Annotation of src/usr.bin/ssh/PROTOCOL.agent, Revision 1.5
1.1 djm 1: This describes the protocol used by OpenSSH's ssh-agent.
2:
3: OpenSSH's agent supports managing keys for the standard SSH protocol
4: 2 as well as the legacy SSH protocol 1. Support for these key types
5: is almost completely disjoint - in all but a few cases, operations on
6: protocol 2 keys cannot see or affect protocol 1 keys and vice-versa.
7:
8: Protocol 1 and protocol 2 keys are separated because of the differing
9: cryptographic usage: protocol 1 private RSA keys are used to decrypt
10: challenges that were encrypted with the corresponding public key,
11: whereas protocol 2 RSA private keys are used to sign challenges with
12: a private key for verification with the corresponding public key. It
13: is considered unsound practice to use the same key for signing and
14: encryption.
15:
16: With a couple of exceptions, the protocol message names used in this
17: document indicate which type of key the message relates to. SSH_*
18: messages refer to protocol 1 keys only. SSH2_* messages refer to
1.4 stevesk 19: protocol 2 keys. Furthermore, the names also indicate whether the
20: message is a request to the agent (*_AGENTC_*) or a reply from the
21: agent (*_AGENT_*). Section 3 below contains the mapping of the
22: protocol message names to their integer values.
1.1 djm 23:
24: 1. Data types
25:
1.4 stevesk 26: Because of support for legacy SSH protocol 1 keys, OpenSSH's agent
1.1 djm 27: protocol makes use of some data types not defined in RFC 4251.
28:
29: 1.1 uint16
30:
31: The "uint16" data type is a simple MSB-first 16 bit unsigned integer
32: encoded in two bytes.
33:
34: 1.2 mpint1
35:
36: The "mpint1" type represents an arbitrary precision integer (bignum).
37: Its 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
43: eight bits per byte in big-endian (MSB first) format.
44:
1.4 stevesk 45: Note the difference between the "mpint1" encoding and the "mpint"
1.1 djm 46: encoding defined in RFC 4251. Also note that the length of the encoded
1.4 stevesk 47: integer is specified in bits, not bytes and that the byte length of
1.1 djm 48: the integer must be calculated by rounding up the number of bits to the
49: nearest eight.
50:
51: 2. Protocol Messages
52:
53: All protocol messages are prefixed with their length in bytes, encoded
54: as a 32 bit unsigned integer. Specifically:
55:
56: uint32 message_length
57: byte[message_length] message
58:
1.4 stevesk 59: The following message descriptions refer only to the content the
1.1 djm 60: "message" field.
61:
62: 2.1 Generic server responses
63:
64: The following generic messages may be sent by the server in response to
65: requests from the client. On success the agent may reply either with:
66:
67: byte SSH_AGENT_SUCCESS
68:
69: or a request-specific success message.
70:
71: On failure, the agent may reply with:
72:
73: byte SSH_AGENT_FAILURE
74:
75: SSH_AGENT_FAILURE messages are also sent in reply to unknown request
76: types.
77:
78: 2.2 Adding keys to the agent
79:
80: Keys are added to the agent using the SSH_AGENTC_ADD_RSA_IDENTITY and
81: SSH2_AGENTC_ADD_IDENTITY requests for protocol 1 and protocol 2 keys
82: respectively.
83:
84: Two variants of these requests are SSH_AGENTC_ADD_RSA_ID_CONSTRAINED
85: and SSH2_AGENTC_ADD_ID_CONSTRAINED - these add keys with optional
86: "constraints" on their usage.
87:
88: OpenSSH may be built with support for keys hosted on a smartcard
1.4 stevesk 89: or other hardware security module. These keys may be added
1.1 djm 90: to the agent using the SSH_AGENTC_ADD_SMARTCARD_KEY and
1.4 stevesk 91: SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED requests.
1.1 djm 92:
93: 2.2.1 Key constraints
94:
95: The OpenSSH agent supports some basic optional constraints on key usage.
96: At present there are two constraints defined.
