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Network Working Group T. Ylonen INTERNET-DRAFT T. Kivinen draft-ietf-secsh-transport-02.txt M. Saarinen Expires in six months SSH 14 October 1997 SSH Transport Layer Protocol 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 obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract SSH is a protocol for secure remote login and other secure network ser- vices over an insecure network. This document describes the SSH transport layer protocol which typically runs on top of TCP/IP. The protocol can be used as a basis for a number of secure network services. It provides strong encryption, server authentication, and integrity protection. It may also provide compression. Key exchange method, public key algorithm, symmetric encryption algorithm, message authentication algorithm, and hash algorithm are all negotiated. This document also describes the Diffie-Hellman key exchange method and the minimal set of algorithms that are needed to implement the SSH transport layer protocol. T. Ylonen, T. Kivinen, and M. Saarinen [page 1] INTERNET-DRAFT 14 October 1997 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions Used in This Document . . . . . . . . . . . . . . . 3 3. Connection Setup . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Use over TCP/IP . . . . . . . . . . . . . . . . . . . . . . 3 3.2. Protocol Version Exchange . . . . . . . . . . . . . . . . . 3 3.3. Compatibility with Old SSH Versions . . . . . . . . . . . . 4 3.3.1. Old Client, New Server . . . . . . . . . . . . . . . . . 4 3.3.2. New Client, Old Server . . . . . . . . . . . . . . . . . 4 4. Binary Packet Protocol . . . . . . . . . . . . . . . . . . . . . 4 4.1. Maximum Packet Length . . . . . . . . . . . . . . . . . . . 5 4.2. Compression . . . . . . . . . . . . . . . . . . . . . . . . 5 4.3. Encryption . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.4. Data Integrity . . . . . . . . . . . . . . . . . . . . . . . 7 4.5. Key Exchange Methods . . . . . . . . . . . . . . . . . . . . 8 4.6. Public Key Algorithms . . . . . . . . . . . . . . . . . . . 8 5. Key Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.1. Algorithm Negotiation . . . . . . . . . . . . . . . . . . . 10 5.2. Output from Key Exchange . . . . . . . . . . . . . . . . . . 12 5.3. Taking Keys into Use . . . . . . . . . . . . . . . . . . . . 13 6. Diffie-Hellman Key Exchange . . . . . . . . . . . . . . . . . . 13 6.1. diffie-hellman-group1-sha1 . . . . . . . . . . . . . . . . . 15 7. Key Re-Exchange . . . . . . . . . . . . . . . . . . . . . . . . 15 8. Service Request . . . . . . . . . . . . . . . . . . . . . . . . 16 9. Additional Messages . . . . . . . . . . . . . . . . . . . . . . 16 9.1. Disconnection Message . . . . . . . . . . . . . . . . . . . 16 9.2. Ignored Data Message . . . . . . . . . . . . . . . . . . . . 17 9.3. Debug Message . . . . . . . . . . . . . . . . . . . . . . . 17 9.4. Reserved Messages . . . . . . . . . . . . . . . . . . . . . 18 10. Summary of Message Numbers . . . . . . . . . . . . . . . . . . 18 11. Security Considerations . . . . . . . . . . . . . . . . . . . . 18 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 13. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction The SSH transport layer is a secure low level transport protocol. It provides strong encryption, cryptographic host authentication, and integrity protection. Authentication in this protocol level is host-based; this protocol does not perform user authentication. A higher level protocol for user authentication can be designed on top of this protocol. The protocol has been designed to be simple, flexible, to allow parameter negotiation, and to minimize the number of round-trips. Key exchange method, public key algorithm, symmetric encryption algorithm, message authentication algorithm, and hash algorithm are all negotiated. It is expected that in most environments, only 2 round-trips will be needed for full key exchange, server authentication, service request, T. Ylonen, T. Kivinen, and M. Saarinen [page 2] INTERNET-DRAFT 14 October 1997 and acceptance notification of service request. The worst case is 3 round-trips. 2. Conventions Used in This Document The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", and "MAY" that appear in this document are to be interpreted as described in [RFC-2119]. The used data types and terminology are specified in the architecture document [SSH-ARCH]. The architecture document also discusses the algorithm naming conventions that MUST be used with the SSH protocols. 3. Connection Setup SSH works over any 8-bit clean, binary-transparent packet-based transport. The underlying transport SHOULD protect against transmission errors as such errors cause the SSH connection to terminate. The client initiates the connection. 