03 }; CipherSuite SSL_RSA_WITH_RC4_128_MD5 = { 0x00,0x04 }; CipherSuite SSL_RSA_WITH_RC4_128_SHA = { 0x00,0x05 }; CipherSuite SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x06 }; CipherSuite SSL_RSA_WITH_IDEA_CBC_SHA = { 0x00,0x07 }; CipherSuite SSL_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x08 }; CipherSuite SSL_RSA_WITH_DES_CBC_SHA = { 0x00,0x09 }; CipherSuite SSL_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0A }; The following CipherSuite definitions are used for server- authenticated [and optionally client-authenticated] Diffie-Hellman. DH denotes cipher suites in which the server's certificate contains the Diffie-Hellman parameters signed by the certificate authority [CA]. DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman parameters are signed by a DSS or RSA certificate, which has been signed by the CA. The signing algorithm used is specified after the DH or DHE parameter. In all cases, the client must have the same type of certificate, and must use the Diffie-Hellman parameters chosen by the server. CipherSuite SSL_DH_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0B }; CipherSuite SSL_DH_DSS_WITH_DES_CBC_SHA = { 0x00,0x0C }; CipherSuite SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0D }; CipherSuite SSL_DH_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0E }; CipherSuite SSL_DH_RSA_WITH_DES_CBC_SHA = { 0x00,0x0F }; CipherSuite SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x10 }; CipherSuite SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x11 }; CipherSuite SSL_DHE_DSS_WITH_DES_CBC_SHA = { 0x00,0x12 }; CipherSuite SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x13 }; CipherSuite SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x14 }; CipherSuite SSL_DHE_RSA_WITH_DES_CBC_SHA = { 0x00,0x15 }; CipherSuite SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x16 }; The following cipher suites are used for completely anonymous Diffie- Hellman communications in which neither party is authenticated. Note that this mode is vulnerable to man-in-the-middle attacks and is therefore strongly discouraged. CipherSuite SSL_DH_anon_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x17 }; CipherSuite SSL_DH_anon_WITH_RC4_128_MD5 = { 0x00,0x18 }; CipherSuite SSL_DH_anon_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x19 }; CipherSuite SSL_DH_anon_WITH_DES_CBC_SHA = { 0x00,0x1A }; CipherSuite SSL_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1B }; Freier, et al. Historic [Page 48] RFC 6101 The SSL Protocol Version 3.0 August 2011 The final cipher suites are for the FORTEZZA token. CipherSuite SSL_FORTEZZA_KEA_WITH_NULL_SHA = { 0X00,0X1C }; CipherSuite SSL_FORTEZZA_KEA_WITH_FORTEZZA_CBC_SHA = { 0x00,0x1D }; CipherSuite SSL_FORTEZZA_KEA_WITH_RC4_128_SHA = { 0x00,0x1E }; Note: All cipher suites whose first byte is 0xFF are considered private and can be used for defining local/experimental algorithms. Interoperability of such types is a local matter. A.7. The CipherSpec A cipher suite identifies a CipherSpec. These structures are part of the SSL session state. The CipherSpec includes: enum { stream, block } CipherType; enum { true, false } IsExportable; enum { null, rc4, rc2, des, 3des, des40, fortezza } BulkCipherAlgorithm; enum { null, md5, sha } MACAlgorithm; struct { BulkCipherAlgorithm bulk_cipher_algorithm; MACAlgorithm mac_algorithm; CipherType cipher_type; IsExportable is_exportable uint8 hash_size; uint8 key_material; uint8 IV_size; } CipherSpec; Freier, et al. Historic [Page 49] RFC 6101 The SSL Protocol Version 3.0 August 2011 Appendix B. Glossary application protocol: An application protocol is a protocol that normally layers directly on top of the transport layer [e.g., TCP/IP [RFC0793]/[RFC0791]]. Examples include HTTP [RFC1945], TELNET [RFC0959], FTP [RFC0854], and SMTP. asymmetric cipher: See public key cryptography. authentication: Authentication is the ability of one entity to determine the identity of another entity. block cipher: A block cipher is an algorithm that operates on plaintext in groups of bits, called blocks. 