97:
98: The first constraint limits the validity duration of a key. It is
99: encoded as:
100:
101: byte SSH_AGENT_CONSTRAIN_LIFETIME
102: uint32 seconds
103:
104: Where "seconds" contains the number of seconds that the key shall remain
105: valid measured from the moment that the agent receives it. After the
106: validity period has expired, OpenSSH's agent will erase these keys from
107: memory.
108:
109: The second constraint requires the agent to seek explicit user
110: confirmation before performing private key operations with the loaded
111: key. This constraint is encoded as:
112:
113: byte SSH_AGENT_CONSTRAIN_CONFIRM
114:
115: Zero or more constraints may be specified when adding a key with one
116: of the *_CONSTRAINED requests. Multiple constraints are appended
117: consecutively to the end of the request:
118:
119: byte constraint1_type
1.3 djm 120: .... constraint1_data
1.1 djm 121: byte constraint2_type
1.3 djm 122: .... constraint2_data
1.1 djm 123: ....
124: byte constraintN_type
1.3 djm 125: .... constraintN_data
1.1 djm 126:
127: Such a sequence of zero or more constraints will be referred to below
128: as "constraint[]". Agents may determine whether there are constraints
1.4 stevesk 129: by checking whether additional data exists in the "add key" request
1.1 djm 130: after the key data itself. OpenSSH will refuse to add a key if it
131: contains unknown constraints.
132:
133: 2.2.2 Add protocol 1 key
134:
135: A client may add a protocol 1 key to an agent with the following
136: request:
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:
150: Note that there is some redundancy in the key parameters; a key could be
151: fully specified using just rsa_q, rsa_p and rsa_e at the cost of extra
152: computation.
153:
154: "key_constraints" may only be present if the request type is
155: SSH_AGENTC_ADD_RSA_IDENTITY.
156:
157: The agent will reply with a SSH_AGENT_SUCCESS if the key has been
158: successfully added or a SSH_AGENT_FAILURE if an error occurred.
159:
160: 2.2.3 Add protocol 2 key
161:
162: The OpenSSH agent supports DSA and RSA keys for protocol 2. DSA keys may
163: 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:
1.5 ! djm 176: DSA 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:
1.1 djm 185: RSA keys may be added with this request:
186:
187: byte SSH2_AGENTC_ADD_IDENTITY or
188: SSH2_AGENTC_ADD_ID_CONSTRAINED
189: string "ssh-rsa"
190: mpint rsa_n
191: mpint rsa_e
192: mpint rsa_d
193: mpint rsa_iqmp
194: mpint rsa_p
195: mpint rsa_q
196: string key_comment
197: constraint[] key_constraints
198:
1.5 ! djm 199: RSA certificates may be added with this request:
! 200:
! 201: byte SSH2_AGENTC_ADD_IDENTITY or
! 202: SSH2_AGENTC_ADD_ID_CONSTRAINED
! 203: string "ssh-rsa-cert-v00@openssh.com"
! 204: string certificate
! 205: mpint rsa_d
! 206: mpint rsa_iqmp
! 207: mpint rsa_p
! 208: mpint rsa_q
! 209: string key_comment
! 210: constraint[] key_constraints
! 211:
1.4 stevesk 212: Note that the 'rsa_p' and 'rsa_q' parameters are sent in the reverse
1.1 djm 213: order to the protocol 1 add keys message. As with the corresponding
214: protocol 1 "add key" request, the private key is overspecified to avoid
215: redundant processing.
216:
217: For both DSA and RSA key add requests, "key_constraints" may only be
218: present if the request type is SSH2_AGENTC_ADD_ID_CONSTRAINED.
219:
220: The agent will reply with a SSH_AGENT_SUCCESS if the key has been
221: successfully added or a SSH_AGENT_FAILURE if an error occurred.
222:
223: 2.2.4 Loading keys from a smartcard
224:
225: The OpenSSH agent may have optional smartcard support built in to it. If
226: so, it supports an operation to load keys from a smartcard. Technically,
227: only the public components of the keys are loaded into the agent so
228: this operation really arranges for future private key operations to be
229: delegated to the smartcard.