3.1. Use over TCP/IP When used over TCP/IP, the server normally listens for connections on port 22. This port number has been registered with the IANA, and has been officially assigned for SSH. 3.2. Protocol Version Exchange When the connection has been established, both sides MUST send an identification string of the form "SSH-protoversion-softwareversion comments", followed by carriage return and newline characters (ascii 13 and 10, respectively). No null character is sent. The maximum length of the string is 255 characters, including the carriage return and newline. The part of the identification string preceding carriage return and newline is used in the Diffie-Hellman key exchange (Section ``Diffie- Hellman Key Exchange''). The server MAY send other lines of data before sending the version string. Each line SHOULD be terminated by a carriage return and newline. Such lines MUST NOT begin with "SSH-", and SHOULD be encoded in ISO-10646 UTF-8 [RFC-2044] (language is not specified). Clients MUST be able to process such lines; they MAY be silently ignored, or MAY be displayed to the client user; if they are displayed, control character filtering discussed in [SSH-ARCH] SHOULD be used. The primary use of this feature is to allow TCP-wrappers to display an error message before disconnecting. Version strings MUST consist of printable US-ASCII characters, not including whitespaces or a minus sign (-). The version string is T. Ylonen, T. Kivinen, and M. Saarinen [page 3] INTERNET-DRAFT 14 October 1997 primarily used to trigger compatible extensions and to indicate the capabilities of an implementation. The comment string should contain additional information that might be useful in solving user problems. The protocol version described in this document is 2.0. Key exchange will begin immediately after sending this identifier. All packets following the identification string SHALL use the binary packet protocol, to be described below. 3.3. Compatibility with Old SSH Versions During a transition period, it is important to be able to work compatibly with installed SSH clients and servers using an older version of the protocol. Information in this section is only relevant for implementations supporting compatibility with SSH versions 1.x. 3.3.1. Old Client, New Server Server implementations MAY support a configurable "compatibility" flag that enables compatibility with old versions. When this flag is on, the server SHOULD not send any further data after its initialization string until it has received an identification string from the client. The server can then determine whether the client is using an old protocol, and can revert to the old protocol if desired. When compatibility with old clients is not needed, the server MAY send its initial key exchange data immediately after the identification string. 3.3.2. New Client, Old Server Since the new client MAY immediately send additional data after its identification string (before receiving server's identification), the old protocol may already have been corrupted when the client learns that the server is old. When this happens, the client SHOULD close the connection to the server, and reconnect using the old protocol. 4. Binary Packet Protocol Each packet is of the following format. uint32 packet_length byte padding_length byte[n1] payload; n1 = packet_length - padding_length - 1 byte[n2] random padding; n2 = padding_length byte[m] mac (message authentication code); m = mac_length packet_length The length of the packet (bytes), not including MAC or the packet_length field itself. padding_length T. Ylonen, T. Kivinen, and M. Saarinen [page 4] INTERNET-DRAFT 14 October 1997 Length of padding (bytes). payload The useful contents of the packet. If compression has been negotiated, this field is compressed. Initially, compression MUST be "none". Padding Arbitrary-length padding, such that the total length of (packet_length || padding_length || payload || padding) is a multiple of the cipher block size or 8, whichever is larger. There MUST be at least four bytes of padding. The padding SHOULD consist of random bytes. The maximum amount of padding is 255 bytes. MAC Message authentication code. If message authentication has been negotiated, this field contains the MAC bytes. Initially, the MAC algorithm MUST be "none". Note that length of the concatenation of packet length, padding length, payload, and padding MUST be a multiple of the cipher block size or 8, whichever is larger. This constraint MUST be enforced even when using stream ciphers. Note that the packet length field is also encrypted, and processing it requires special care when sending or receiving packets. The minimum size of a packet is 16 (or the cipher block size, whichever is larger) bytes (plus MAC); implementations SHOULD decrypt the length after receiving the first 8 (or cipher block size, whichever is larger) bytes of a packet. 4.1. Maximum Packet Length All implementations MUST be able to process packets with uncompressed payload length of 32768 bytes or less and total packet size of 35000 bytes or less (including length, padding length, payload, padding, and MAC). Implementations SHOULD support longer packets, where they might be needed e.g. if an implementation wants to send a very large number of certificates. Such packets MAY be sent if the version string indicates that the other party is able to process them. However, implementations SHOULD check that the packet length is reasonable for the implementation to avoid denial-of-service and/or buffer overflow attacks. 