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PROTOCOL.agent
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This describes the protocol used by OpenSSH's ssh-agent. OpenSSH's agent supports managing keys for the standard SSH protocol 2 as well as the legacy SSH protocol 1. Support for these key types is almost completely disjoint - in all but a few cases, operations on protocol 2 keys cannot see or affect protocol 1 keys and vice-versa. Protocol 1 and protocol 2 keys are separated because of the differing cryptographic usage: protocol 1 private RSA keys are used to decrypt challenges that were encrypted with the corresponding public key, whereas protocol 2 RSA private keys are used to sign challenges with a private key for verification with the corresponding public key. It is considered unsound practice to use the same key for signing and encryption. With a couple of exceptions, the protocol message names used in this document indicate which type of key the message relates to. SSH_* messages refer to protocol 1 keys only. SSH2_* messages refer to protocol 2 keys. Furthermore, the names also indicate whether the message is a request to the agent (*_AGENTC_*) or a reply from the agent (*_AGENT_*). Section 3 below contains the mapping of the protocol message names to their integer values. 1. Data types Because of support for legacy SSH protocol 1 keys, OpenSSH's agent protocol makes use of some data types not defined in RFC 4251. 1.1 uint16 The "uint16" data type is a simple MSB-first 16 bit unsigned integer encoded in two bytes. 1.2 mpint1 The "mpint1" type represents an arbitrary precision integer (bignum). Its format is as follows: uint16 bits byte[(bits + 7) / 8] bignum "bignum" contains an unsigned arbitrary precision integer encoded as eight bits per byte in big-endian (MSB first) format. Note the difference between the "mpint1" encoding and the "mpint" encoding defined in RFC 4251. Also note that the length of the encoded integer is specified in bits, not bytes and that the byte length of the integer must be calculated by rounding up the number of bits to the nearest eight. 2. Protocol Messages All protocol messages are prefixed with their length in bytes, encoded as a 32 bit unsigned integer. Specifically: uint32 message_length byte[message_length] message The following message descriptions refer only to the content the "message" field. 2.1 Generic server responses The following generic messages may be sent by the server in response to requests from the client. On success the agent may reply either with: byte SSH_AGENT_SUCCESS or a request-specific success message. On failure, the agent may reply with: byte SSH_AGENT_FAILURE SSH_AGENT_FAILURE messages are also sent in reply to unknown request types. 2.2 Adding keys to the agent Keys are added to the agent using the SSH_AGENTC_ADD_RSA_IDENTITY and SSH2_AGENTC_ADD_IDENTITY requests for protocol 1 and protocol 2 keys respectively. Two variants of these requests are SSH_AGENTC_ADD_RSA_ID_CONSTRAINED and SSH2_AGENTC_ADD_ID_CONSTRAINED - these add keys with optional "constraints" on their usage. OpenSSH may be built with support for keys hosted on a smartcard or other hardware security module. These keys may be added to the agent using the SSH_AGENTC_ADD_SMARTCARD_KEY and SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED requests. 2.2.1 Key constraints The OpenSSH agent supports some basic optional constraints on key usage. At present there are two constraints defined. The first constraint limits the validity duration of a key. It is encoded as: byte SSH_AGENT_CONSTRAIN_LIFETIME uint32 seconds Where "seconds" contains the number of seconds that the key shall remain valid measured from the moment that the agent receives it. After the validity period has expired, OpenSSH's agent will erase these keys from memory. The second constraint requires the agent to seek explicit user confirmation before performing private key operations with the loaded key. This constraint is encoded as: byte SSH_AGENT_CONSTRAIN_CONFIRM Zero or more constraints may be specified when adding a key with one of the *_CONSTRAINED requests. Multiple constraints are appended consecutively to the end of the request: byte constraint1_type .... constraint1_data byte constraint2_type .... constraint2_data .... byte constraintN_type .... constraintN_data Such a sequence of zero or more constraints will be referred to below as "constraint[]". Agents may determine whether there are constraints by checking whether additional data exists in the "add key" request after the key data itself. OpenSSH will refuse to add a key if it contains unknown constraints. 