64 bits is a typical block size. bulk cipher: A symmetric encryption algorithm used to encrypt large quantities of data. cipher block chaining [CBC] mode: CBC is a mode in which every plaintext block encrypted with the block cipher is first exclusive-ORed with the previous ciphertext block [or, in the case of the first block, with the initialization vector]. certificate: As part of the X.509 protocol [a.k.a. ISO Authentication framework], certificates are assigned by a trusted certificate authority and provide verification of a party's identity and may also supply its public key. client: The application entity that initiates a connection to a server. 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 SSL, such connections are peer-to-peer relationships. The connections are transient. Every connection is associated with one session. Data Encryption Standard [DES]: DES is a very widely used symmetric encryption algorithm. DES is a block cipher [DES] [3DES]. Freier, et al. Historic [Page 50] RFC 6101 The SSL Protocol Version 3.0 August 2011 Digital Signature Standard: [DSS] A standard for digital signing, including the Digital Signature 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. 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. FORTEZZA: A PCMCIA card that provides both encryption and digital signing. handshake: An initial negotiation between client and server that establishes the parameters of their transactions. Initialization Vector [IV]: When a block cipher is used in CBC mode, the initialization vector is exclusive-ORed with the first plaintext block prior to encryption. IDEA: A 64-bit block cipher designed by Xuejia Lai and James Massey [IDEA]. Message Authentication Code [MAC]: A Message Authentication Code is a one-way hash computed from a message and some secret data. 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 [RFC1321] is a secure hashing function that converts an arbitrarily long data stream into a digest of fixed size. 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. Freier, et al. Historic [Page 51] RFC 6101 The SSL Protocol Version 3.0 August 2011 RC2, RC4: Proprietary bulk ciphers from RSA Data Security, Inc. [There is no good reference to these as they are unpublished works; however, see [RSADSI]]. RC2 is a block cipher and RC4 is a stream cipher. RSA: A very widely used public key algorithm that can be used for either encryption or digital signing. salt: Non-secret random data used to make export encryption keys resist precomputation attacks. server: The server is the application entity that responds to requests for connections from clients. The server is passive, waiting for requests from clients. session: An SSL 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, which 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 180-1. It produces a 20-byte output [SHA]. 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. Freier, et al. Historic [Page 52] RFC 6101 The SSL Protocol Version 3.0 August 2011 Appendix C. CipherSuite Definitions CipherSuite Is Key Cipher Hash Exportable Exchange SSL_NULL_WITH_NULL_NULL * NULL NULL NULL SSL_RSA_WITH_NULL_MD5 * RSA NULL MD5 SSL_RSA_WITH_NULL_SHA * RSA NULL SHA SSL_RSA_EXPORT_WITH_RC4_40_MD5 * RSA_EXPORT RC4_40 MD5 SSL_RSA_WITH_RC4_128_MD5 RSA RC4_128 MD5 SSL_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5 * RSA_EXPORT RC2_CBC_40 MD5 SSL_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA SSL_RSA_EXPORT_WITH_DES40_CBC_SHA * RSA_EXPORT DES40_CBC SHA SSL_RSA_WITH_DES_CBC_SHA RSA DES_CBC SHA SSL_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA SSL_DH_DSS_EXPORT_WITH_DES40_CBC_SHA * DH_DSS_EXPORT DES40_CBC SHA SSL_DH_DSS_WITH_DES_CBC_SHA DH_DSS DES_CBC SHA SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA SSL_DH_RSA_EXPORT_WITH_DES40_CBC_SHA * DH_RSA_EXPORT DES40_CBC SHA SSL_DH_RSA_WITH_DES_CBC_SHA DH_RSA DES_CBC SHA SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA * DHE_DSS_EXPORT DES40_CBC SHA SSL_DHE_DSS_WITH_DES_CBC_SHA DHE_DSS DES_CBC SHA SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA * DHE_RSA_EXPORT DES40_CBC SHA SSL_DHE_RSA_WITH_DES_CBC_SHA DHE_RSA DES_CBC SHA SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA DHE_RSA 3DES_EDE_CBC SHA SSL_DH_anon_EXPORT_WITH_RC4_40_MD5 * DH_anon_EXPORT RC4_40 MD5 SSL_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5 