4.2. Compression If compression has been negotiated, the payload field (and only it) will be compressed using the negotiated algorithm. The length field and MAC will be computed from the compressed payload. Compression MAY be stateful, depending on the method. Compression MUST be independent for each direction, and implementations MUST allow independently choosing the algorithm for each direction. T. Ylonen, T. Kivinen, and M. Saarinen [page 5] INTERNET-DRAFT 14 October 1997 The following compression methods are currently defined: none REQUIRED no compression zlib OPTIONAL GNU ZLIB (LZ77) compression The "zlib" compression is described in [RFC-1950] and in [RFC-1951]. The compression context is initialized after each key exchange, and is passed from one packet to the next with only a partial flush being performed at the end of each packet. A partial flush means that all data will be output, but the next packet will continue using compression tables from the end of the previous packet. Additional methods may be defined as specified in [SSH-ARCH]. 4.3. Encryption An encryption algorithm and a key will be negotiated during the key exchange. When encryption is in effect, the packet length, padding length, payload and padding fields of each packet MUST be encrypted with the given algorithm. The encrypted data in all packets sent in one direction SHOULD be considered a single data stream. For example, initialization vectors SHOULD be passed from the end of one packet to the beginning of the next packet. All ciphers SHOULD use keys with an effective key length of 128 bits or more. The ciphers in each direction MUST run independently of each other, and implementations MUST allow independently choosing the algorithm for each direction (if multiple algorithms are allowed by local policy). The following ciphers are currently defined: 3des-cbc REQUIRED three-key 3DES in CBC mode blowfish-cbc RECOMMENDED Blowfish in CBC mode arcfour OPTIONAL the ARCFOUR stream cipher idea-cbc OPTIONAL IDEA in CBC mode cast128-cbc OPTIONAL CAST-128 in CBC mode none OPTIONAL no encryption; NOT RECOMMENDED The "3des-cbc" cipher is three-key triple-DES (encrypt-decrypt-encrypt), where the first 8 bytes of the key are used for the first encryption, the next 8 bytes for the decryption, and the following 8 bytes for the final encryption. This requires 24 bytes of key data (of which 168 bits are actually used). To implement CBC mode, outer chaining MUST be used (i.e., there is only one initialization vector). This is a block cipher with 8 byte blocks. This algorithm is defined in [Schneier]. The "blowfish-cbc" cipher is Blowfish in CBC mode, with 128 bit keys [Schneier]. This is a block cipher with 8 byte blocks. The "arcfour" is the Arcfour stream cipher with 128 bit keys. The Arcfour cipher is believed to be compatible with the RC4 cipher T. Ylonen, T. Kivinen, and M. Saarinen [page 6] INTERNET-DRAFT 14 October 1997 [Schneier]. RC4 is a registered trademark of RSA Data Security Inc. The "idea-cbc" cipher is the IDEA cipher in CBC mode [Schneier]. IDEA is patented by Ascom AG. The "cast128-cbc" cipher is the CAST-128 cipher in CBC mode [RFC-2144]. The "none" algorithm specifies that no encryption is to be done. Note that this method provides no confidentiality protection, and it is not recommended. Some functionality (e.g. password authentication) may be disabled for security reasons if this cipher is chosen. Additional methods may be defined as specified in [SSH-ARCH]. 4.4. Data Integrity Data integrity is protected by including with each packet a message authentication code (MAC) that is computed from a shared secret, packet sequence number, and the contents of the packet. The message authentication algorithm and key are negotiated during key exchange. Initially, no MAC will be in effect, and its length MUST be zero. After key exchange, the selected MAC will be computed before encryption from the concatenation of packet data: mac = MAC(key, sequence_number || unencrypted_packet) where unencrypted_packet is the entire packet without MAC (the length fields, payload and padding), and sequence_number is an implicit packet sequence number represented as uint32. The sequence number is initial- ized to zero for the first packet, and is incremented after every packet (regardless of whether encryption or MAC is in use). It is never reset, even if keys/algorithms are renegotiated later. It wraps around to zero after every 2^32 packets. The packet sequence number itself is not included in the packet sent over the wire. The MAC algorithms for each direction MUST run independently, and implementations MUST allow choosing the algorithm independently for both directions. The MAC bytes resulting from the MAC algorithm MUST be transmitted without encryption as the last part of the packet. The number of MAC bytes depends on the algorithm chosen. The following MAC algorithms are currently defined: hmac-sha1 REQUIRED HMAC-SHA1 (length = 20) hmac-sha-96 RECOMMENDED first 96 bits of HMAC-SHA1 (length = 12) hmac-md5 OPTIONAL HMAC-MD5 (length = 16) hmac-md5-96 OPTIONAL first 96 bits of HMAC-MD5 (length = 12) none OPTIONAL no MAC; NOT RECOMMENDED The "hmac-*" algorithms are described in [RFC-2104]. The "*-n" MACs use T. Ylonen, T. Kivinen, and M. Saarinen [page 7] INTERNET-DRAFT 14 October 1997 only the first n bits of the resulting value. The hash algorithms are described in [Schneier]. The "none" method is NOT RECOMMENDED. An active attacker may be able to modify transmitted data if this is used. Additional methods may be defined as specified in [SSH-ARCH]. 