2.2.2 Add protocol 1 key A client may add a protocol 1 key to an agent with the following request: byte SSH_AGENTC_ADD_RSA_IDENTITY or SSH_AGENTC_ADD_RSA_ID_CONSTRAINED uint32 ignored mpint1 rsa_n mpint1 rsa_e mpint1 rsa_d mpint1 rsa_iqmp mpint1 rsa_q mpint1 rsa_p string key_comment constraint[] key_constraints Note that there is some redundancy in the key parameters; a key could be fully specified using just rsa_q, rsa_p and rsa_e at the cost of extra computation. "key_constraints" may only be present if the request type is SSH_AGENTC_ADD_RSA_ID_CONSTRAINED. The agent will reply with a SSH_AGENT_SUCCESS if the key has been successfully added or a SSH_AGENT_FAILURE if an error occurred. 2.2.3 Add protocol 2 key The OpenSSH agent supports DSA, ECDSA and RSA keys for protocol 2. DSA keys may be added using the following request byte SSH2_AGENTC_ADD_IDENTITY or SSH2_AGENTC_ADD_ID_CONSTRAINED string "ssh-dss" mpint dsa_p mpint dsa_q mpint dsa_g mpint dsa_public_key mpint dsa_private_key string key_comment constraint[] key_constraints DSA certificates may be added with: byte SSH2_AGENTC_ADD_IDENTITY or SSH2_AGENTC_ADD_ID_CONSTRAINED string "ssh-dss-cert-v00@openssh.com" string certificate mpint dsa_private_key string key_comment constraint[] key_constraints ECDSA keys may be added using the following request byte SSH2_AGENTC_ADD_IDENTITY or SSH2_AGENTC_ADD_ID_CONSTRAINED string "ecdsa-sha2-nistp256" | "ecdsa-sha2-nistp384" | "ecdsa-sha2-nistp521" string ecdsa_curve_name string ecdsa_public_key mpint ecdsa_private string key_comment constraint[] key_constraints ECDSA certificates may be added with: byte SSH2_AGENTC_ADD_IDENTITY or SSH2_AGENTC_ADD_ID_CONSTRAINED string "ecdsa-sha2-nistp256-cert-v01@openssh.com" | "ecdsa-sha2-nistp384-cert-v01@openssh.com" | "ecdsa-sha2-nistp521-cert-v01@openssh.com" string certificate mpint ecdsa_private_key string key_comment constraint[] key_constraints ED25519 keys may be added using the following request byte SSH2_AGENTC_ADD_IDENTITY or SSH2_AGENTC_ADD_ID_CONSTRAINED string "ssh-ed25519" string ed25519_public_key string ed25519_private_key || ed25519_public_key string key_comment constraint[] key_constraints ED25519 certificates may be added with: byte SSH2_AGENTC_ADD_IDENTITY or SSH2_AGENTC_ADD_ID_CONSTRAINED string "ssh-ed25519-cert-v01@openssh.com" string certificate string ed25519_public_key string ed25519_private_key || ed25519_public_key string key_comment constraint[] key_constraints For both ssh-ed25519 and ssh-ed25519-cert-v01@openssh.com keys, the private key has the public key appended (for historical reasons). RSA keys may be added with this request: byte SSH2_AGENTC_ADD_IDENTITY or SSH2_AGENTC_ADD_ID_CONSTRAINED string "ssh-rsa" mpint rsa_n mpint rsa_e mpint rsa_d mpint rsa_iqmp mpint rsa_p mpint rsa_q string key_comment constraint[] key_constraints RSA certificates may be added with this request: byte SSH2_AGENTC_ADD_IDENTITY or SSH2_AGENTC_ADD_ID_CONSTRAINED string "ssh-rsa-cert-v00@openssh.com" string certificate mpint rsa_d mpint rsa_iqmp mpint rsa_p mpint rsa_q string key_comment constraint[] key_constraints Note that the 'rsa_p' and 'rsa_q' parameters are sent in the reverse order to the protocol 1 add keys message. As with the corresponding protocol 1 "add key" request, the private key is overspecified to avoid redundant processing. For DSA, ECDSA and RSA key add requests, "key_constraints" may only be present if the request type is SSH2_AGENTC_ADD_ID_CONSTRAINED. The agent will reply with a SSH_AGENT_SUCCESS if the key has been successfully added or a SSH_AGENT_FAILURE if an error occurred. 2.2.4 Loading keys from a smartcard The OpenSSH agent may have optional smartcard support built in to it. If so, it supports an operation to load keys from a smartcard. Technically, only the public components of the keys are loaded into the agent so this operation really arranges for future private key operations to be delegated to the smartcard. byte SSH_AGENTC_ADD_SMARTCARD_KEY or SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED string reader_id string pin constraint[] key_constraints "reader_id" is an identifier to a smartcard reader and "pin" is a PIN or passphrase used to unlock the private key(s) on the device. "key_constraints" may only be present if the request type is SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED. This operation may load all SSH keys that are unlocked using the "pin" on the specified reader. The type of key loaded (protocol 1 or protocol 2) will be specified by the smartcard itself, it is not client-specified. The agent will reply with a SSH_AGENT_SUCCESS if one or more keys have been successfully loaded or a SSH_AGENT_FAILURE if an error occurred. The agent will also return SSH_AGENT_FAILURE if it does not support smartcards. 