SSL_DH_anon_EXPORT_WITH_DES40_CBC_SHA DH_anon DES40_CBC SHA SSL_DH_anon_WITH_DES_CBC_SHA DH_anon DES_CBC SHA SSL_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA SSL_FORTEZZA_KEA_WITH_NULL_SHA FORTEZZA_KEA NULL SHA SSL_FORTEZZA_KEA_WITH_FORTEZZA_CBC_SHA FORTEZZA_KEA FORTEZZA_CBC SHA SSL_FORTEZZA_KEA_WITH_RC4_128_SHA FORTEZZA_KEA RC4_128 SHA Freier, et al. Historic [Page 53] RFC 6101 The SSL Protocol Version 3.0 August 2011 +----------------+------------------------------+-------------------+ | Key Exchange | Description | Key Size Limit | | Algorithm | | | +----------------+------------------------------+-------------------+ | DHE_DSS | Ephemeral DH with DSS | None | | | signatures | | | DHE_DSS_EXPORT | Ephemeral DH with DSS | DH = 512 bits | | | signatures | | | DHE_RSA | Ephemeral DH with RSA | None | | | signatures | | | DHE_RSA_EXPORT | Ephemeral DH with RSA | DH = 512 bits, | | | signatures | RSA = none | | DH_anon | Anonymous DH, no signatures | None | | DH_anon_EXPORT | Anonymous DH, no signatures | DH = 512 bits | | DH_DSS | DH with DSS-based | None | | | certificates | | | DH_DSS_EXPORT | DH with DSS-based | DH = 512 bits | | | certificates | | | DH_RSA | DH with RSA-based | None | | | certificates | | | DH_RSA_EXPORT | DH with RSA-based | DH = 512 bits, | | | certificates | RSA = none | | FORTEZZA_KEA | FORTEZZA KEA. Details | N/A | | | unpublished | | | NULL | No key exchange | N/A | | RSA | RSA key exchange | None | | RSA_EXPORT | RSA key exchange | RSA = 512 bits | +----------------+------------------------------+-------------------+ Table 1 Key size limit: The key size limit gives the size of the largest public key that can be legally used for encryption in cipher suites that are exportable. Freier, et al. Historic [Page 54] RFC 6101 The SSL Protocol Version 3.0 August 2011 +--------------+--------+-----+-------+-------+-------+------+------+ | Cipher | Cipher | IsE | Key | Exp. | Effec | IV | Bloc | | | Type | xpo | Mater | Key | tive | Size | k | | | | rta | ial | Mater | Key | | Size | | | | ble | | ial | Bits | | | +--------------+--------+-----+-------+-------+-------+------+------+ | NULL | Stream | * | 0 | 0 | 0 | 0 | N/A | | FORTEZZA_CBC | Block | | NA | 12 | 96 | 20 | 8 | | | | | [**] | [**] | [**] | [**] | | | IDEA_CBC | Block | | 16 | 16 | 128 | 8 | 8 | | RC2_CBC_40 | Block | * | 5 | 16 | 40 | 8 | 8 | | RC4_40 | Stream | * | 5 | 16 | 40 | 0 | N/A | | RC4_128 | Stream | | 16 | 16 | 128 | 0 | N/A | | DES40_CBC | Block | * | 5 | 8 | 40 | 8 | 8 | | DES_CBC | Block | | 8 | 8 | 56 | 8 | 8 | | 3DES_EDE_CBC | Block | | 24 | 24 | 168 | 8 | 8 | +--------------+--------+-----+-------+-------+-------+------+------+ * Indicates IsExportable is true. ** FORTEZZA uses its own key and IV generation algorithms. Table 2 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. Effective Key Bits: How much entropy material is in the key material being fed into the encryption routines. +---------------+-----------+--------------+ | Hash Function | Hash Size | Padding Size | +---------------+-----------+--------------+ | NULL | 0 | 0 | | MD5 | 16 | 48 | | SHA | 20 | 40 | +---------------+-----------+--------------+ Table 3 Freier, et al. Historic [Page 55] RFC 6101 The SSL Protocol Version 3.0 August 2011 Appendix D. Implementation Notes The SSL protocol cannot prevent many common security mistakes. This section provides several recommendations to assist implementers. D.1. Temporary RSA Keys US export restrictions limit RSA keys used for encryption to 512 bits, but do not place any limit on lengths of RSA keys used for signing operations. Certificates often need to be larger than 512 bits, since 512-bit RSA keys are not secure enough for high-value transactions or for applications requiring long-term security. Some certificates are also designated signing-only, in which case they cannot be used for key exchange. When the public key in the certificate cannot be used for encryption, the server signs a temporary RSA key, which is then exchanged. In exportable applications, the temporary RSA key should be the maximum allowable length [i.e., 512 bits]. Because 512-bit RSA keys are relatively insecure, they should be changed often. For typical electronic commerce