4.5. Key Exchange Methods The key exchange method specifies how one-time session keys are generated for encryption and for authentication, and how the server authentication is done. Only one REQUIRED key exchange method has been defined: diffie-hellman-group1-sha1 REQUIRED This method is described later in this document. Additional methods may be defined as specified in [SSH-ARCH]. 4.6. Public Key Algorithms This protocol has been designed to be able to operate with almost any public key format, encoding, and algorithm (signature and/or encryption). There are several aspects that define a public key type: o Key format: how is the key encoded and how are certificates represented. The key blobs in this protocol MAY contain certificates in addition to keys. o Signature and/or encryption algorithms. Some key types may not support both signing and encryption. Key usage may also be restricted by policy statements in e.g. certificates. In this case, different key types SHOULD be defined for the different policy alternatives. o Encoding of signatures and/or encrypted data. This includes but is not limited to padding, byte order, and data formats. The following public key and/or certificate formats are currently defined: ssh-dss REQUIRED sign Simple DSS x509v3 RECOMMENDED sign X.509 certificates spki OPTIONAL sign SPKI certificates pgp OPTIONAL sign OpenPGP certificates Additional key types may be defined as specified in [SSH-ARCH]. T. Ylonen, T. Kivinen, and M. Saarinen [page 8] INTERNET-DRAFT 14 October 1997 The key type MUST always be explicitly known (from algorithm negotiation or some other source). It is not normally included in the key blob. Certificates and public keys are encoded as: uint32 the combined length of the format identifier and associated data string certificate or public key format identifier byte[n] key/certificate data The certificate part may have be a zero length string, but a public key is required. This is the public key that will be used for authentication; the certificate sequence contained in the certificate blob can be used to provide authorization. The "ssh-dss" key format has the following specific encoding: uint32 length string "ssh-dss" mpint p mpint q mpint g mpint y Here the "p", "q", "g", and "y" parameters form the signature key blob. Signing and verifying using this key format are done according to the Digital Signature Standard [FIPS-186] using the SHA-1 hash. A description can also be found in [Schneier]. The resulting signature is encoded as: uint32 length string "ssh-dss" mpint r mpint s The "x509v3" method indicates that the certificates, the public key, and the resulting signature are in X.509v3 compatible DER-encoded format. The formats used in X.509v3 is described in [PKIX-Part1]. The "spki" method indicates that the certificate blob contains a sequence of SPKI certificates. The format of SPKI certificates is described in [SPKI]. The "pgp" method indicates the the certificates, the public key, and the signature are in OpenPGP compatible binary format. [PGP] 5. Key Exchange Key exchange begins by each side sending lists of supported algorithms. Each side has a preferred algorithm in each category, and it is assumed that most implementations at any given time will use the same preferred algorithm. Each side MAY guess which algorithm the other side is using, T. Ylonen, T. Kivinen, and M. Saarinen [page 9] INTERNET-DRAFT 14 October 1997 and MAY send an initial key exchange packet according to the algorithm if appropriate for the preferred method. If all algorithms were guessed right, the optimistically sent packet MUST be handled as the first key exchange packet. However, if the guess was wrong, and a packet was optimistically sent by one or both parties, such packets MUST be ignored (even if the error in the guess would not affect the contents of the initial packet(s)), and the appropriate side MUST send the correct initial packet. Server authentication in the key exchange MAY be implicit. After a key exchange with implicit server authentication, the client MUST wait for response to its service request message before sending any further data. 5.1. Algorithm Negotiation Key exchange begins by each side sending the following packet: byte SSH_MSG_KEXINIT byte[16] cookie (random bytes) string kex_algorithms string server_host_key_algorithms string encryption_algorithms_client_to_server string encryption_algorithms_server_to_client string mac_algorithms_client_to_server string mac_algorithms_server_to_client string compression_algorithms_client_to_server string compression_algorithms_server_to_client