Netscape Communications Corporation Internet Draft December 1995 (Expires 6/96) Alan O. Freier, Netscape Communications Philip Karlton, Netscape Communications Paul C. Kocher, Independent Consultant SSL Version 3.0 12/5/95 Abstract This document specifies Version 3.0 of the Secure Sockets Layer (SSL V3.0) protocol, a security protocol that provides privacy over the Internet. The protocol allows client/server applications to communicate in a way that prevents eavesdropping, tampering, or message forgery. Table of Contents 1. Status of this memo 3 2. Abstract 3 3. Introduction 3 4. Presentation language 4 4.1 Basic block size 4 4.2 Miscellaneous 4 4.3 Vectors 4 4.4 Numbers 5 4.5 Enumerateds 5 4.6 Constructed types 6 4.6.1 Variants 7 4.7 Cryptographic attributes 8 4.8 Constants 8 5. SSL protocol 8 5.1 Session and connection states 8 5.2 Record layer 10 5.2.1 Fragmentation 10 5.2.2 Record compression and decompression 11 5.2.3 Record payload protection and the CipherSpec 12 5.2.3.1 Null or standard stream cipher 12 5.2.3.2 CBC block cipher 13 5.3 Change cipher spec protocol 14 Freier, Karlton, Kocher [ Page 1] 5.4 Alert protocol 14 5.4.1 Closure alerts 15 5.4.2 Error alerts 15 5.5 Handshake protocol overview 16 5.6 Handshake protocol 17 5.6.1 Hello messages 18 5.6.1.1 Hello request 18 5.6.1.2 Client hello 19 5.6.1.3 Server hello 21 5.6.2 Server certificate 22 5.6.3 Server key exchange message 22 5.6.4 Certificate request 24 5.6.5 Server hello done 24 5.6.6 Client certificate 25 5.6.7 Client key exchange message 25 5.6.7.1 RSA encrypted premaster secret message 25 5.6.7.2 Fortezza key exchange message 26 5.6.7.3 Client Diffie-Hellman public value 27 5.6.8 Certificate verify 28 5.6.9 Finished 28 5.7 Application data protocol 29 6. Cryptographic computations 29 6.1 Asymmetric cryptographic computations 29 6.1.1 RSA 29 6.1.2 Diffie-Hellman 30 6.1.3 Fortezza 30 6.2 Symmetric cryptographic calculations and the CipherSpec 30 6.2.1 The master secret 30 6.2.2 Converting master secret into keys and MAC secrets 30 6.2.2.1 Export key generation example 32 A. Protocol constant values 33 A.1 Reserved port assignments 33 A.1.1 Record layer 33 A.2 Change cipher specs message 34 A.3 Alert messages 34 A.4 Handshake protocol 34 A.4.1 Hello messages 35 A.4.2 Server authentication and key exchange messages 35 A.5 Client authentication and key exchange messages 37 A.5.1 Handshake finalization message 37 A.6 The CipherSuite 38 A.7 The CipherSpec 39 B. Glossary 40 C. Version 2.0 Backward Compatibility 41 C.1 Version 2 client hello 42 D. Security analysis 44 D.1 Handshake protocol 44 D.1.1 Authentication and key exchange 44 D.1.1.1 Anonymous key exchange 45 D.1.1.2 RSA key exchange and authentication 45 D.1.1.3 Diffie-Hellman key exchange with authentication 46 D.1.1.4 Fortezza 46 D.1.2 Version rollback attacks 47 D.1.3 Detecting attacks against the handshake protocol 47 D.1.4 Resuming sessions 47 D.1.5 MD5 and SHA 48 D.2 Protecting application data 48 D.3 Final notes 49 E. Patent Statement 49 Freier, Karlton, Kocher [ Page 2 ] SSL 3.0 December 1995 1. Status of this memo This document is an Internet-Draft. 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 made obsolete 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. To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). 2. Abstract This document specifies Version 3.0 of the Secure Sockets Layer (SSL V3.0) protocol, a security protocol that provides privacy over the Internet. The protocol allows client/server applications to communicate in a way that prevents eavesdropping, tampering, or message forgery. 3. Introduction The 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[TCP]), 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. The 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], RC4[RC4], etc.) o The connection can be authenticated using asymmetric, or public key, cryptography (e.g., RSA[RSA], DSS[DSS], etc.). o The connection is reliable. Message transport includes a message integrity check using a keyed MAC. Secure hash Freier, Karlton, Kocher [ Page 3 ] SSL 3.0 December 1995 functions (e.g., SHA, MD5, etc.) are used for MAC computations. 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 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 the 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 (italicized) [ ] brackets. Single byte entities containing uninterpreted data are of type opaque. 4.3 Vectors A vector (single dimensioned array) is a stream of homogeneous data elements. The size of the vector may be specified at documentation time or left unspecified until runtime. In either case the length declares the number of bytes, not the number of elements, in the vector. The syntax for specifying a new type T' that is a fixed length vector of type T is T T'[n]; Here T' occupies n bytes in the data stream, where n is a multiple of the size of T. The length of the vector is not Freier, Karlton, Kocher [ Page 4 ] SSL 3.0 December 1995 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 . When 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'; In the following example, mandatory is a vector that must contain between 300 and 400 elements 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. 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 vectors 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]; 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 Freier, Karlton, Kocher [ Page 5 ] SSL 3.0 December 1995 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 example above, 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, but legal* Color color = blue; /*correct, type is implicit*/ For enumerateds that are never converted to external representation, the numerical information may be omitted. enum { low, medium, high } Amount; 4.6 Constructed types Structure types may be constructed from primitive types for convenience. Each specification declares a new, unique type. The syntax for definition is much like that of C. struct { T1 f1; T2 f2; ... Tn f3; } [T]; The fields within a structure may be qualified using the type's name using 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. Freier, Karlton, Kocher [ Page 6 ] SSL 3.0 December 1995 4.6.1 Variants Defined structures may have variants based on some knowledge that is available within the environment. The selector must be an enumerated type that defines the possible variants the structure defines. There must be a case arm for every element of the enumeration declared in the select. The body of the variant structure may be given a label for reference. The mechanism by which the variant is selected at runtime is not prescribed by the presentation language. struct { T1 f1; T2 f2; .... Tn fn; select (E) { case e1: Te1; case e2: Te2; .... case en: Ten; } [fv]; } [Tv]; For example enum { apple, orange } VariantTag; struct { uint16 number; opaque string<0..10>; /* variable length */ } V1; struct { uint32 number; opaque string[10]; /* fixed length */ } V2; struct { select (VariantTag) { case apple: V1; case orange: V2; } variant_body; } VariantRecord; Variant structures may be qualified (narrowed) by specifying a value for the selector prior to the type. For example, a orange VariantRecord is a narrowed type of a VariantRecord containing a variant_body of type V2. Freier, Karlton, Kocher [ Page 7 ] SSL 3.0 December 1995 4.7 Cryptographic attributes The four cryptographic operations digital signing, stream cipher encryption, block cipher encryption, and public key encryption are designated digitally-signed, stream-ciphered, block-ciphered, and public-key-encrypted, respectively. A field's cryptographic processing is specified by prepending an appropriate key word designation before the field's type specification. Cryptographic keys are implied by the current session state (see Section 5.1). In the following example: stream-ciphered struct { uint8 field1; uint8 field2; digitally-signed opaque field3[20]; } UserType; The contents of field3 are signed, then the entire structure is encrypted with a stream cipher. 4.8 Constants Typed constants can be defined for purposes of specification by declaring a symbol of the desired type and assigning values to it. Under-specified types (opaque, variable length vectors, and structures that contain opaque) cannot be assigned values. No fields of a multi-element structure or vector may be elided. For example, struct { uint8 f1; uint8 f2; } Example1; Example1 ex1 = {1, 4}; /*assigns f1 = 1, f2 = 4*/ 5. SSL protocol SSL is a layered protocol. At each layer, messages may include fields for length, description, and content. SSL 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, and reassembled, then delivered to higher level clients. 5.1 Session and connection states An SSL session is stateful. It is the responsibility of the SSL Handshake protocol to coordinate the states of the client and server, thereby allowing the protocol state Freier, Karlton, Kocher [ Page 8 ] SSL 3.0 December 1995 machines of each to operate consistently, despite the fact that the state is not exactly parallel. Logically the state is represented twice, once as the current operating state, and (during the handshake protocol) again as the pending state. When the handshake negotiation is complete, the client and server exchange change cipher spec messages (see Section 5.3). At that time, the pending state is copied into the operating state. State information used only during the handshake protocol is not listed. An SSL session may include multiple secure connections; in addition, parties may have multiple simultaneous sessions. The session state includes the following elements: session identifier An arbitrary byte sequence chosen by the server to identify an active or resumable session state. peer certificate X509.v3[X509] certificate of the peer. This element of the state may be null. compression method The algorithm used to compress data prior to encryption. cipher spec Specifies the bulk data encryption algorithm (such as null, DES, etc.) and a MAC algorithm (such as MD5 or SHA). It also defines cryptographic attributes such as the hash_size. 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. The connection state includes the following elements: server and client random Byte sequences that are chosen by the server and client for each connection. server write MAC secret The secret used in MAC operations on data written by the server. client write MAC secret The secret used in MAC operations on data written by the client. server write key The bulk cipher key for data encrypted Freier, Karlton, Kocher [ Page 9 ] SSL 3.0 December 1995 by the server and decrypted by the client. client write key The bulk cipher key for data encrypted by the client and decrypted by the server. initialization vectors When a block cipher in CBC mode is used, an initialization vector (IV) is maintained for each key. This field is first initialized by the SSL handshake protocol. Thereafter the final ciphertext block from each record is preserved for use with the following record. sequence numbers Each party maintains separate sequence numbers for transmitted and received messages for each connection. When a party sends or receives a change cipher specs message, the appropriate sequence number is set to zero. Sequence numbers are of type uint64 and may not exceed 2^64-1. 5.2 Record layer The SSL Record Layer receives uninterpreted data from clients in non-empty blocks of arbitrary size. 5.2.1 Fragmentation The record layer fragments client blocks into SSLPlaintext records 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 SSLPlaintext record). struct { uint8 major, minor; } ProtocolVersion; enum { change_cipher_spec(20), alert(21), handshake(22), application_data(23), (255) } ContentType; struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[SSLPlaintext.length]; } SSLPlaintext; Freier, Karlton, Kocher [ Page 10 ] SSL 3.0 December 1995 type The higher level protocol used to process the enclosed fragment. version The version of protocol being employed. This document describes SSL Version 3.0 (See Appendix A.1.1). length The length (in bytes) of the following SSLPlaintext.fragment. The length should 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. Note: Data of different SSL Record layer content types may be interleaved. Application data is generally of lower precedence for transmission than other content types. 