applications, it is suggested that keys be changed daily or every 500 transactions, and more often if possible. Note that while it is acceptable to use the same temporary key for multiple transactions, it must be signed each time it is used. RSA key generation is a time-consuming process. In many cases, a low-priority process can be assigned the task of key generation. Whenever a new key is completed, the existing temporary key can be replaced with the new one. D.2. Random Number Generation and Seeding SSL requires a cryptographically secure pseudorandom number generator [PRNG]. Care must be taken in designing and seeding PRNGs. PRNGs based on secure hash operations, most notably MD5 and/or SHA, are acceptable, but cannot provide more security than the size of the random number generator state. [For example, MD5-based PRNGs usually provide 128 bits of state.] To estimate the amount of seed material being produced, add the number of bits of unpredictable information in each seed byte. For example, keystroke timing values taken from a PC-compatible's 18.2 Hz timer provide 1 or 2 secure bits each, even though the total size of the counter value is 16 bits or more. To seed a 128-bit PRNG, one would thus require approximately 100 such timer values. Freier, et al. Historic [Page 56] RFC 6101 The SSL Protocol Version 3.0 August 2011 Note: The seeding functions in RSAREF and versions of BSAFE prior to 3.0 are order independent. For example, if 1000 seed bits are supplied, one at a time, in 1000 separate calls to the seed function, the PRNG will end up in a state that depends only on the number of 0 or 1 seed bits in the seed data [i.e., there are 1001 possible final states]. Applications using BSAFE or RSAREF must take extra care to ensure proper seeding. D.3. 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.4. CipherSuites SSL supports a range of key sizes and security levels, including some that provide no or minimal security. A proper implementation will probably not support many cipher suites. For example, 40-bit encryption is easily broken, so implementations requiring strong security should not allow 40-bit keys. Similarly, anonymous Diffie- Hellman is strongly discouraged because it cannot prevent man-in-the- middle attacks. Applications should also enforce minimum and maximum key sizes. For example, certificate chains containing 512-bit RSA keys or signatures are not appropriate for high-security applications. D.5. FORTEZZA This section describes implementation details for cipher suites that make use of the FORTEZZA hardware encryption system. D.5.1. Notes on Use of FORTEZZA Hardware A complete explanation of all issues regarding the use of FORTEZZA hardware is outside the scope of this document. However, there are a few special requirements of SSL that deserve mention. Because SSL is a full duplex protocol, two crypto states must be maintained, one for reading and one for writing. There are also a number of circumstances that can result in the crypto state in the FORTEZZA card being lost. For these reasons, it's recommended that the current crypto state be saved after processing a record, and loaded before processing the next. Freier, et al. Historic [Page 57] RFC 6101 The SSL Protocol Version 3.0 August 2011 After the client generates the TEK, it also generates two message encryption keys [MEKs], one for reading and one for writing. After generating each of these keys, the client must generate a corresponding IV and then save the crypto state. The client also uses the TEK to generate an IV and encrypt the premaster secret. All three IVs are sent to the server, along with the wrapped keys and the encrypted premaster secret in the client key exchange message. At this point, the TEK is no longer needed, and may be discarded. On the server side, the server uses the master IV and the TEK to decrypt the premaster secret. It also loads the wrapped MEKs into the card. The server loads both IVs to verify that the IVs match the keys. However, since the card is unable to encrypt after loading an IV, the server must generate a new IV for the server write key. This IV is discarded. When encrypting the first encrypted record [and only that record], the server adds 8 bytes of random data to the beginning of the fragment. These 8 bytes are discarded by the client after decryption. The purpose of this is to synchronize the state on the client and server resulting from the different IVs. D.5.2. FORTEZZA Cipher Suites 5] FORTEZZA_NULL_WITH_NULL_SHA: Uses the full FORTEZZA key exchange, including sending server and client write keys and IVs. D.5.3. FORTEZZA Session Resumption There are two possibilities for FORTEZZA session restart: 1] Never restart a FORTEZZA session. 