boolean first_kex_packet_follows uint32 0 (reserved for future extension) Each of the algorithm strings MUST be a comma-separated list of algorithm names (see ``Algorithm Naming'' in [SSH-ARCH]). Each supported (allowed) algorithm MUST be listed, in order of preference. The first algorithm in each list MUST be the preferred (guessed) algorithm. Each string MUST contain at least one algorithm name. cookie The cookie MUST be a random value generated by the sender. Its purpose is to make it impossible for either side to fully determine the keys and the session identifier. kex_algorithms Key exchange algorithms were defined above. The first algorithm MUST be the preferred (and guessed) algorithm. If both sides make the same guess, that algorithm MUST used. Otherwise, the following algorithm MUST be used to choose a key exchange method: iterate over client's kex algorithms, one at a time. Choose the first algorithm that satisfies the following conditions: o the server also supports the algorithm, o if the algorithm requires an encryption-capable host key, there is T. Ylonen, T. Kivinen, and M. Saarinen [page 10] INTERNET-DRAFT 14 October 1997 an encryption-capable algorithm on the server's server_host_key_algorithms that is also supported by the client, and o if the algorithm requires a signature-capable host key, there is a signature-capable algorithm on the server's server_host_key_algorithms that is also supported by the client. If no algorithm satisfying all these conditions can be found, connection fails, and both sides MUST disconnect. server_host_key_algorithms List of the algorithms supported for the server host key. The server lists the algorithms for which it has host keys; the client lists the algorithms that it is willing to accept. (There MAY be multiple host keys for a host, possibly with different algorithms.) Some host keys may not support both signatures and encryption (this can be determined from the algorithm), and thus not all host keys are valid for all key exchange methods. Algorithm selection depends on whether the chosen key exchange algorithm requires a signature- or encryption capable host key. It MUST be possible to determine this from the public key algorithm name. The first algorithm on the client's list that satisfies the requirements and is also supported by the server MUST be chosen. If there is no such algorithm, both sides MUST disconnect. encryption_algorithms Lists the acceptable symmetric encryption algorithms in order of preference. The chosen encryption algorithm to each direction MUST be the first algorithm on the client's list that is also on the server's list. If there is no such algorithm, both sides MUST disconnect. Note that "none" must be explicitly listed if it is to be acceptable. The defined algorithm names are listed in Section ``Encryption''. mac_algorithms Lists the acceptable MAC algorithms in order of preference. The chosen MAC algorithm MUST be the first algorithm on the client's list that is also on the server's list. If there is no such algorithm, both sides MUST disconnect. Note that "none" must be explicitly listed if it is to be acceptable. The MAC algorithm names are listed in Section ``Data Integrity''. compression_algorithms Lists the acceptable compression algorithms in order of preference. The chosen compression algorithm MUST be the first T. Ylonen, T. Kivinen, and M. Saarinen [page 11] INTERNET-DRAFT 14 October 1997 algorithm on the client's list that is also on the server's list. If there is no such algorithm, both sides MUST disconnect. Note that "none" must be explicitly listed if it is to be acceptable. The compression algorithm names are listed in Section ``Compression''. first_kex_packet_follows Indicates whether a guessed key exchange packet follows. If a guessed packet will be sent, this MUST be true. If no guessed packet will be sent, this MUST be false. After receiving the SSH_MSG_KEXINIT packet from the other side, each party will know whether their guess was right. If the other party's guess was wrong, and this field was true, the next packet MUST be silently ignored, and both sides MUST then act as determined by the negotiated key exchange method. If the guess was right, key exchange MUST continue using the guessed packet. After the KEXINIT packet exchange, the key exchange algorithm is run. It may involve several packet exchanges, as specified by the key exchange method. 5.2. Output from Key Exchange The key exchange produces two values: a shared secret K, and an exchange hash H. Encryption and authentication keys are derived from these. The exchange hash H from the first key exchange is additionally used as the session identifier, which is a unique identifier for this connection. It is used by authentication methods as a part of the data that is signed as a proof of possession of a private key. Once computed, the session identifier is not changed, even if keys are later re-exchanged. Each key exchange method specifies a hash function that is used in the key exchange. The same hash algorithm MUST be used in key derivation. Here, we'll call it HASH. Encryption keys MUST be computed as HASH of a known value and K as follows: o Initial IV client to server: HASH(K || "A" || session_id) ("A" means the single character A, ascii 65). o Initial IV server to client: HASH(K || "B" || session_id) o Encryption key client to server: HASH(K || "C" || session_id) o Encryption key server to client: HASH(K || "D" || session_id) o Integrity key client to server: HASH(K || "E" || session_id) o Integrity key server to client: HASH(K || "F" || session_id) T. Ylonen, T. Kivinen, and M. Saarinen [page 12] INTERNET-DRAFT 14 October 1997 Key data MUST be taken from the beginning of the hash output. 