PROTOCOL.agent revision 323121
1This describes the protocol used by OpenSSH's ssh-agent. 2 3OpenSSH's agent supports managing keys for the standard SSH protocol 42 as well as the legacy SSH protocol 1. Support for these key types 5is almost completely disjoint - in all but a few cases, operations on 6protocol 2 keys cannot see or affect protocol 1 keys and vice-versa. 7 8Protocol 1 and protocol 2 keys are separated because of the differing 9cryptographic usage: protocol 1 private RSA keys are used to decrypt 10challenges that were encrypted with the corresponding public key, 11whereas protocol 2 RSA private keys are used to sign challenges with 12a private key for verification with the corresponding public key. It 13is considered unsound practice to use the same key for signing and 14encryption. 15 16With a couple of exceptions, the protocol message names used in this 17document indicate which type of key the message relates to. SSH_* 18messages refer to protocol 1 keys only. SSH2_* messages refer to 19protocol 2 keys. Furthermore, the names also indicate whether the 20message is a request to the agent (*_AGENTC_*) or a reply from the 21agent (*_AGENT_*). Section 3 below contains the mapping of the 22protocol message names to their integer values. 23 241. Data types 25 26Because of support for legacy SSH protocol 1 keys, OpenSSH's agent 27protocol makes use of some data types not defined in RFC 4251. 28 291.1 uint16 30 31The "uint16" data type is a simple MSB-first 16 bit unsigned integer 32encoded in two bytes. 33 341.2 mpint1 35 36The "mpint1" type represents an arbitrary precision integer (bignum). 37Its format is as follows: 38 39 uint16 bits 40 byte[(bits + 7) / 8] bignum 41 42"bignum" contains an unsigned arbitrary precision integer encoded as 43eight bits per byte in big-endian (MSB first) format. 44 45Note the difference between the "mpint1" encoding and the "mpint" 46encoding defined in RFC 4251. Also note that the length of the encoded 47integer is specified in bits, not bytes and that the byte length of 48the integer must be calculated by rounding up the number of bits to the 49nearest eight. 50 512. Protocol Messages 52 53All protocol messages are prefixed with their length in bytes, encoded 54as a 32 bit unsigned integer. Specifically: 55 56 uint32 message_length 57 byte[message_length] message 58 59The following message descriptions refer only to the content the 60"message" field. 61 622.1 Generic server responses 63 64The following generic messages may be sent by the server in response to 65requests from the client. On success the agent may reply either with: 66 67 byte SSH_AGENT_SUCCESS 68 69or a request-specific success message. 70 71On failure, the agent may reply with: 72 73 byte SSH_AGENT_FAILURE 74 75SSH_AGENT_FAILURE messages are also sent in reply to unknown request 76types. 77 782.2 Adding keys to the agent 79 80Keys are added to the agent using the SSH_AGENTC_ADD_RSA_IDENTITY and 81SSH2_AGENTC_ADD_IDENTITY requests for protocol 1 and protocol 2 keys 82respectively. 83 84Two variants of these requests are SSH_AGENTC_ADD_RSA_ID_CONSTRAINED 85and SSH2_AGENTC_ADD_ID_CONSTRAINED - these add keys with optional 86"constraints" on their usage. 87 88OpenSSH may be built with support for keys hosted on a smartcard 89or other hardware security module. These keys may be added 90to the agent using the SSH_AGENTC_ADD_SMARTCARD_KEY and 91SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED requests. 92 932.2.1 Key constraints 94 95The OpenSSH agent supports some basic optional constraints on key usage. 96At present there are two constraints defined. 97 98The first constraint limits the validity duration of a key. It is 99encoded as: 100 101 byte SSH_AGENT_CONSTRAIN_LIFETIME 102 uint32 seconds 103 104Where "seconds" contains the number of seconds that the key shall remain 105valid measured from the moment that the agent receives it. After the 106validity period has expired, OpenSSH's agent will erase these keys from 107memory. 