2] Restart a session with the previously negotiated keys and IVs. Never restarting a FORTEZZA session: Clients who never restart FORTEZZA sessions should never send session IDs that were previously used in a FORTEZZA session as part of the ClientHello. Servers who never restart FORTEZZA sessions should never send a previous session id on the ServerHello if the negotiated session is FORTEZZA. Restart a session: You cannot restart FORTEZZA on a session that has never done a complete FORTEZZA key exchange [that is, you cannot restart FORTEZZA if the session was an RSA/RC4 session renegotiated for FORTEZZA]. If you wish to restart a FORTEZZA session, you must save the MEKs and Freier, et al. Historic [Page 58] RFC 6101 The SSL Protocol Version 3.0 August 2011 IVs from the initial key exchange for this session and reuse them for any new connections on that session. This is not recommended, but it is possible. Appendix E. Version 2.0 Backward Compatibility Version 3.0 clients that support version 2.0 servers must send version 2.0 client hello messages [SSL-2]. Version 3.0 servers should accept either client hello format. The only deviations from the version 2.0 specification are the ability to specify a version with a value of three and the support for more ciphering types in the CipherSpec. Warning: The ability to send version 2.0 client hello messages will be phased out with all due haste. Implementers should make every effort to move forward as quickly as possible. Version 3.0 provides better mechanisms for transitioning to newer versions. The following cipher specifications are carryovers from SSL version 2.0. These are assumed to use RSA for key exchange and authentication. V2CipherSpec SSL_RC4_128_WITH_MD5 = { 0x01,0x00,0x80 }; V2CipherSpec SSL_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 }; V2CipherSpec SSL_RC2_CBC_128_CBC_WITH_MD5 = { 0x03,0x00,0x80 }; V2CipherSpec SSL_RC2_CBC_128_CBC_EXPORT40_WITH_MD5 = { 0x04,0x00,0x80 }; V2CipherSpec SSL_IDEA_128_CBC_WITH_MD5 = { 0x05,0x00,0x80 }; V2CipherSpec SSL_DES_64_CBC_WITH_MD5 = { 0x06,0x00,0x40 }; V2CipherSpec SSL_DES_192_EDE3_CBC_WITH_MD5 = { 0x07,0x00,0xC0 }; Cipher specifications introduced in version 3.0 can be included in version 2.0 client hello messages using the syntax below. Any V2CipherSpec element with its first byte equal to zero will be ignored by version 2.0 servers. Clients sending any of the above V2CipherSpecs should also include the version 3.0 equivalent [see Appendix A.6]: V2CipherSpec [see Version 3.0 name] = { 0x00, CipherSuite }; E.1. Version 2 Client Hello The version 2.0 client hello message is presented below using this document's presentation model. The true definition is still assumed to be the SSL version 2.0 specification. Freier, et al. Historic [Page 59] RFC 6101 The SSL Protocol Version 3.0 August 2011 uint8 V2CipherSpec[3]; struct { unit8 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]; Random challenge; } V2ClientHello; session msg_type: This field, in conjunction with the version field, identifies a version 2 client hello message. The value should equal one [1]. version: The highest version of the protocol supported by the client [equals ProtocolVersion.version; see Appendix A.1]. cipher_spec_length: This field is the total length of the field cipher_specs. It cannot be zero and must be a multiple of the V2CipherSpec length [3]. session_id_length: This field must have a value of either zero or 16. If zero, the client is creating a new session. If 16, the session_id field will contain the 16 bytes of session identification. challenge_length: The length in bytes of the client's challenge to the server to authenticate itself. This value must be 32. cipher_specs: This is a list of all CipherSpecs the client is willing and able to use. There must be at least one CipherSpec acceptable to the server. session_id: If this field's length is not zero, it will contain the identification for a session that the client wishes to resume. challenge: The client's challenge to the server for the server to identify itself is a [nearly] arbitrary length random. The version 3.0 server will right justify the challenge data to become the ClientHello.random data [padded with leading zeroes, if necessary], as specified in this version 3.0 protocol. If the length of the challenge is greater than 32 bytes, then only the last 32 bytes are used. It is legitimate [but not necessary] for a V3 server to reject a V2 ClientHello that has fewer than 16 bytes of challenge data. Freier, et al. Historic [Page 60] RFC 6101 The SSL Protocol Version 3.0 August 2011 Note: Requests to resume an SSL 3.0 session should use an SSL 3.0 client hello. E.2. Avoiding Man-in-the-Middle Version Rollback When SSL version 3.0 clients fall back to version 2.0 compatibility mode, they use special PKCS #1 block formatting. This is done so that version 3.0 servers will reject version 2.0 sessions with version 3.0-capable clients. When version 3.0 clients are in version 2.0 compatibility mode, they set the right-hand [least-significant] 8 random bytes of the PKCS padding [not including the terminal null of the padding] for the RSA encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY to 0x03 [the other padding bytes are random]. After decrypting the ENCRYPTED-KEY-DATA field, servers that support SSL 3.0 should issue an error if these eight padding bytes are 0x03. Version 2.0 servers receiving blocks padded in this manner will proceed normally. Appendix F. Security Analysis The SSL 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 SSL 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 MasterSecret, 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 SSL supports three authentication modes: authentication of both parties, server authentication with an unauthenticated client, and total anonymity. Whenever the server is authenticated, the channel should be secure against man-in-the-middle attacks, but completely anonymous sessions are inherently vulnerable to such attacks. Freier, et al. Historic [Page 61] RFC 6101 The SSL Protocol Version 3.0 August 2011 Anonymous servers cannot authenticate clients, since the client signature in the certificate verify message may require a server certificate to bind the signature to a particular server. 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 6.1]. The master_secret is required to generate the finished messages, encryption keys, and MAC secrets [see Sections 5.6.9 and 6.2.2]. 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, Diffie- Hellman, or FORTEZZA for key exchange. With anonymous RSA, the client encrypts a pre_master_secret with the server's uncertified public key extracted from the server key exchange message. The result is sent in a client key exchange message. Since eavesdroppers do not know the server's private key, it will be infeasible for them to decode the pre_master_secret. With Diffie-Hellman or FORTEZZA, 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] or the FORTEZZA token encryption key [TEK]. Warning: Completely anonymous connections only provide protection against passive eavesdropping. Unless an independent tamper-proof channel is used to verify that the finished messages were not replaced by an attacker, server authentication is required in environments where active man-in-the-middle attacks are a concern. F.1.1.2. RSA Key Exchange and Authentication With RSA, key exchange and server authentication are combined. The public key either may be contained in the server's certificate or may be a temporary RSA key sent in a server key exchange message. When temporary RSA keys are used, they are signed by the server's RSA or DSS certificate. The signature includes the current Freier, et al. Historic [Page 62] RFC 6101 The SSL Protocol Version 3.0 August 2011 ClientHello.random, so old signatures and temporary keys cannot be replayed. Servers may use a single temporary RSA key for multiple negotiation sessions. Note: The temporary RSA key option is useful if servers need large certificates but must comply with government-imposed size limits on keys used for key exchange. 