2.3 Removing multiple keys A client may request that an agent delete all protocol 1 keys using the following request: byte SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES This message requests the deletion of all protocol 2 keys: byte SSH2_AGENTC_REMOVE_ALL_IDENTITIES On success, the agent will delete all keys of the requested type and reply with a SSH_AGENT_SUCCESS message. If an error occurred, the agent will reply with SSH_AGENT_FAILURE. Note that, to delete all keys (both protocol 1 and 2), a client must send both a SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES and a SSH2_AGENTC_REMOVE_ALL_IDENTITIES request. 2.4 Removing specific keys 2.4.1 Removing a protocol 1 key Removal of a protocol 1 key may be requested with the following message: byte SSH_AGENTC_REMOVE_RSA_IDENTITY uint32 key_bits mpint1 rsa_e mpint1 rsa_n Note that key_bits is strictly redundant, as it may be inferred by the length of rsa_n. The agent will delete any private key matching the specified public key and return SSH_AGENT_SUCCESS. If no such key was found, the agent will return SSH_AGENT_FAILURE. 2.4.2 Removing a protocol 2 key Protocol 2 keys may be removed with the following request: byte SSH2_AGENTC_REMOVE_IDENTITY string key_blob Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key Algorithms" for any of the supported protocol 2 key types. The agent will delete any private key matching the specified public key and return SSH_AGENT_SUCCESS. If no such key was found, the agent will return SSH_AGENT_FAILURE. 2.4.3 Removing keys loaded from a smartcard A client may request that a server remove one or more smartcard-hosted keys using this message: byte SSH_AGENTC_REMOVE_SMARTCARD_KEY string reader_id string pin "reader_id" the an identifier to a smartcard reader and "pin" is a PIN or passphrase used to unlock the private key(s) on the device. When this message is received, and if the agent supports smartcard-hosted keys, it will delete all keys that are hosted on the specified smartcard that may be accessed with the given "pin". The agent will reply with a SSH_AGENT_SUCCESS if one or more keys have been successfully removed or a SSH_AGENT_FAILURE if an error occurred. The agent will also return SSH_AGENT_FAILURE if it does not support smartcards. 2.5 Requesting a list of known keys An agent may be requested to list which keys it holds. Different requests exist for protocol 1 and protocol 2 keys. 2.5.1 Requesting a list of protocol 1 keys To request a list of protocol 1 keys that are held in the agent, a client may send the following message: byte SSH_AGENTC_REQUEST_RSA_IDENTITIES The agent will reply with the following message: byte SSH_AGENT_RSA_IDENTITIES_ANSWER uint32 num_keys Followed by zero or more consecutive keys, encoded as: uint32 bits mpint1 rsa_e mpint1 rsa_n string key_comment 2.5.2 Requesting a list of protocol 2 keys A client may send the following message to request a list of protocol 2 keys that are stored in the agent: byte SSH2_AGENTC_REQUEST_IDENTITIES The agent will reply with the following message header: byte SSH2_AGENT_IDENTITIES_ANSWER uint32 num_keys Followed by zero or more consecutive keys, encoded as: string key_blob string key_comment Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key Algorithms" for any of the supported protocol 2 key types. 2.6 Private key operations The purpose of the agent is to perform private key operations, such as signing and encryption without requiring a passphrase to unlock the key and without allowing the private key itself to be exposed. There are separate requests for the protocol 1 and protocol 2 private key operations. 