5.2.2 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 an SSLPlaintext structure into an SSLCompressed structure. Compression functions erase their state information whenever the CipherSpec is replaced. Note: The CipherSpec is part of the session state described in Section 5.1. References to fields of the CipherSpec are made throughout this document using presentation syntax. A more complete description of the CipherSpec is shown in Appendix A.7. Compression must be lossless and may not increase the content length by more than 1024 bytes. If the decompression function encounters an SSLCompressed.fragment that would decompress to a length in excess of 2^14 bytes, it should issue a fatal decompression_failure alert (Section 5.4.2). struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[SSLCompressed.length]; } SSLCompressed; length The length (in bytes) of the following Freier, Karlton, Kocher [ Page 11 ] SSL 3.0 December 1995 SSLCompressed.fragment. The length should not exceed 2^14 + 1024. fragment The compressed form of SSLPlaintext.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. 5.2.3 Record payload protection and the CipherSpec All records are protected using the encryption and MAC algorithms defined in the current CipherSpec. There is always an active CipherSpec, however initially it is SSL_NULL_WITH_NULL, which does not provide any security. Once the handshake is complete, the two parties have shared secrets, which are used to encrypt records and compute keyed message authenticity checks (MACs) on their contents. The techniques used to perform the encryption and MAC operations are defined by the CipherSpec and constrained by CipherSpec.cipher_type. The encryption and MAC functions translate an SSLCompressed structure into an SSLCiphertext. The decryption functions reverse the process. struct { ContentType type; ProtocolVersion version; uint16 length; select (CipherSpec.cipher_type) { case stream: GenericStreamCipher; case block: GenericBlockCipher; } fragment; } SSLCiphertext; type The type field is identical to SSLCompressed.type. version The version field is identical to SSLCompressed.version. length The length (in bytes) of the following SSLCiphertext.fragment. The length may not exceed 2^14 + 2048. fragment The encrypted form of SSLCompressed.fragment, including the MAC. 5.2.3.1 Null or standard stream cipher Freier, Karlton, Kocher [ Page 12 ] SSL 3.0 December 1995 Stream ciphers (including BulkCipherAlgorithm.null) convert SSLCompressed.fragment structures to and from stream SSLCiphertext.fragment structures. stream-ciphered struct { opaque content[SSLCompressed.length]; opaque MAC[CipherSpec.hash_size]; } GenericStreamCipher; The MAC is generated as: hash(MAC secret + hash MAC secret + write sequence number + SSLCompressed.type + SSLCompressed.length + SSLCompressed.fragment)); where '+' denotes concatenation. Note that the MAC is computed before encryption. The stream cipher encrypts the entire block, including the MAC. For stream ciphers that do not use a synchronization vector (such as RC4), the stream cipher state from the end of one record is simply used on the subsequent packet. If the CipherSuite is SSL_NULL_WITH_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). SSLCiphertext.length is SSLCompressed.length plus CipherSpec.hash_size. 5.2.3.2 CBC block cipher For block ciphers (such as RC2 or DES), the encryption and MAC functions convert SSLCompressed.fragment structures to and from block SSLCiphertext.fragment structures. block-ciphered struct { opaque content[SSLCompressed.length]; opaque MAC[CipherSpec.hash_size]; uint8 padding[GenericBlockCipher.padding_length]; uint8 padding_length; } GenericBlockCipher; The MAC is generated as described in Section 5.2.3.1. padding Padding that is added to force the length of the plaintext to be a multiple of the block cipher's block length. padding_length The length of the padding must be less than the cipher's block length and may be zero. The padding length should be such that the total size of the GenericBlockCipher structure is a Freier, Karlton, Kocher [ Page 13 ] SSL 3.0 December 1995 multiple of the cipher's block length. The encrypted data length (SSLCiphertext.length) is one more than the sum of SSLCompressed.length, CipherSpec.hash_size, and padding_length. Note: With CBC block chaining the initialization vector (IV) for the first record is provided by the handshake protocol. The IV for subsequent records is the last ciphertext block from the previous record. 5.3 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) CipherSpec. The message consists of a single byte of value 1. struct { enum { change_cipher_spec(1), (255) } type; } ChangeCipherSpec; The change cipher spec message is sent by both the client and server to notify the receiving party that subsequent records will be protected under the just-negotiated CipherSpec and keys. Reception of this message causes the receiver to copy the pending state into the current state. The client sends a change cipher spec message immediately following a handshake key exchange and certificate verify (if any) messages, and the server sends one after successfully processing the key exchange message it received from the client. An unexpected change cipher spec message should generate an unexpected_message alert (Section 5.4.2). 5.4 Alert protocol One of the content types supported by the SSL 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), Freier, Karlton, Kocher [ Page 14 ] SSL 3.0 December 1995 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), (255) } AlertDescription; struct { AlertLevel level; AlertDescription description; } Alert; 5.4.1 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. The session becomes unresumable if any connection is terminated without proper close_notify messages with level equal to warning. 5.4.2 Error alerts Error handling in the SSL 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 an unrecoverable error alert message, both parties immediately close the connection. Servers and clients are required to forget any session-identifiers, keys, and secrets associated with a failed 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 MAC. 