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 5.6.8]. The client signs a value derived from the master_secret and all preceding handshake messages. These handshake messages include the server certificate, which binds the signature to the server, and ServerHello.random, which binds the signature to the current handshake process. F.1.1.3. Diffie-Hellman Key Exchange with Authentication When Diffie-Hellman key exchange is used, the server either can supply a certificate containing fixed Diffie-Hellman parameters or can use the server key exchange message to send a set of temporary Diffie-Hellman parameters signed with a DSS or RSA certificate. Temporary parameters are hashed with the hello.random values before signing to ensure that attackers do not replay old parameters. In either case, the client can verify the certificate or signature to ensure that the parameters belong to the server. If the client has a certificate containing fixed Diffie-Hellman parameters, its certificate contains the information required to complete the key exchange. Note that in this case, the client and server will generate the same Diffie-Hellman result [i.e., pre_master_secret] every time they communicate. To prevent the pre_master_secret from staying in memory any longer than necessary, it should be converted into the master_secret as soon as possible. Client Diffie-Hellman parameters must be compatible with those supplied by the server for the key exchange to work. If the client has a standard DSS or RSA certificate or is unauthenticated, it sends a set of temporary parameters to the server in the client key exchange message, then optionally uses a certificate verify message to authenticate itself. Freier, et al. Historic [Page 63] RFC 6101 The SSL Protocol Version 3.0 August 2011 F.1.1.4. FORTEZZA FORTEZZA's design is classified, but at the protocol level it is similar to Diffie-Hellman with fixed public values contained in certificates. The result of the key exchange process is the token encryption key [TEK], which is used to wrap data encryption keys, client write key, server write key, and master secret encryption key. The data encryption keys are not derived from the pre_master_secret because unwrapped keys are not accessible outside the token. The encrypted pre_master_secret is sent to the server in a client key exchange message. F.1.2. Version Rollback Attacks Because SSL version 3.0 includes substantial improvements over SSL version 2.0, attackers may try to make version 3.0-capable clients and servers fall back to version 2.0. This attack is occurring if [and only if] two version 3.0-capable parties use an SSL 2.0 handshake. Although the solution using non-random PKCS #1 block type 2 message padding is inelegant, it provides a reasonably secure way for version 3.0 servers to detect the attack. This solution is not secure against attackers who can brute force the key and substitute a new ENCRYPTED-KEY-DATA message containing the same key [but with normal padding] before the application specified wait threshold has expired. Parties concerned about attacks of this scale should not be using 40- bit encryption keys anyway. Altering the padding of the least significant 8 bytes of the PKCS padding does not impact security, 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 choose. Because many implementations will support 40-bit exportable encryption and some may even support null encryption or MAC algorithms, this attack is of particular concern. 