108 109The second constraint requires the agent to seek explicit user 110confirmation before performing private key operations with the loaded 111key. This constraint is encoded as: 112 113 byte SSH_AGENT_CONSTRAIN_CONFIRM 114 115Zero or more constraints may be specified when adding a key with one 116of the *_CONSTRAINED requests. Multiple constraints are appended 117consecutively to the end of the request: 118 119 byte constraint1_type 120 .... constraint1_data 121 byte constraint2_type 122 .... constraint2_data 123 .... 124 byte constraintN_type 125 .... constraintN_data 126 127Such a sequence of zero or more constraints will be referred to below 128as "constraint[]". Agents may determine whether there are constraints 129by checking whether additional data exists in the "add key" request 130after the key data itself. OpenSSH will refuse to add a key if it 131contains unknown constraints. 132 1332.2.2 Add protocol 1 key 134 135A client may add a protocol 1 key to an agent with the following 136request: 137 138 byte SSH_AGENTC_ADD_RSA_IDENTITY or 139 SSH_AGENTC_ADD_RSA_ID_CONSTRAINED 140 uint32 ignored 141 mpint1 rsa_n 142 mpint1 rsa_e 143 mpint1 rsa_d 144 mpint1 rsa_iqmp 145 mpint1 rsa_q 146 mpint1 rsa_p 147 string key_comment 148 constraint[] key_constraints 149 150Note that there is some redundancy in the key parameters; a key could be 151fully specified using just rsa_q, rsa_p and rsa_e at the cost of extra 152computation. 153 154"key_constraints" may only be present if the request type is 155SSH_AGENTC_ADD_RSA_ID_CONSTRAINED. 156 157The agent will reply with a SSH_AGENT_SUCCESS if the key has been 158successfully added or a SSH_AGENT_FAILURE if an error occurred. 159 1602.2.3 Add protocol 2 key 161 162The OpenSSH agent supports DSA, ECDSA and RSA keys for protocol 2. DSA 163keys may be added using the following request 164 165 byte SSH2_AGENTC_ADD_IDENTITY or 166 SSH2_AGENTC_ADD_ID_CONSTRAINED 167 string "ssh-dss" 168 mpint dsa_p 169 mpint dsa_q 170 mpint dsa_g 171 mpint dsa_public_key 172 mpint dsa_private_key 173 string key_comment 174 constraint[] key_constraints 175 176DSA certificates may be added with: 177 byte SSH2_AGENTC_ADD_IDENTITY or 178 SSH2_AGENTC_ADD_ID_CONSTRAINED 179 string "ssh-dss-cert-v00@openssh.com" 180 string certificate 181 mpint dsa_private_key 182 string key_comment 183 constraint[] key_constraints 184 185ECDSA keys may be added using the following request 186 187 byte SSH2_AGENTC_ADD_IDENTITY or 188 SSH2_AGENTC_ADD_ID_CONSTRAINED 189 string "ecdsa-sha2-nistp256" | 190 "ecdsa-sha2-nistp384" | 191 "ecdsa-sha2-nistp521" 192 string ecdsa_curve_name 193 string ecdsa_public_key 194 mpint ecdsa_private 195 string key_comment 196 constraint[] key_constraints 197 198ECDSA certificates may be added with: 199 byte SSH2_AGENTC_ADD_IDENTITY or 200 SSH2_AGENTC_ADD_ID_CONSTRAINED 201 string "ecdsa-sha2-nistp256-cert-v01@openssh.com" | 202 "ecdsa-sha2-nistp384-cert-v01@openssh.com" | 203 "ecdsa-sha2-nistp521-cert-v01@openssh.com" 204 string certificate 205 mpint ecdsa_private_key 206 string key_comment 207 constraint[] key_constraints 208 209RSA keys may be added with this request: 210 211 byte SSH2_AGENTC_ADD_IDENTITY or 212 SSH2_AGENTC_ADD_ID_CONSTRAINED 213 string "ssh-rsa" 214 mpint rsa_n 215 mpint rsa_e 216 mpint rsa_d 217 mpint rsa_iqmp 218 mpint rsa_p 219 mpint rsa_q 220 string key_comment 221 constraint[] key_constraints 222 223RSA certificates may be added with this request: 224 225 byte SSH2_AGENTC_ADD_IDENTITY or 226 SSH2_AGENTC_ADD_ID_CONSTRAINED 227 string "ssh-rsa-cert-v00@openssh.com" 228 string certificate 229 mpint rsa_d 230 mpint rsa_iqmp 231 mpint rsa_p 232 mpint rsa_q 233 string key_comment 234 constraint[] key_constraints 235 236Note that the 'rsa_p' and 'rsa_q' parameters are sent in the reverse 237order to the protocol 1 add keys message. As with the corresponding 238protocol 1 "add key" request, the private key is overspecified to avoid 239redundant processing. 