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. Freier, Karlton, Kocher [ Page 15 ] SSL 3.0 December 1995 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 A no_certificate alert message may be sent in response to a certification request if no appropriate certificate is available. 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. 5.5 Handshake protocol overview The cryptographic parameters of the session state are produced by the SSL Handshake Protocol, a client of the Record Layer. When a 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. These processes are performed in the handshake protocol, which can consist of the following messages: Client Server ClientHello --------> ServerHello Certificate* ServerKeyExchange* CertificateRequest* <-------- ServerHelloDone Certificate* ClientKeyExchange Freier, Karlton, Kocher [ Page 16 ] SSL 3.0 December 1995 CertificateVerify* ChangeCipherSpec [Begin new CipherSpec] Finished --------> ChangeCipherSpec [Begin new CipherSpec] <-------- Finished Application Data <-------> Application Data Notes: Messages marked with an asterisk (*) are not always sent. To help avoid pipeline stalls, ChangeCipherSpec is an independent SSL Protocol content type, and is not actually an SSL 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: Client Server ClientHello --------> ServerHello ChangeCipherSpec [Resume CipherSpec] <-------- Finished ChangeCipherSpec [Resume CipherSpec] Finished --------> Application Data <-------> Application Data The contents and significance of each message will be presented in the subsequent 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_done(14), certificate_verify(15), client_key_exchange(16), finished(20), (255) Freier, Karlton, Kocher [ Page 17 ] SSL 3.0 December 1995 } 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; 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: Freier, Karlton, Kocher [ Page 18 ] SSL 3.0 December 1995 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. 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 identifer 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<0..32>; Warning: Servers must not place confidential information in session identifers or let the contents of fake session identifers 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 Freier, Karlton, Kocher [ Page 19 ] SSL 3.0 December 1995 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: Compression methods to support is under investigation. The structure of the client hello is as follows. struct { ProtocolVersion client_version; Random random; SessionID session_id; CipherSuite cipher_suites<2..2^216-1>; CompressionMethod compression_methods<1..2^8-1>; } 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 (See Appendix C 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 Freier, Karlton, Kocher [ Page 20 ] SSL 3.0 December 1995 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. The security of transmitted application data is unknown until a valid finished message has been received. 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 C 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 Freier, Karlton, Kocher [ Page 21 ] SSL 3.0 December 1995 must proceed directly to the finished messages. Otherwise this field will contain a different value identifying the new session, or 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. 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 agreement algorithm, and is generally an X.509.v3 certificate (or a modified X.509 certificate in the case of Fortezza [FOR]). The same message type will be used for the client's response to a server certificate request. opaque ASN.1Cert<2^24-1>; struct { ASN.1Cert certificate_list<1..2^24-1>; } Certificate; certificate_list This is a sequence (chain) of X.509.v3 certificates, ordered with the sender's certificate first and the root certificate authority last. 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 ServerKeyExchange message is sent by the server if it has no certificate or has a certificate only used for Freier, Karlton, Kocher [ Page 22 ] SSL 3.0 December 1995 signing (e.g., DSS [DSS] certificates, signing-only RSA [RSA] certificates) This message is not used if the server certificate contains Diffie-Hellman [DH1] parameters. Note: RSA moduli larger than 512 bits may not be used for key exchange in export software, but with this message larger RSA keys may be used as signature-only certificates to sign temporary shorter RSA keys for key exchange. enum { anonymous, certified } ServerIdentity; enum { rsa, diffie_hellman, fortezza } KeyExchangeAlgorithm; struct { opaque rsa_modulus<1..2^16-1>; opaque rsa_exponent<1..2^16-1>; } ServerRSAParams; rsa_modulus The modulus of the server's temporary RSA key. rsa_exponent The public exponent of the server's temporary RSA key. struct { opaque dh_p<1..2^16-1>; opaque dh_g<1..2^16-1>; opaque dh_Ys<1..2^16-1>; } ServerDHParams; /* Ephemeral DH parameters */ dh_p The prime modulus used for the Diffie- Hellman operation. dh_g The generator used for the Diffie- Hellman operation. dh_Ys The server's Diffie-Hellman public value (gX mod p). struct { select (KeyExchangeAlgorithm) { case diffie_hellman: ServerDHParams; case rsa: ServerRSAParams; case fortezza: struct { }; /*will not occur*/ } } ServerParams; struct { ServerParams params; select (ServerIdentity) { case anonymous: struct { }; case certified: digitally-signed struct { opaque md5_hash[16]; Freier, Karlton, Kocher [ Page 23 ] SSL 3.0 December 1995 opaque sha_hash[20]; }; }; } ServerKeyExchange; params The server's key exchange parameters. md5_hash MD5(ClientHello.random + ServerHello.random + ServerParams); sha_hash SHA(ClientHello.random + ServerHello.random + ServerParams); 5.6.4 Certificate request A non-anonymous server can optionally request a certificate from the client, if appropriate for the selected cipher suite. opaque CertificateAuthority <0..2^24-1>; enum { rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), rsa_ephemeral_dh(5), dss_ephemeral_dh(6), fortezza(20), (255) } ClientCertificateType; opaque DistinquishedName<1..2^8-1>; struct { ClientCertificateType certificate_types<1..2^8-1>; DistinquishedName certificate_authorities<3..