2.6.1 Protocol 1 private key challenge The private key operation used in version 1 of the SSH protocol is decrypting a challenge that has been encrypted with a public key. It may be requested using this message: byte SSH_AGENTC_RSA_CHALLENGE uint32 ignored mpint1 rsa_e mpint1 rsa_n mpint1 encrypted_challenge byte[16] session_id uint32 response_type /* must be 1 */ "rsa_e" and "rsa_n" are used to identify which private key to use. "encrypted_challenge" is a challenge blob that has (presumably) been encrypted with the public key and must be in the range 1 <= encrypted_challenge < 2^256. "session_id" is the SSH protocol 1 session ID (computed from the server host key, the server semi-ephemeral key and the session cookie). "ignored" and "response_type" exist for compatibility with legacy implementations. "response_type" must be equal to 1; other response types are not supported. On receiving this request, the server decrypts the "encrypted_challenge" using the private key matching the supplied (rsa_e, rsa_n) values. For the response derivation, the decrypted challenge is represented as an unsigned, big-endian integer encoded in a 32 byte buffer (i.e. values smaller than 2^248 will have leading 0 bytes). The response value is then calculated as: response = MD5(decrypted_challenge || session_id) and returned in the following message byte SSH_AGENT_RSA_RESPONSE byte[16] response If the agent cannot find the key specified by the supplied (rsa_e, rsa_n) then it will return SSH_AGENT_FAILURE. 2.6.2 Protocol 2 private key signature request A client may use the following message to request signing of data using a protocol 2 key: byte SSH2_AGENTC_SIGN_REQUEST string key_blob string data uint32 flags Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key Algorithms" for any of the supported protocol 2 key types. "flags" is a bit-mask, but at present only one possible value is defined (see below for its meaning): SSH_AGENT_OLD_SIGNATURE 1 Upon receiving this request, the agent will look up the private key that corresponds to the public key contained in key_blob. It will use this private key to sign the "data" and produce a signature blob using the key type-specific method described in RFC 4253 section 6.6 "Public Key Algorithms". An exception to this is for "ssh-dss" keys where the "flags" word contains the value SSH_AGENT_OLD_SIGNATURE. In this case, a legacy signature encoding is used in lieu of the standard one. In this case, the DSA signature blob is encoded as: byte[40] signature The signature will be returned in the response message: byte SSH2_AGENT_SIGN_RESPONSE string signature_blob If the agent cannot find the key specified by the supplied key_blob then it will return SSH_AGENT_FAILURE. 2.7 Locking or unlocking an agent The agent supports temporary locking with a passphrase to suspend processing of sensitive operations until it has been unlocked with the same passphrase. To lock an agent, a client send the following request: byte SSH_AGENTC_LOCK string passphrase Upon receipt of this message and if the agent is not already locked, it will suspend processing requests and return a SSH_AGENT_SUCCESS reply. If the agent is already locked, it will return SSH_AGENT_FAILURE. While locked, the agent will refuse all requests except SSH_AGENTC_UNLOCK, SSH_AGENTC_REQUEST_RSA_IDENTITIES and SSH2_AGENTC_REQUEST_IDENTITIES. The "request identities" requests are treated specially by a locked agent: it will always return an empty list of keys. To unlock an agent, a client may request: byte SSH_AGENTC_UNLOCK string passphrase If the passphrase matches and the agent is locked, then it will resume processing all requests and return SSH_AGENT_SUCCESS. If the agent is not locked or the passphrase does not match then it will return SSH_AGENT_FAILURE. Locking and unlocking affects both protocol 1 and protocol 2 keys. 3. Protocol message numbers 3.1 Requests from client to agent for protocol 1 key operations SSH_AGENTC_REQUEST_RSA_IDENTITIES 1 SSH_AGENTC_RSA_CHALLENGE 3 SSH_AGENTC_ADD_RSA_IDENTITY 7 SSH_AGENTC_REMOVE_RSA_IDENTITY 8 SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES 9 SSH_AGENTC_ADD_RSA_ID_CONSTRAINED 24 3.2 Requests from client to agent for protocol 2 key operations SSH2_AGENTC_REQUEST_IDENTITIES 11 SSH2_AGENTC_SIGN_REQUEST 13 SSH2_AGENTC_ADD_IDENTITY 17 SSH2_AGENTC_REMOVE_IDENTITY 18 SSH2_AGENTC_REMOVE_ALL_IDENTITIES 19 SSH2_AGENTC_ADD_ID_CONSTRAINED 25 3.3 Key-type independent requests from client to agent SSH_AGENTC_ADD_SMARTCARD_KEY 20 SSH_AGENTC_REMOVE_SMARTCARD_KEY 21 SSH_AGENTC_LOCK 22 SSH_AGENTC_UNLOCK 23 SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED 26 3.4 Generic replies from agent to client SSH_AGENT_FAILURE 5 SSH_AGENT_SUCCESS 6 3.5 Replies from agent to client for protocol 1 key operations SSH_AGENT_RSA_IDENTITIES_ANSWER 2 SSH_AGENT_RSA_RESPONSE 4 3.6 Replies from agent to client for protocol 2 key operations SSH2_AGENT_IDENTITIES_ANSWER 12 SSH2_AGENT_SIGN_RESPONSE 14 3.7 Key constraint identifiers SSH_AGENT_CONSTRAIN_LIFETIME 1 SSH_AGENT_CONSTRAIN_CONFIRM 2 $OpenBSD: PROTOCOL.agent,v 1.11 2016/05/19 07:45:32 djm Exp $
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