128 bits (16 bytes) SHOULD be used for algorithms with variable-length keys. For other algorithms, as many bytes as are needed are taken from the beginning of the hash value. If the key length in longer than the output of the HASH, the key is extended by computing HASH of the concatenation of K and the entire key so far, and appending the resulting bytes (as many as HASH generates) to the key. This process is repeated until enough key material is available; the key is taken from the beginning of this value. In other words, K1 = HASH(K || X || session_id) (X is e.g. "A") K2 = HASH(K || K1) K3 = HASH(K || K1 || K2) ... key = K1 || K2 || K3 || ... 5.3. Taking Keys into Use Key exchange ends by each side sending an SSH_MSG_NEWKEYS message. This message is sent with the old keys and algorithms. All messages sent after this message MUST use the new keys and algorithms. When this message is received, the new keys and algorithms MUST be taken into use for receiving. This message is the only valid message after key exchange, in addition to SSH_MSG_DEBUG, SSH_MSG_DISCONNECT and SSH_MSG_IGNORE messages. The purpose of this message is to ensure that a party is able to respond with a disconnect message that the other party can understand if something goes wrong with the key exchange. Implementations MUST NOT accept any other messages after key exchange before receiving SSH_MSG_NEWKEYS. byte SSH_MSG_NEWKEYS 6. Diffie-Hellman Key Exchange The Diffie-Hellman key exchange provides a shared secret that can not be determined by either party alone. The key exchange is combined with a signature with the host key to provide host authentication. In the following description (C is the client, S is the server; p is a large safe prime, g is a generator for a subgroup of GF(p), and q is the order of the subgroup; V_S is S's version string; V_C is C's version string; K_S is S's public host key; I_C is C's KEXINIT message and I_S S's KEXINIT message which have been exchanged before this part begins): 1. C generates a random number x (1 < x < q) and computes e = g^x mod p. C sends "e" to S. 2. S generates a random number y (0 < y < q) and computes f = g^y mod p. S receives "e". It computes K = e^y mod p, H = hash(V_C || V_S || I_C || I_S || K_S || e || f || K), and signature s on H with its T. Ylonen, T. Kivinen, and M. Saarinen [page 13] INTERNET-DRAFT 14 October 1997 private host key. S sends "K_S || f || s" to C. The signing operation may involve a second hashing operation. 3. C verifies that K_S really is the host key for S (e.g. using certificates or a local database). C is also allowed to accept the key without verification; however, doing so will render the protocol insecure against active attacks (but may be desirable for practical reasons in the short term in many environments). C then computes K = f^x mod p, H = hash(V_C || V_S || I_C || I_S || K_S || e || f || K), and verifies the signature s on H. Either side MUST NOT send or accept e or f values that are not in the range [1, p-1]. If this condition is violated, the key exchange fails. This is implemented with the following messages. The hash algorithm for computing the exchange hash is defined by the method name, and is called HASH. The public key algorithm for signing is negotiated with the KEXINIT messages. First, the client sends: byte SSH_MSG_KEXDH_INIT mpint e The server responds with: byte SSH_MSG_KEXDH_REPLY string server public host key and certificates (K_S) mpint f string signature of H The hash H is computed as the HASH hash of the concatenation of the following: string V_C, the client's version string (CR and NL excluded) string V_S, the server's version string (CR and NL excluded) string I_C, the payload of the client's SSH_MSG_KEXINIT string I_S, the payload of the server's SSH_MSG_KEXINIT string K_S, the host key mpint e, exchange value sent by the client mpint f, exchange value sent by the server mpint K, the shared secret This value is called the exchange hash, and it is used to authenticate the key exchange. The signature algorithm MUST be applied over H, not the original data. Most signature algorithms include hashing and additional padding. For example, a "ssh-dss" specifies SHA-1 hashing; in that case, the data is first hashed with HASH to compute H, and H is then hashed with SHA-1 as part of the signing operation. T. Ylonen, T. Kivinen, and M. Saarinen [page 14] INTERNET-DRAFT 14 October 1997 6.1. diffie-hellman-group1-sha1 The "diffie-hellman-group1-sha1" method specifies Diffie-Hellman key exchange with