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_authortie A list of the distinguished names of acceptable certificate authorities. Note: DistinquishedName is derived from [X509]. Note: It is a fatal handshake_failure alert for an anonymous server to request client identification. 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 Freier, Karlton, Kocher [ Page 24 ] SSL 3.0 December 1995 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 client certificate alert instead. This error is only a warning, however the server may respond with a fatal handshake failure if client authentication is required. Client certificates are sent using the Certificate message type 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. struct { select (KeyExchangeAlgorithm) { case rsa: EncryptedPreMasterSecret; case diffie_hellman: ClientDiffieHellmanPublic; case fortezza: FortezzaKeys; } exchange_keys; } ClientKeyExchange; The information to select the appropriate record structure is in the pending session state (see Section 5.1). 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 pre-master 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]; Freier, Karlton, Kocher [ Page 25 ] SSL 3.0 December 1995 } 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 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 also generates a master secret key, wraps it using the TEK, and sends it to the server. The client generates IV's for the session keys and master secret key and sends them also. The client generates a random 48-byte master secret, encrypts it using the master secret key, and sends the result: struct { opaque y_c<0..128>; opaque r_c[128]; opaque wrapped_client_write_key[12]; opaque wrapped_server_write_key[12]; opaque wrapped_master_secret_key[12]; opaque client_write_iv[24]; opaque server_write_iv[24]; opaque master_secret_iv[24]; EncryptedPreMasterSecret encrypted_pre_master_secret[48]; } FortezzaKeys; y_c The client's Yc value (public key) for the KEA calculation. If the client sent a certificate, this value must be empty since the certificate already contains this value. However, for anonymous Fortezza, this value is required and must be between 64 and 128 bytes, Freier, Karlton, Kocher [ Page 26 ] SSL 3.0 December 1995 inclusively. 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. wrapped_master_secret_key This is the master secret key, wrapped by the TEK. This key is only used for the encryption of the pre_master_secret. client_write_iv This is the IV for the client write key. server_write_iv This is the IV for the server write key. master_secret_iv This is the IV for the master secret key. encrypted_pre_master_secret This is a random value, generated by the client, and encrypted using the master secret key. It is used to generate the master secret, as specified in Section 6.1. Note: The server's R value for KEA is the integer 1. It is fixed and not sent in any message. 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 { }; Freier, Karlton, Kocher [ Page 27 ] SSL 3.0 December 1995 case explicit: opaque dh_Yc<1..2^16-1>; } 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). digitally-signed struct { opaque md5_hash[16]; opaque sha_hash[20]; } CertificateVerify; md5_hash MD5(master_secret + SHA(handshake_messages + master_secret)); sha_hash SHA(master_secret + MD5(handshake_messages + master_secret)); Here handshake_messages refers to all handshake messages starting at client hello up to but not including this message. 5.6.9 Finished A finished message is always sent immediately after a change cipher specs 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 confidential 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 + SHA(handshake_messages + Sender + master_secret)); Freier, Karlton, Kocher [ Page 28 ] SSL 3.0 December 1995 sha_hash SHA(master_secret + MD5(handshake_messages + Sender + master_secret)); The hash containd in finished messages sent by the server incorporate Sender.server; those sent by the client incoporate Sender.client. The value handshake_messages includes to all handshake messages starting at client hello up to but not including the finished messages. 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. 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 Freier, Karlton, Kocher [ Page 29 ] SSL 3.0 December 1995 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. 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 an additional key and an IV reserved for this task. 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 default to SSL_NULL_WITH_NULL at the beginning of the SSL Handshake Protocol, indicating that no 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 master secret into keys and MAC secrets Freier, Karlton, Kocher [ Page 30 ] SSL 3.0 December 1995 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 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(master_secret + ServerHello.random + ClientHello.random + 'A')) + MD5(master_secret + SHA(master_secret + ServerHello.random + ClientHello.random + 'BB')) + MD5(master_secret + SHA(master_secret + ServerHello.random + ClientHello.random + 'CCC')) + [...]; 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_size] server_write_key[CipherSPec.key_size] client_write_IV[CipherSpec.IV_size] server_write_IV[CipherSpec.IV_size] 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); Exportable encryption algorithms derive their IVs from the Freier, Karlton, Kocher [ Page 31 ] SSL 3.0 December 1995 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_WITH_RC2_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; Freier, Karlton, Kocher [ Page 32 ] SSL 3.0 December 1995 Appendix A A. Protocol constant values This section describes protocol types and constants. A.1 Reserved port assignments At the present time SSL is implemented using TCP/IP as the base networking technology. The IANA reserved the following Internet Protocol [IP] port numbers for use in conjunction with SSL. 443 Reserved for use by Hypertext Transfer Protocol with SSL (https). 