240 241For DSA, ECDSA and RSA key add requests, "key_constraints" may only be 242present if the request type is SSH2_AGENTC_ADD_ID_CONSTRAINED. 243 244The agent will reply with a SSH_AGENT_SUCCESS if the key has been 245successfully added or a SSH_AGENT_FAILURE if an error occurred. 246 2472.2.4 Loading keys from a smartcard 248 249The OpenSSH agent may have optional smartcard support built in to it. If 250so, it supports an operation to load keys from a smartcard. Technically, 251only the public components of the keys are loaded into the agent so 252this operation really arranges for future private key operations to be 253delegated to the smartcard. 254 255 byte SSH_AGENTC_ADD_SMARTCARD_KEY or 256 SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED 257 string reader_id 258 string pin 259 constraint[] key_constraints 260 261"reader_id" is an identifier to a smartcard reader and "pin" 262is a PIN or passphrase used to unlock the private key(s) on the 263device. "key_constraints" may only be present if the request type is 264SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED. 265 266This operation may load all SSH keys that are unlocked using the 267"pin" on the specified reader. The type of key loaded (protocol 1 268or protocol 2) will be specified by the smartcard itself, it is not 269client-specified. 270 271The agent will reply with a SSH_AGENT_SUCCESS if one or more keys have 272been successfully loaded or a SSH_AGENT_FAILURE if an error occurred. 273The agent will also return SSH_AGENT_FAILURE if it does not support 274smartcards. 275 2762.3 Removing multiple keys 277 278A client may request that an agent delete all protocol 1 keys using the 279following request: 280 281 byte SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES 282 283This message requests the deletion of all protocol 2 keys: 284 285 byte SSH2_AGENTC_REMOVE_ALL_IDENTITIES 286 287On success, the agent will delete all keys of the requested type and 288reply with a SSH_AGENT_SUCCESS message. If an error occurred, the agent 289will reply with SSH_AGENT_FAILURE. 290 291Note that, to delete all keys (both protocol 1 and 2), a client 292must send both a SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES and a 293SSH2_AGENTC_REMOVE_ALL_IDENTITIES request. 294 2952.4 Removing specific keys 296 2972.4.1 Removing a protocol 1 key 298 299Removal of a protocol 1 key may be requested with the following message: 300 301 byte SSH_AGENTC_REMOVE_RSA_IDENTITY 302 uint32 key_bits 303 mpint1 rsa_e 304 mpint1 rsa_n 305 306Note that key_bits is strictly redundant, as it may be inferred by the 307length of rsa_n. 308 309The agent will delete any private key matching the specified public key 310and return SSH_AGENT_SUCCESS. If no such key was found, the agent will 311return SSH_AGENT_FAILURE. 312 3132.4.2 Removing a protocol 2 key 314 315Protocol 2 keys may be removed with the following request: 316 317 byte SSH2_AGENTC_REMOVE_IDENTITY 318 string key_blob 319 320Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key 321Algorithms" for any of the supported protocol 2 key types. 322 323The agent will delete any private key matching the specified public key 324and return SSH_AGENT_SUCCESS. If no such key was found, the agent will 325return SSH_AGENT_FAILURE. 326 3272.4.3 Removing keys loaded from a smartcard 328 329A client may request that a server remove one or more smartcard-hosted 330keys using this message: 331 332 byte SSH_AGENTC_REMOVE_SMARTCARD_KEY 333 string reader_id 334 string pin 335 336"reader_id" the an identifier to a smartcard reader and "pin" is a PIN 337or passphrase used to unlock the private key(s) on the device. 