230:
231: byte SSH_AGENTC_ADD_SMARTCARD_KEY or
232: SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED
233: string reader_id
234: string pin
235: constraint[] key_constraints
236:
1.4 stevesk 237: "reader_id" is an identifier to a smartcard reader and "pin"
1.1 djm 238: is a PIN or passphrase used to unlock the private key(s) on the
239: device. "key_constraints" may only be present if the request type is
240: SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED.
241:
242: This operation may load all SSH keys that are unlocked using the
243: "pin" on the specified reader. The type of key loaded (protocol 1
244: or protocol 2) will be specified by the smartcard itself, it is not
245: client-specified.
246:
247: The agent will reply with a SSH_AGENT_SUCCESS if one or more keys have
248: been successfully loaded or a SSH_AGENT_FAILURE if an error occurred.
249: The agent will also return SSH_AGENT_FAILURE if it does not support
250: smartcards.
251:
252: 2.3 Removing multiple keys
253:
254: A client may request that an agent delete all protocol 1 keys using the
255: following request:
256:
257: byte SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES
258:
259: This message requests the deletion of all protocol 2 keys:
260:
261: byte SSH2_AGENTC_REMOVE_ALL_IDENTITIES
262:
263: On success, the agent will delete all keys of the requested type and
264: reply with a SSH_AGENT_SUCCESS message. If an error occurred, the agent
265: will reply with SSH_AGENT_FAILURE.
266:
267: Note that, to delete all keys (both protocol 1 and 2), a client
268: must send both a SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES and a
269: SSH2_AGENTC_REMOVE_ALL_IDENTITIES request.
270:
271: 2.4 Removing specific keys
272:
273: 2.4.1 Removing a protocol 1 key
274:
275: Removal of a protocol 1 key may be requested with the following message:
276:
277: byte SSH_AGENTC_REMOVE_RSA_IDENTITY
278: uint32 key_bits
279: mpint1 rsa_e
280: mpint1 rsa_n
281:
282: Note that key_bits is strictly redundant, as it may be inferred by the
283: length of rsa_n.
284:
285: The agent will delete any private key matching the specified public key
286: and return SSH_AGENT_SUCCESS. If no such key was found, the agent will
287: return SSH_AGENT_FAILURE.
288:
289: 2.4.2 Removing a protocol 2 key
290:
291: Protocol 2 keys may be removed with the following request:
292:
293: byte SSH2_AGENTC_REMOVE_IDENTITY
1.2 djm 294: string key_blob
1.1 djm 295:
296: Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key
297: Algorithms" for either of the supported key types: "ssh-dss" or
298: "ssh-rsa".
299:
300: The agent will delete any private key matching the specified public key
301: and return SSH_AGENT_SUCCESS. If no such key was found, the agent will
302: return SSH_AGENT_FAILURE.
303:
304: 2.4.3 Removing keys loaded from a smartcard
305:
306: A client may request that a server remove one or more smartcard-hosted
307: keys using this message:
308:
309: byte SSH_AGENTC_REMOVE_SMARTCARD_KEY
310: string reader_id
311: string pin
312:
313: "reader_id" the an identifier to a smartcard reader and "pin" is a PIN
314: or passphrase used to unlock the private key(s) on the device.
315:
316: When this message is received, and if the agent supports
317: smartcard-hosted keys, it will delete all keys that are hosted on the
318: specified smartcard that may be accessed with the given "pin".
319:
320: The agent will reply with a SSH_AGENT_SUCCESS if one or more keys have
321: been successfully removed or a SSH_AGENT_FAILURE if an error occurred.
322: The agent will also return SSH_AGENT_FAILURE if it does not support
323: smartcards.
324:
325: 2.5 Requesting a list of known keys
326:
327: An agent may be requested to list which keys it holds. Different
328: requests exist for protocol 1 and protocol 2 keys.