465 Reserved (pending) for use by Simple Mail Transfer Protocol with SSL (ssmtp). 563 Reserved (pending) for use by Network News Transfer Protocol (snntp). A.1.1 Record layer struct { uint8 major, minor; } ProtocolVersion; ProtocolVersion version = { 3,0 }; /*SSL 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; Freier, Karlton, Kocher [ Page 33 ] SSL 3.0 December 1995 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]; 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), (255) } AlertDescription; struct { AlertLevel level; AlertDescription description; } Alert; A.4 Handshake protocol One of the content types predefined to be carried by the SSL Record Protocol is the 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; Freier, Karlton, Kocher [ Page 34 ] SSL 3.0 December 1995 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[2^8]; } Random; opaque SessionID<0..32>; uint8 CipherSuite[2]; /* Cryptographic suite selector */ enum { null(0), (255) } CompressionMethod; struct { ProtocolVersion client_version; Random random; SessionID session_id; CipherSuite cipher_suites<0..2^16-1>; CompressionMethod compression_methods<0..2^8-1>; } ClientHello; struct { ProtocolVersion server_version; Random random; SessionID session_id; CipherSuite cipher_suite; CompressionMethod compression_method; } ServerHello; A.4.2 Server authentication and key exchange messages opaque ASN.1Cert<2^24-1>; struct { ASN.1Cert certificate_list<1..2^24-1>; Freier, Karlton, Kocher [ Page 35 ] SSL 3.0 December 1995 } Certificate; enum { anonymous, certified } ServerIdentity; enum { rsa, diffie_hellman, fortezza } KeyExchangeAlgorithm; struct { opaque RSA_modulus<1..2^16-1>; opaque RSA_exponent<1..2^16-1>; } ServerRSAParams; RSA_modulus The modulus of the servers temporary RSA key. RSA_exponent The public exponent of the servers temporary RSA key. struct { opaque DH_p<1..2^16-1>; opaque DH_g<1..2^16-1>; opaque DH_Ys<1..2^16-1>; } ServerDHParams; struct select (KeyExchangeAlgorithm) { case diffie_hellman: ServerDHParams; case rsa: ServerRSAParams; case fortezza: { }; /* illegal, will not occur */ } } ServerParams; struct { ServerParams params; select (ServerIdentity) { case anonymous: empty; case certified: digitally-signed struct { uint8 MD5_hash[16]; uint8 SHA_hash[20]; }; }; } ServerKeyExchange; opaque CertificateAuthority <0..224-1>; enum { RSA_sign(1), DSS_sign(2), RSA_fixed_DH(3), DSS_fixed_DH(4), RSA_ephemeral_DH(5), DSS_ephemeral_DH(6) Fortezza(20), (255) } CertificateType; opaque DistinquishedName<1..2^8-1>; struct { CertificateType certificate_types<1..2^8-1>; DistinquishedName certificate_authorities<3..2^16-1>; } CertificateRequest; Freier, Karlton, Kocher [ Page 36 ] SSL 3.0 December 1995 struct { } ServerHelloDone; A.5 Client authentication and key exchange messages struct { select (KeyExchangeAlgorithm) { case rsa: EncryptedPreMasterSecret; case diffie_hellman: DiffieHellmanClientPublicValue; case fortezza: FortezzaKeys; } exchange_keys; } ClientKeyExchange; struct { ProtocolVersion client_version; opaque random[46]; } PreMasterSecret; struct { public-key-encrypted PreMasterSecret pre_master_secret; } EncryptedPreMasterSecret; struct { opaque y_c<0..128>; opaque r_c[128]; opaque wrapped_client_write_key[12]; opaque wrapped_server_write_key[12]; opaque wrapped_master_secret_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<1..2^16-1>; } dh_public; } ClientDiffieHellmanPublic; digitally-signed struct { uint8 MD5_hash[16]; uint8 SHA_hash[20]; } CertificateVerify; A.5.1 Handshake finalization message struct { opaque md5_hash[16]; opaque sha_hash[20]; } Finished; Freier, Karlton, Kocher [ Page 37 ] SSL 3.0 December 1995 A.6 The CipherSuite The following values define the CipherKind 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. CipherSuite SSL_RSA_WITH_NULL_MD5 = { 0x00,0x01 }; CipherSuite SSL_RSA_WITH_NULL_SHA = { 0x00,0x02 }; CipherSuite SSL_RSA_WITH_RC4_40_MD5 = { 0x01,0x01 }; CipherSuite SSL_RSA_WITH_RC4_40_SHA = { 0x01,0x02 }; CipherSuite SSL_RSA_WITH_RC4_128_MD5 = { 0x01,0x03 }; CipherSuite SSL_RSA_WITH_RC4_128_SHA = { 0x01,0x04 }; CipherSuite SSL_RSA_WITH_RC2_40_MD5 = { 0x01,0x05 }; CipherSuite SSL_RSA_WITH_RC2_40_SHA = { 0x01,0x06 }; CipherSuite SSL_RSA_WITH_RC2_128_MD5 = { 0x01,0x07 }; CipherSuite SSL_RSA_WITH_RC2_128_SHA = { 0x01,0x08 }; CipherSuite SSL_RSA_WITH_IDEA_CBC_MD5= { 0x01,0x09 }; CipherSuite SSL_RSA_WITH_IDEA_CBC_SHA= { 0x01,0x0A }; CipherSuite SSL_RSA_WITH_DES40_CBC_MD5= { 0x01,0x0B }; CipherSuite SSL_RSA_WITH_DES40_CBC_SHA= { 0x01,0x0C }; CipherSuite SSL_RSA_WITH_DES_CBC_MD5 = { 0x01,0x0D }; CipherSuite SSL_RSA_WITH_DES_CBC_SHA = { 0x01,0x0E }; CipherSuite SSL_RSA_WITH_3DES_EDE_CBC_MD5= { 0x01,0x0F }; CipherSuite SSL_RSA_WITH_3DES_EDE_CBC_SHA= { 0x01,0x10 }; The following CipherSuite definitions are used for server- authenticated (and optionally client-authenticated) Diffie- Hellman. "DH" denotes cipher suites in which the servers 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_WITH_DES40_CBC_SHA = { 0x02,0x01 }; CipherSuite SSL_DH_DSS_WITH_DES_CBC_SHA = { 0x02,0x02 }; CipherSuite SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA= { 0x02,0x03 }; CipherSuite SSL_DH_RSA_WITH_DES40_CBC_SHA = { 0x02,0x04 }; CipherSuite SSL_DH_RSA_WITH_DES_CBC_SHA = { 0x02,0x05 }; CipherSuite SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA= { 0x02,0x06 }; Freier, Karlton, Kocher [ Page 38 ] SSL 3.0 December 1995 CipherSuite SSL_DHE_DSS_WITH_DES40_CBC_SHA = { 0x03,0x01 }; CipherSuite SSL_DHE_DSS_WITH_DES_CBC_SHA = { 0x03,0x02 }; CipherSuite SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA= { 0x03,0x03 }; CipherSuite SSL_DHE_RSA_WITH_DES40_CBC_SHA = { 0x03,0x04 }; CipherSuite SSL_DHE_RSA_WITH_DES_CBC_SHA = { 0x03,0x05 }; CipherSuite SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA= { 0x03,0x06 }; 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_WITH_RC4_40_MD5 = { 0x04,0x01 }; CipherSuite SSL_DH_anon_WITH_RC4_40_SHA = { 0x04,0x02 }; CipherSuite SSL_DH_anon_WITH_RC4_128_MD5 = { 0x04,0x03 }; CipherSuite SSL_DH_anon_WITH_RC4_128_SHA = { 0x04,0x04 }; CipherSuite SSL_DH_anon_WITH_RC2_40_MD5 = { 0x04,0x05 }; CipherSuite SSL_DH_anon_WITH_RC2_40_SHA = { 0x04,0x06 }; CipherSuite SSL_DH_anon_WITH_RC2_128_MD5 = { 0x04,0x07 }; CipherSuite SSL_DH_anon_WITH_RC2_128_SHA = { 0x04,0x08 }; CipherSuite SSL_DH_anon_WITH_IDEA_CBC_MD5 = { 0x04,0x09 }; CipherSuite SSL_DH_anon_WITH_IDEA_CBC_SHA = { 0x04,0x0A }; CipherSuite SSL_DH_anon_WITH_DES40_CBC_MD5= { 0x04,0x0B }; CipherSuite SSL_DH_anon_WITH_DES40_CBC_SHA= { 0x04,0x0C }; CipherSuite SSL_DH_anon_WITH_DES_CBC_MD5 = { 0x04,0x0D }; CipherSuite SSL_DH_anon_WITH_DES_CBC_SHA = { 0x04,0x0E }; CipherSuite SSL_DH_anon_WITH_3DES_EDE_CBC_MD5={ 0x04,0x0F }; CipherSuite SSL_DH_anon_WITH_3DES_EDE_CBC_SHA={ 0x04,0x10 }; The final cipher suite is for the Fortezza token. CipherSuite SSL_FORTEZZA_WITH_FORTEZZA_CBC_SHA { 0x10,0x00 }; Note: Any cipher types 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 defines 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; Freier, Karlton, Kocher [ Page 39 ] SSL 3.0 December 1995 struct { BulkCipherAlgorithm bulk_cipher_algorithm; MACAlgorithm mac_algorithm; CipherType cipher_type; IsExportable is_exportable uint8 hash_size; uint8 key_size; uint8 IV_size; opaque IV[CipherSpec.IV_size]; } CipherSpec; Appendix B B. Glossary application protocol An application protocol is a protocol that normally layers directly on top of the transport layer (e.g., TCP/IP). Examples include HTTP, TELNET, FTP, and SMTP. asymmetric cipher See public key cryptography. authentication Authentication is the ability of one entity to determine the identity of another entity. bulk cipher A symmetric encryption algorithm used to encrypt large quantities of data. 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. MAC A Message Authentication Code (MAC) 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 [7] 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 Freier, Karlton, Kocher [ Page 40 ] SSL 3.0 December 1995 encrypted with the public key can only be decrypted with the associated private key. Conversely, messages signed with the private key canbe verified with the public key. 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 block cipher and RC4 is a stream cipher. salt Non-secret random data used to make export encryption keys resist precomputation attacks. secure hash function A one-way transformation that converts an arbitrary amount of data into a fixed length hash. It is hard to reverse the transformation or to find collisions. MD5 and SHA are examples of secure hash functions. 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 A 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]. symmetric cipher See bulk cipher. Appendix C C. Version 2.0 Backward Compatibility Version 3.0 clients that support Version 2.0 servers must Freier, Karlton, Kocher [ Page 41 ] SSL 3.0 December 1995 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. Implementors 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_128_CBC_WITH_MD5 = { 0x03,0x00,0x80 }; V2CipherSpec SSL_RC2_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 }; C.1 Version 2 client hello The Version 2.0 client hello message is presented below using this documents presentation model. The true definition is still assumed to be the SSL Version 2.0 specification. uint8 V2CipherSpec[3]; struct { unit8 msg_type; Version version; Freier, Karlton, Kocher [ Page 42 ] SSL 3.0 December 1995 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; 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 highers version of protocol supported by the client (equals ProtocolVersion.version, see Appendix A.1.1). 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 clients 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. The CipherSpecs are given first. There must be at least one CipherSpec acceptable to the server. session_id If this fields length is not zero, it will contain the identification for a session that the client wishes to resume. challenge The clients challenge to the server for the server to identify itself. The Version 3.0 server will use the challenge data as the client random data as specified in this Version 3.0 protocol. Note: Requests to resume an SSL 3.0 session should Freier, Karlton, Kocher [ Page 43 ] SSL 3.0 December 1995 use an SSL 3.0 client hello. C.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 #7 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 bytes of the random PKCS 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 D D. 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. D.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. D.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. Anonymous servers cannot authenticate clients, since the client signature in the certificate verify message may require a server Freier, Karlton, Kocher [ Page 44 ] SSL 3.0 December 1995 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 others 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. D.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 servers 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 servers private key, it will be infeasible for them to decode the pre_master_secret. With Diffie-Hellman or Fortezza, the servers public parameters are contained in the server key exchange message and the clients 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. D.1.1.2 RSA key exchange and authentication With RSA, key exchange and server authentication are combined. The public key may be either contained in the servers 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 servers RSA or DSS certificate. The signature includes the current Freier, Karlton, Kocher [ Page 45 ] SSL 3.0 December 1995 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 servers certificate, the client encrypts a pre_master_secret with the servers 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. D.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 can use the client 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. D.1.1.4 Fortezza Freier, Karlton, Kocher [ Page 46 ] SSL 3.0 December 1995 Fortezzas 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 accessable outside the token. The encrypted pre_master_secret is sent to the server in a client key exchange message. D.