338 339When this message is received, and if the agent supports 340smartcard-hosted keys, it will delete all keys that are hosted on the 341specified smartcard that may be accessed with the given "pin". 342 343The agent will reply with a SSH_AGENT_SUCCESS if one or more keys have 344been successfully removed or a SSH_AGENT_FAILURE if an error occurred. 345The agent will also return SSH_AGENT_FAILURE if it does not support 346smartcards. 347 3482.5 Requesting a list of known keys 349 350An agent may be requested to list which keys it holds. Different 351requests exist for protocol 1 and protocol 2 keys. 352 3532.5.1 Requesting a list of protocol 1 keys 354 355To request a list of protocol 1 keys that are held in the agent, a 356client may send the following message: 357 358 byte SSH_AGENTC_REQUEST_RSA_IDENTITIES 359 360The agent will reply with the following message: 361 362 byte SSH_AGENT_RSA_IDENTITIES_ANSWER 363 uint32 num_keys 364 365Followed by zero or more consecutive keys, encoded as: 366 367 uint32 bits 368 mpint1 rsa_e 369 mpint1 rsa_n 370 string key_comment 371 3722.5.2 Requesting a list of protocol 2 keys 373 374A client may send the following message to request a list of 375protocol 2 keys that are stored in the agent: 376 377 byte SSH2_AGENTC_REQUEST_IDENTITIES 378 379The agent will reply with the following message header: 380 381 byte SSH2_AGENT_IDENTITIES_ANSWER 382 uint32 num_keys 383 384Followed by zero or more consecutive keys, encoded as: 385 386 string key_blob 387 string key_comment 388 389Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key 390Algorithms" for any of the supported protocol 2 key types. 391 3922.6 Private key operations 393 394The purpose of the agent is to perform private key operations, such as 395signing and encryption without requiring a passphrase to unlock the 396key and without allowing the private key itself to be exposed. There 397are separate requests for the protocol 1 and protocol 2 private key 398operations. 399 4002.6.1 Protocol 1 private key challenge 401 402The private key operation used in version 1 of the SSH protocol is 403decrypting a challenge that has been encrypted with a public key. 404It may be requested using this message: 405 406 byte SSH_AGENTC_RSA_CHALLENGE 407 uint32 ignored 408 mpint1 rsa_e 409 mpint1 rsa_n 410 mpint1 encrypted_challenge 411 byte[16] session_id 412 uint32 response_type /* must be 1 */ 413 414"rsa_e" and "rsa_n" are used to identify which private key to use. 415"encrypted_challenge" is a challenge blob that has (presumably) 416been encrypted with the public key and must be in the range 4171 <= encrypted_challenge < 2^256. "session_id" is the SSH protocol 1 418session ID (computed from the server host key, the server semi-ephemeral 419key and the session cookie). 420 421"ignored" and "response_type" exist for compatibility with legacy 422implementations. "response_type" must be equal to 1; other response 423types are not supported. 424 425On receiving this request, the server decrypts the "encrypted_challenge" 426using the private key matching the supplied (rsa_e, rsa_n) values. For 427the response derivation, the decrypted challenge is represented as an 428unsigned, big-endian integer encoded in a 32 byte buffer (i.e. values 429smaller than 2^248 will have leading 0 bytes). 430 431The response value is then calculated as: 432 433 response = MD5(decrypted_challenge || session_id) 434 435and returned in the following message 436 437 byte SSH_AGENT_RSA_RESPONSE 438 byte[16] response 439 440If the agent cannot find the key specified by the supplied (rsa_e, 441rsa_n) then it will return SSH_AGENT_FAILURE. 