SHA-1 as HASH, and the following group: The prime p is equal to 2^1024 - 2^960 - 1 + 2^64 * floor( 2^894 Pi + 129093 ). Its hexadecimal value is FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE65381 FFFFFFFF FFFFFFFF. In decimal, this value is 179769313486231590770839156793787453197860296048756011706444 423684197180216158519368947833795864925541502180565485980503 646440548199239100050792877003355816639229553136239076508735 759914822574862575007425302077447712589550957937778424442426 617334727629299387668709205606050270810842907692932019128194 467627007. The generator used with this prime is g = 2. The group order q is (p - 1) / 2. This group was taken from the ISAKMP/Oakley specification, and was originally generated by Richard Schroeppel at the University of Arizona. Properties of this prime are described in [Orm96]. 7. Key Re-Exchange Key re-exchange is started by sending a SSH_MSG_KEXINIT packet when not already doing a key exchange (as described in Section ``Algorithm Negotiation''). When this message is received, a party MUST respond with its own SSH_MSG_KEXINIT message except when the received SSH_MSG_KEXINIT already was a reply. Either party MAY initiate the re- exchange, but roles MUST NOT be changed (i.e., the server remains the server, and the client remains the client). Key re-exchange is performed using whatever encryption was in effect when the exchange was started. Encryption, compression, and MAC methods are not changed before a new SSH_MSG_NEWKEYS is sent after the key exchange (as in the initial key exchange). Re-exchange is processed identically to the initial key exchange, except for the session identifier that will remain unchanged. It is permissible to change some or all of the algorithms during the re-exchange. Host keys can also change. All keys and initialization vectors are recomputed after the exchange. Compression and encryption contexts are reset. It is recommended that the keys are changed after each gigabyte of transmitted data or after each hour of connection time, whichever comes T. Ylonen, T. Kivinen, and M. Saarinen [page 15] INTERNET-DRAFT 14 October 1997 sooner. However, since the re-exchange is a public key operation, it requires a fair amount of processing power and should not be performed too often. More application data may be sent after the SSH_MSG_NEWKEYS packet has been sent; key exchange does not affect the protocols that lie above the SSH transport layer. 8. Service Request After the key exchange, the client requests a service. The service is identified by a name. The format of names and procedures for defining new names are defined in [SSH-ARCH]. Currently, the following names have been reserved: ssh-userauth ssh-connection Similar local naming policy is applied to the service names that is applied to the algorithm names; a local service should use the servicename@domain syntax. byte SSH_MSG_SERVICE_REQUEST string service name If the server rejects the service request, it SHOULD send an appropriate SSH_MSG_DISCONNECT message and MUST disconnect. When the service starts, it may have access to the session identifier generated during the key exchange. If the server supports the service (and permits the client to use it), it MUST respond with byte SSH_MSG_SERVICE_ACCEPT string service name Message numbers used by services should be in the area reserved for them (see Section ``Summary of Message Numbers''). The transport level will continue to process its own messages. Note that after a key exchange with implicit server authentication, the client MUST wait for response to its service request message before sending any further data. 9. Additional Messages Either party may send any of the following messages at any time. 9.1. Disconnection Message byte SSH_MSG_DISCONNECT T. Ylonen, T. Kivinen, and M. Saarinen [page 16] INTERNET-DRAFT 14 October 1997 uint32 reason code string description [[RFC-2044]] string language tag [[RFC-1766]] This message causes immediate termination of the connection. All implementations MUST be able to process this message; they SHOULD be able to send this message. The sender MUST NOT send or receive any data after this message, and the recipient MUST NOT accept any data after receiving this message. The description field gives a more specific explanation in a human-readable form. The error code gives the reason in a more machine-readable format (suitable for localization), and can have the following values: #define SSH_DISCONNECT_HOST_NOT_ALLOWED_TO_CONNECT 1 #define SSH_DISCONNECT_PROTOCOL_ERROR 2 #define SSH_DISCONNECT_KEY_EXCHANGE_FAILED 3 #define SSH_DISCONNECT_HOST_AUTHENTICATION_FAILED 4 #define SSH_DISCONNECT_MAC_ERROR 5 #define SSH_DISCONNECT_COMPRESSION_ERROR 6 #define SSH_DISCONNECT_SERVICE_NOT_AVAILABLE 7 #define SSH_DISCONNECT_PROTOCOL_VERSION_NOT_SUPPORTED 8 #define SSH_DISCONNECT_HOST_KEY_NOT_VERIFIABLE 9 #define SSH_DISCONNECT_CONNECTION_LOST 10 #define SSH_DISCONNECT_BY_APPLICATION 11 If the description string is displayed, control character filtering discussed in [SSH-ARCH] should be used to avoid attacks by sending terminal control characters. 