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. D.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 others finished messages. Without the master_secret, the attacker cannot repair the finished messages, so the attack will be discovered. D.1.4 Resuming sessions When a connection is established by resuming a session, new ClientHello.random and ServerHello.random values are hashed Freier, Karlton, Kocher [ Page 47 ] SSL 3.0 December 1995 with the sessions 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. D.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. D.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 type, the message length, and the message contents. 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 others 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 Freier, Karlton, Kocher [ Page 48 ] SSL 3.0 December 1995 as the MAC. Note: MAC secrets may be larger than encryption keys, so messages can remain tamper resistant even if encryption keys are broken. D.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 E E. Patent Statement This version of the SSL protocol relies on the use of patented public key encryption technology for authentication and encryption. The Internet Standards Process as defined in RFC 1310 requires a written statement from the Patent holder that a license will be made available to applicants under reasonable terms and conditions prior to approving a specification as a Proposed, Draft or Internet Standard. The Massachusetts Institute of Technology has granted RSA Data Security, Inc., exclusive sub-licensing rights to the following patent issued in the United States: Cryptographic Communications System and Method ("RSA"), No. 4,405,829 The Board of Trustees of the Leland Stanford Junior University have granted Caro-Kann Corporation, a wholly owned subsidiary corporation, exclusive sub-licensing rights to the following patents issued in the United States, and all of their corresponding foreign patents: Cryptographic Apparatus and Method ("Diffie-Hellman"), No. 4,200,770 Public Key Cryptographic Apparatus and Method ("Hellman- Merkle"), No. 4,218,582 The Internet Society, Internet Architecture Board, Internet Engineering Steering Group and the Corporation for National Freier, Karlton, Kocher [ Page 49 ] SSL 3.0 December 1995 Research Initiatives take no position on the validity or scope of the patents and patent applications, nor on the appropriateness of the terms of the assurance. The Internet Society and other groups mentioned above have not made any determination as to any other intellectual property rights which may apply to the practice of this standard. Any further consideration of these matters is the users own responsibility. Freier, Karlton, Kocher [ Page 50 ] SSL 3.0 December 1995 References [DH1] W. Diffie and M. E. Hellman, "New Directons in Cryptography," IEEE Transactions on Information Theory, V. IT-22, n. 6, Jun 1977, pp. 74-84. [3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES," IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41. [DES] ANSI X3.106, "American National Standard for Information Systems-Data Link Encryption," American National Standards Institute, 1983. [DSS] NIST FIPS PUB XX, "Digital Signature Standard," National Institute of Standards and Technology, U.S. Department of Commerce, DRAFT, 1 Feb 1993 [FTP] J. Postel and J. Reynolds, RFC 959: File Transfer Protocol, October 1985. [HTTP] T. Berners-Lee, R. Fielding, H. Frystyk, Hypertext Transfer Protocol -- HTTP/1.0, October, 1995. [IDEA] X. Lai, "On the Design and Security of Block Ciphers," ETH Series in Information Processing, v. 1, Konstanz: Hartung-Gorre Verlag, 1992. [FOR] NSA X22, Document # PD4002103-1.01, "Fortezza: Application Implementors Guide," April 6, 1995. [MD2] R. Rivest. RFC 1319: The MD2 Message Digest Algorithm. April 1992. [MD5] R. Rivest. RFC 1321: The MD5 Message Digest Algorithm. April 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] 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. [RSADSI] Contact RSA Data Security, Inc., Tel: 415-595-8782 Freier, Karlton, Kocher [ Page 51 ] SSL 3.0 December 1995 [SCH] B. Schneier. Applied Cryptography: Protocols, Algorithms, and Source Code in C, Published by John Wiley & Sons, Inc. 1994. [SHA] NIST FIPS PUB XX, "Secure Hash Standard," National Institute of Standards and Technology, U.S. Department of Commerce, DRAFT, 1 Feb 1993 [TCP] ISI for DARPA, RFC 793: Transport Control Protocol, September 1981. [TEL] J. Postel and J. Reynolds, RFC 854/5, May, 1993. [X509] CCITT. Recommendation X.509: "The Directory - Authentication Framework". 1988. [XDR] R. Srinivansan, Sun Microsystems, RFC-1832: XDR: External Data Representation Standard, August 1995. Authors Alan O. Freier Paul C. Kocher Netscape Communications Independent Consultant 501 East Middlefield Rd. Box 8243 Mountain View, CA 94043 Stanford, CA 94039 freier@netscape.com pck@netcom.com Philip L. Karlton Netscape Communications 501 East Middlefield Rd. Mountain View, CA 94043 karlton@netscape.com Other contributors Martin Abadi Kipp E.B. Hickman Digital Equipment Corporation Netscape Communications ma@pa.dec.com 501 East Middlefield Rd. Mountain View, CA 94043 kipp@netscape.com Taher Elgamal Jim Roskind Netscape Communications Netscape Communications 501 East Middlefield Rd. 501 East Middlefield Rd Mountain View, CA 94043 Mountain View, CA 94043 elgamal@netscape.com jar@netscape.com Anil Gangolli Micheal J. Sabin, Ph. D. Netscape Communications Consulting Engineer 501 East Middlefield Rd 833 Mango Ave. Mountain View, CA 94043 Sunnyvale, CA 94087 gangolli@netscape.com msabin@netcom.com Freier, Karlton, Kocher [ Page 52 ] SSL 3.0 December 1995 Early reviewers Robert Baldwin Eric Murray RSA Data Security, Inc. ericm@lne.com baldwin@rsa.com George Cox Don Stephenson Intel Corporation Sun Microsystems cox@ibeam.jf.intel.com don.stephenson@eng.sun.com Cheri Dowell Joe Tardo Sun Microsystems General Magic cheri@eng.sun.com tardo@genmagic.com Burt Kaliski RSA Data Security, Inc. burt@rsa.com Freier, Karlton, Kocher [ Page 53 ]