329:
330: 2.5.1 Requesting a list of protocol 1 keys
331:
332: To request a list of protocol 1 keys that are held in the agent, a
333: client may send the following message:
334:
335: byte SSH_AGENTC_REQUEST_RSA_IDENTITIES
336:
337: The agent will reply with the following message:
338:
339: byte SSH_AGENT_RSA_IDENTITIES_ANSWER
340: uint32 num_keys
341:
342: Followed by zero or more consecutive keys, encoded as:
343:
344: uint32 bits
345: mpint1 rsa_e
346: mpint1 rsa_n
347: string key_comment
348:
349: 2.5.2 Requesting a list of protocol 2 keys
350:
1.4 stevesk 351: A client may send the following message to request a list of
1.1 djm 352: protocol 2 keys that are stored in the agent:
353:
354: byte SSH2_AGENTC_REQUEST_IDENTITIES
355:
356: The agent will reply with the following message header:
357:
358: byte SSH2_AGENT_IDENTITIES_ANSWER
359: uint32 num_keys
360:
361: Followed by zero or more consecutive keys, encoded as:
362:
1.2 djm 363: string key_blob
1.1 djm 364: string key_comment
365:
366: Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key
367: Algorithms" for either of the supported key types: "ssh-dss" or
368: "ssh-rsa".
369:
370: 2.6 Private key operations
371:
372: The purpose of the agent is to perform private key operations, such as
373: signing and encryption without requiring a passphrase to unlock the
374: key and without allowing the private key itself to be exposed. There
375: are separate requests for the protocol 1 and protocol 2 private key
376: operations.
377:
378: 2.6.1 Protocol 1 private key challenge
379:
380: The private key operation used in version 1 of the SSH protocol is
381: decrypting a challenge that has been encrypted with a public key.
382: It may be requested using this message:
383:
384: byte SSH_AGENTC_RSA_CHALLENGE
385: uint32 ignored
386: mpint1 rsa_e
387: mpint1 rsa_n
388: mpint1 encrypted_challenge
389: byte[16] session_id
390: uint32 response_type /* must be 1 */
391:
392: "rsa_e" and "rsa_n" are used to identify which private key to use.
393: "encrypted_challenge" is a challenge blob that has (presumably)
394: been encrypted with the public key and must be in the range
395: 1 <= encrypted_challenge < 2^256. "session_id" is the SSH protocol 1
396: session ID (computed from the server host key, the server semi-ephemeral
1.4 stevesk 397: key and the session cookie).
1.1 djm 398:
399: "ignored" and "response_type" exist for compatibility with legacy
400: implementations. "response_type" must be equal to 1; other response
401: types are not supported.
402:
403: On receiving this request, the server decrypts the "encrypted_challenge"
1.4 stevesk 404: using the private key matching the supplied (rsa_e, rsa_n) values. For
1.1 djm 405: the response derivation, the decrypted challenge is represented as an
406: unsigned, big-endian integer encoded in a 32 byte buffer (i.e. values
407: smaller than 2^248 will have leading 0 bytes).
408:
409: The response value is then calculated as:
410:
411: response = MD5(decrypted_challenge || session_id)
412:
413: and returned in the following message
414:
415: byte SSH_AGENT_RSA_RESPONSE
416: byte[16] response
417:
418: If the agent cannot find the key specified by the supplied (rsa_e,
419: rsa_n) then it will return SSH_AGENT_FAILURE.
420:
421: 2.6.2 Protocol 2 private key signature request
422:
423: A client may use the following message to request signing of data using
424: a protocol 2 key:
425:
426: byte SSH2_AGENTC_SIGN_REQUEST
427: string key_blob
428: string data
429: uint32 flags
430:
431: Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key
432: Algorithms" for either of the supported key types: "ssh-dss" or
433: "ssh-rsa". "flags" is a bit-mask, but at present only one possible value
434: is defined (see below for its meaning):
435:
436: SSH_AGENT_OLD_SIGNATURE 1
437:
438: Upon receiving this request, the agent will look up the private key that
439: corresponds to the public key contained in key_blob. It will use this
440: private key to sign the "data" and produce a signature blob using the
441: key type-specific method described in RFC 4253 section 6.6 "Public Key
442: Algorithms".