442 4432.6.2 Protocol 2 private key signature request 444 445A client may use the following message to request signing of data using 446a protocol 2 key: 447 448 byte SSH2_AGENTC_SIGN_REQUEST 449 string key_blob 450 string data 451 uint32 flags 452 453Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key 454Algorithms" for any of the supported protocol 2 key types. "flags" is 455a bit-mask, but at present only one possible value is defined (see below 456for its meaning): 457 458 SSH_AGENT_OLD_SIGNATURE 1 459 460Upon receiving this request, the agent will look up the private key that 461corresponds to the public key contained in key_blob. It will use this 462private key to sign the "data" and produce a signature blob using the 463key type-specific method described in RFC 4253 section 6.6 "Public Key 464Algorithms". 465 466An exception to this is for "ssh-dss" keys where the "flags" word 467contains the value SSH_AGENT_OLD_SIGNATURE. In this case, a legacy 468signature encoding is used in lieu of the standard one. In this case, 469the DSA signature blob is encoded as: 470 471 byte[40] signature 472 473The signature will be returned in the response message: 474 475 byte SSH2_AGENT_SIGN_RESPONSE 476 string signature_blob 477 478If the agent cannot find the key specified by the supplied key_blob then 479it will return SSH_AGENT_FAILURE. 480 4812.7 Locking or unlocking an agent 482 483The agent supports temporary locking with a passphrase to suspend 484processing of sensitive operations until it has been unlocked with the 485same passphrase. To lock an agent, a client send the following request: 486 487 byte SSH_AGENTC_LOCK 488 string passphrase 489 490Upon receipt of this message and if the agent is not already locked, 491it will suspend processing requests and return a SSH_AGENT_SUCCESS 492reply. If the agent is already locked, it will return SSH_AGENT_FAILURE. 493 494While locked, the agent will refuse all requests except 495SSH_AGENTC_UNLOCK, SSH_AGENTC_REQUEST_RSA_IDENTITIES and 496SSH2_AGENTC_REQUEST_IDENTITIES. The "request identities" requests are 497treated specially by a locked agent: it will always return an empty list 498of keys. 499 500To unlock an agent, a client may request: 501 502 byte SSH_AGENTC_UNLOCK 503 string passphrase 504 505If the passphrase matches and the agent is locked, then it will resume 506processing all requests and return SSH_AGENT_SUCCESS. If the agent 507is not locked or the passphrase does not match then it will return 508SSH_AGENT_FAILURE. 509 510Locking and unlocking affects both protocol 1 and protocol 2 keys. 511 5123. Protocol message numbers 513 5143.1 Requests from client to agent for protocol 1 key operations 515 516 SSH_AGENTC_REQUEST_RSA_IDENTITIES 1 517 SSH_AGENTC_RSA_CHALLENGE 3 518 SSH_AGENTC_ADD_RSA_IDENTITY 7 519 SSH_AGENTC_REMOVE_RSA_IDENTITY 8 520 SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES 9 521 SSH_AGENTC_ADD_RSA_ID_CONSTRAINED 24 522 5233.2 Requests from client to agent for protocol 2 key operations 524 525 SSH2_AGENTC_REQUEST_IDENTITIES 11 526 SSH2_AGENTC_SIGN_REQUEST 13 527 SSH2_AGENTC_ADD_IDENTITY 17 528 SSH2_AGENTC_REMOVE_IDENTITY 18 529 SSH2_AGENTC_REMOVE_ALL_IDENTITIES 19 530 SSH2_AGENTC_ADD_ID_CONSTRAINED 25 531 5323.3 Key-type independent requests from client to agent 533 534 SSH_AGENTC_ADD_SMARTCARD_KEY 20 535 SSH_AGENTC_REMOVE_SMARTCARD_KEY 21 536 SSH_AGENTC_LOCK 22 537 SSH_AGENTC_UNLOCK 23 538 SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED 26 539 5403.4 Generic replies from agent to client 541 542 SSH_AGENT_FAILURE 5 543 SSH_AGENT_SUCCESS 6 544 5453.5 Replies from agent to client for protocol 1 key operations 546 547 SSH_AGENT_RSA_IDENTITIES_ANSWER 2 548 SSH_AGENT_RSA_RESPONSE 4 549 5503.6 Replies from agent to client for protocol 2 key operations 551 552 SSH2_AGENT_IDENTITIES_ANSWER 12 553 SSH2_AGENT_SIGN_RESPONSE 14 554 5553.7 Key constraint identifiers 556 557 SSH_AGENT_CONSTRAIN_LIFETIME 1 558 SSH_AGENT_CONSTRAIN_CONFIRM 2 559 560$OpenBSD: PROTOCOL.agent,v 1.8 2015/05/08 03:56:51 djm Exp $ 561