9.2. Ignored Data Message byte SSH_MSG_IGNORE string data All implementations MUST understand (and ignore) this message at any time (after receiving the protocol version). No implementation is required to send them. This message can be used as an additional protection measure against advanced traffic analysis techniques. 9.3. Debug Message byte SSH_MSG_DEBUG boolean always_display string message [[RFC-2044]] string language tag [[RFC-1766]] All implementations MUST understand this message, but they are allowed to ignore it. This message is used to pass information to the other side that may help debugging. If always_display is true, the message SHOULD be displayed. Otherwise, it SHOULD NOT be displayed unless debugging information has been explicitly requested by the user. T. Ylonen, T. Kivinen, and M. Saarinen [page 17] INTERNET-DRAFT 14 October 1997 The message doesn't need to contain a newline. It is, however, allowed to consist of multiple lines separated by newlines. If the message string is displayed, terminal control character filtering discussed in [SSH-ARCH] should be used to avoid attacks by sending terminal control characters. 9.4. Reserved Messages An implementation MUST respond to all unrecognized messages with an SSH_MSG_UNIMPLEMENTED message in the order in which they were received. Such messages MUST be otherwise ignored. Later protocol versions may define other meanings for these message types. byte SSH_MSG_UNIMPLEMENTED uint32 packet sequence number of rejected message 10. Summary of Message Numbers The following message numbers have been defined in this protocol. #define SSH_MSG_DISCONNECT 1 #define SSH_MSG_IGNORE 2 #define SSH_MSG_UNIMPLEMENTED 3 #define SSH_MSG_DEBUG 4 #define SSH_MSG_SERVICE_REQUEST 5 #define SSH_MSG_SERVICE_ACCEPT 6 #define SSH_MSG_KEXINIT 20 #define SSH_MSG_NEWKEYS 21 /* Numbers 30-49 used for kex packets. Different kex methods may reuse message numbers in this range. */ #define SSH_MSG_KEXDH_INIT 30 #define SSH_MSG_KEXDH_REPLY 31 11. Security Considerations This protocol provides a secure encrypted channel over an unsecure network. It performs server host authentication, key exchange, encryption, and integrity protection. It also derives a unique session id that may be by higher-level protocols. It is expected that this protocol will sometimes be used without insisting on reliable association between the server host key and the server host name. Such use is inherently insecure, but may be necessary in non-security critical environments, and still provides protection against passive attacks. However, implementors of protocols running on top of this protocol should keep this possibility in mind. This protocol is designed to be used over an reliable transport. If transmission errors or message manipulation occur, the connection is T. Ylonen, T. Kivinen, and M. Saarinen [page 18] INTERNET-DRAFT 14 October 1997 closed. The connection SHOULD be re-established if this occurs. Denial of service attacks of this type ("wire cutter") are almost impossible to avoid. 12. References [FIPS-186] Federal Information Processing Standards Publication (FIPS PUB) 186, Digital Signature Standard, 18 May 1994. [Orm96] Orman, H., "The Oakley Key Determination Protocol", version 1, TR97-92, Department of Computer Science Technical Report, University of Arizona. [RFC-1034] Mockapetris, P., "Domain Names - Concepts and Facilities", November 1987. [RFC-1766] Alvestrand, H., "Tags for the Identification of Languages", March 1995. [RFC-1950] Deutch, P. and Gailly, J-L., "ZLIB Compressed Data Format Specification version 3.3", May 1996. [RFC-1951] Deutch, P., "DEFLATE Compressed Data Format Specification version 1.3", May 1996. [RFC-2044] Yergeau, F., "UTF-8, a Transformation Format of Unicode and ISO 10646", October 1996. [RFC-2104] Krawczyk, H., Bellare, M., and Canetti, R., "HMAC: Keyed- Hashing for Message Authentication", February 1997 [RFC-2119] Bradner, S., "Key words for use in RFCs to indicate Requirement Levels", March 1997. [RFC-2144] Adams, C., "The CAST-128 Encryption Algorithm", May 1997. [Schneier] Schneier, B., "Applied Cryptography Second Edition: protocols, algorithms, and source code in C", 2nd edition, John Wiley & Sons, New York, NY, 1996. [SSH-ARCH] Ylonen, T., Kivinen, T, and Saarinen, M., "SSH Protocol Architecture", Internet Draft, draft-ietf-secsh-architecture-00.txt #91;SSH-USERAUTH] Ylonen, T., Kivinen, T, and Saarinen, M., "SSH Authentication Protocol", Internet Draft, draft-ietf-secsh- userauth-02.txt [SSH-CONNECT] Ylonen, T., Kivinen, T, and Saarinen, M., "SSH Connection Protocol", Internet Draft, draft-ietf-secsh-connect-02.txt [PGP] (XXX n.a.) [PKIX-Part1] (XXX n.a.) T. Ylonen, T. Kivinen, and M. Saarinen [page 19] INTERNET-DRAFT 14 October 1997 13. Authors' Addresses Tatu Ylonen SSH Communications Security Ltd. Tekniikantie 12 FIN-02150 ESPOO Finland E-mail: ylo@ssh.fi Tero Kivinen SSH Communications Security Ltd. Tekniikantie 12 FIN-02150 ESPOO Finland E-mail: kivinen@ssh.fi Markku-Juhani O. Saarinen SSH Communications Security Ltd. Tekniikantie 12 FIN-02150 ESPOO Finland E-mail: mjos@ssh.fi T. Ylonen, T. Kivinen, and M. Saarinen [page 20]