443:
444: An exception to this is for "ssh-dss" keys where the "flags" word
445: contains the value SSH_AGENT_OLD_SIGNATURE. In this case, a legacy
446: signature encoding is used in lieu of the standard one. In this case,
447: the DSA signature blob is encoded as:
448:
449: byte[40] signature
450:
451: The signature will be returned in the response message:
452:
453: byte SSH2_AGENT_SIGN_RESPONSE
454: string signature_blob
455:
456: If the agent cannot find the key specified by the supplied key_blob then
457: it will return SSH_AGENT_FAILURE.
458:
459: 2.7 Locking or unlocking an agent
460:
461: The agent supports temporary locking with a passphrase to suspend
462: processing of sensitive operations until it has been unlocked with the
463: same passphrase. To lock an agent, a client send the following request:
464:
465: byte SSH_AGENTC_LOCK
466: string passphrase
467:
468: Upon receipt of this message and if the agent is not already locked,
469: it will suspend processing requests and return a SSH_AGENT_SUCCESS
470: reply. If the agent is already locked, it will return SSH_AGENT_FAILURE.
471:
472: While locked, the agent will refuse all requests except
473: SSH_AGENTC_UNLOCK, SSH_AGENTC_REQUEST_RSA_IDENTITIES and
474: SSH2_AGENTC_REQUEST_IDENTITIES. The "request identities" requests are
475: treated specially by a locked agent: it will always return an empty list
476: of keys.
477:
478: To unlock an agent, a client may request:
479:
480: byte SSH_AGENTC_UNLOCK
481: string passphrase
482:
483: If the passphrase matches and the agent is locked, then it will resume
484: processing all requests and return SSH_AGENT_SUCCESS. If the agent
485: is not locked or the passphrase does not match then it will return
486: SSH_AGENT_FAILURE.
487:
488: Locking and unlocking affects both protocol 1 and protocol 2 keys.
489:
490: 3. Protocol message numbers
491:
492: 3.1 Requests from client to agent for protocol 1 key operations
493:
494: SSH_AGENTC_REQUEST_RSA_IDENTITIES 1
495: SSH_AGENTC_RSA_CHALLENGE 3
496: SSH_AGENTC_ADD_RSA_IDENTITY 7
497: SSH_AGENTC_REMOVE_RSA_IDENTITY 8
498: SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES 9
499: SSH_AGENTC_ADD_RSA_ID_CONSTRAINED 24
500:
501: 3.2 Requests from client to agent for protocol 2 key operations
502:
503: SSH2_AGENTC_REQUEST_IDENTITIES 11
504: SSH2_AGENTC_SIGN_REQUEST 13
505: SSH2_AGENTC_ADD_IDENTITY 17
506: SSH2_AGENTC_REMOVE_IDENTITY 18
507: SSH2_AGENTC_REMOVE_ALL_IDENTITIES 19
508: SSH2_AGENTC_ADD_ID_CONSTRAINED 25
509:
510: 3.3 Key-type independent requests from client to agent
511:
512: SSH_AGENTC_ADD_SMARTCARD_KEY 20
513: SSH_AGENTC_REMOVE_SMARTCARD_KEY 21
514: SSH_AGENTC_LOCK 22
515: SSH_AGENTC_UNLOCK 23
516: SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED 26
517:
518: 3.4 Generic replies from agent to client
519:
520: SSH_AGENT_FAILURE 5
521: SSH_AGENT_SUCCESS 6
522:
523: 3.5 Replies from agent to client for protocol 1 key operations
524:
525: SSH_AGENT_RSA_IDENTITIES_ANSWER 2
526: SSH_AGENT_RSA_RESPONSE 4
527:
528: 3.6 Replies from agent to client for protocol 2 key operations
529:
530: SSH2_AGENT_IDENTITIES_ANSWER 12
531: SSH2_AGENT_SIGN_RESPONSE 14
532:
533: 3.7 Key constraint identifiers
534:
535: SSH_AGENT_CONSTRAIN_LIFETIME 1
536: SSH_AGENT_CONSTRAIN_CONFIRM 2
537:
1.5 ! djm 538: $OpenBSD: PROTOCOL.agent,v 1.4 2008/07/01 23:12:47 stevesk Exp $
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