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 other's finished messages. Without the master_secret, the attacker cannot repair the finished messages, so the attack will be discovered. Freier, et al. Historic [Page 64] RFC 6101 The SSL Protocol Version 3.0 August 2011 F.1.4. Resuming Sessions When a connection is established by resuming a session, new ClientHello.random and ServerHello.random values are hashed with the session's master_secret. Provided that the master_secret has not been compromised and that the secure hash operations used to produce the encryption keys and MAC secrets are secure, the connection should be secure and effectively independent from previous connections. Attackers cannot use known encryption keys or MAC secrets to compromise the master_secret without breaking the secure hash operations [which use both SHA and MD5]. Sessions cannot be resumed unless both the client and server agree. If either party suspects that the session may have been compromised, or that certificates may have expired or been revoked, it should force a full handshake. An upper limit of 24 hours is suggested for session ID lifetimes, since an attacker who obtains a master_secret may be able to impersonate the compromised party until the corresponding session ID is retired. Applications that may be run in relatively insecure environments should not write session IDs to stable storage. F.1.5. MD5 and SHA SSL uses hash functions very conservatively. Where possible, both MD5 and SHA are used in tandem to ensure that non-catastrophic flaws in one algorithm will not break the overall protocol. F.2. Protecting Application Data The master_secret is hashed with the ClientHello.random and ServerHello.random to produce unique data encryption keys and MAC secrets for each connection. FORTEZZA encryption keys are generated by the token, and are not derived from the master_secret. 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 SSL 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. Freier, et al. Historic [Page 65] RFC 6101 The SSL Protocol Version 3.0 August 2011 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. Final Notes For SSL to be able to provide a secure connection, both the client and server systems, keys, and applications must be secure. In addition, the implementation must be free of security errors. The system is only as strong as the weakest key exchange and authentication algorithm supported, and only trustworthy cryptographic functions should be used. Short public keys, 40-bit bulk encryption keys, and anonymous servers should be used with great caution. Implementations and users must be careful when deciding which certificates and certificate authorities are acceptable; a dishonest certificate authority can do tremendous damage. Appendix G. Acknowledgements G.1. Other Contributors Martin Abadi Robert Relyea Digital Equipment Corporation Netscape Communications Taher Elgamal Jim Roskind Netscape Communications Netscape Communications Anil Gangolli Micheal J. Sabin, Ph.D. Netscape Communications Consulting Engineer Kipp E.B. Hickman Tom Weinstein Netscape Communications Netscape Communications Freier, et al. Historic [Page 66] RFC 6101 The SSL Protocol Version 3.0 August 2011 G.2. Early Reviewers Robert Baldwin Clyde Monma RSA Data Security, Inc. Bellcore George Cox Eric Murray Intel Corporation Cheri Dowell Avi Rubin Sun Microsystems Bellcore Stuart Haber Don Stephenson Bellcore Sun Microsystems Burt Kaliski Joe Tardo RSA Data Security, Inc. General Magic Authors' Addresses Alan O. Freier Netscape Communications Philip Karlton Netscape Communications Paul C. Kocher Independent Consultant Freier, et al. Historic [Page 67]
Which TCP IP protocol is a secure form of HTTP that uses SSL as a sub layer for security?
Hyper Text Transfer Protocol Secure [HTTPS] is a secure form of HTTP that uses either SSL or TLS to encrypt sensitive data before it is transmitted. HTTPS uses port 443.
Which of the following is a purpose of the secure Sockets Layer SSL software?
SSL, or Secure Sockets Layer, is an encryption-based Internet security protocol. It was first developed by Netscape in 1995 for the purpose of ensuring privacy, authentication, and data integrity in Internet communications. SSL is the predecessor to the modern TLS encryption used today.
Which of the following protocols are used to secure HTTP?
Transport Layer Security [TLS] is a cryptographic protocol that is used to secure web [HTTP/HTTPS] connections.
Which Internet protocol is used to transmit encrypted data?
Hypertext transfer protocol secure [HTTPS] is the secure version of HTTP, which is the primary protocol used to send data between a web browser and a website. HTTPS is encrypted in order to increase security of data transfer.