Annotation of src/usr.bin/ssh/PROTOCOL.agent, Revision 1.2
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
19: protocol 2 keys. Furthermore, the names also indicate whether message
20: is a request to the agent (*_AGENTC_*) or a reply from the agent
21: (*_AGENT_*). Section 3 below contains the mapping of the protocol
22: message names to their integer values.
23:
24: 1. Data types
25:
26: Because of it support for legacy SSH protocol 1 keys, OpenSSH's agent
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:
45: Note the difference between the "mpint1" encoding an the the "mpint"
46: encoding defined in RFC 4251. Also note that the length of the encoded
47: integer is specified in bits, not bytes and that the byte length of of
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:
59: The following message description refer only to the content the
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
89: or other hardware security module. These keys may added
90: to the agent using the SSH_AGENTC_ADD_SMARTCARD_KEY and
91: SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED requests
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
120: .... constraint1_date
121: byte constraint2_type
122: .... constraint2_date
123: ....
124: byte constraintN_type
125: .... constraintN_date
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
129: by checking whether additional data exists in the an "add key" request
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:
176: RSA keys may be added with this request:
177:
178: byte SSH2_AGENTC_ADD_IDENTITY or
179: SSH2_AGENTC_ADD_ID_CONSTRAINED
180: string "ssh-rsa"
181: mpint rsa_n
182: mpint rsa_e
183: mpint rsa_d
184: mpint rsa_iqmp
185: mpint rsa_p
186: mpint rsa_q
187: string key_comment
188: constraint[] key_constraints
189:
190: Note that the 'rsa_p' and 'rsa_q' parameters are send in the reverse
191: order to the protocol 1 add keys message. As with the corresponding
192: protocol 1 "add key" request, the private key is overspecified to avoid
193: redundant processing.
194:
195: For both DSA and RSA key add requests, "key_constraints" may only be
196: present if the request type is SSH2_AGENTC_ADD_ID_CONSTRAINED.
197:
198: The agent will reply with a SSH_AGENT_SUCCESS if the key has been
199: successfully added or a SSH_AGENT_FAILURE if an error occurred.
200:
201: 2.2.4 Loading keys from a smartcard
202:
203: The OpenSSH agent may have optional smartcard support built in to it. If
204: so, it supports an operation to load keys from a smartcard. Technically,
205: only the public components of the keys are loaded into the agent so
206: this operation really arranges for future private key operations to be
207: delegated to the smartcard.
208:
209: byte SSH_AGENTC_ADD_SMARTCARD_KEY or
210: SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED
211: string reader_id
212: string pin
213: constraint[] key_constraints
214:
215: "reader_id" the an identifier to a smartcard reader and "pin"
216: is a PIN or passphrase used to unlock the private key(s) on the
217: device. "key_constraints" may only be present if the request type is
218: SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED.
219:
220: This operation may load all SSH keys that are unlocked using the
221: "pin" on the specified reader. The type of key loaded (protocol 1
222: or protocol 2) will be specified by the smartcard itself, it is not
223: client-specified.
224:
225: The agent will reply with a SSH_AGENT_SUCCESS if one or more keys have
226: been successfully loaded or a SSH_AGENT_FAILURE if an error occurred.
227: The agent will also return SSH_AGENT_FAILURE if it does not support
228: smartcards.
229:
230: 2.3 Removing multiple keys
231:
232: A client may request that an agent delete all protocol 1 keys using the
233: following request:
234:
235: byte SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES
236:
237: This message requests the deletion of all protocol 2 keys:
238:
239: byte SSH2_AGENTC_REMOVE_ALL_IDENTITIES
240:
241: On success, the agent will delete all keys of the requested type and
242: reply with a SSH_AGENT_SUCCESS message. If an error occurred, the agent
243: will reply with SSH_AGENT_FAILURE.
244:
245: Note that, to delete all keys (both protocol 1 and 2), a client
246: must send both a SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES and a
247: SSH2_AGENTC_REMOVE_ALL_IDENTITIES request.
248:
249: 2.4 Removing specific keys
250:
251: 2.4.1 Removing a protocol 1 key
252:
253: Removal of a protocol 1 key may be requested with the following message:
254:
255: byte SSH_AGENTC_REMOVE_RSA_IDENTITY
256: uint32 key_bits
257: mpint1 rsa_e
258: mpint1 rsa_n
259:
260: Note that key_bits is strictly redundant, as it may be inferred by the
261: length of rsa_n.
262:
263: The agent will delete any private key matching the specified public key
264: and return SSH_AGENT_SUCCESS. If no such key was found, the agent will
265: return SSH_AGENT_FAILURE.
266:
267: 2.4.2 Removing a protocol 2 key
268:
269: Protocol 2 keys may be removed with the following request:
270:
271: byte SSH2_AGENTC_REMOVE_IDENTITY
1.2 ! djm 272: string key_blob
1.1 djm 273:
274: Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key
275: Algorithms" for either of the supported key types: "ssh-dss" or
276: "ssh-rsa".
277:
278: The agent will delete any private key matching the specified public key
279: and return SSH_AGENT_SUCCESS. If no such key was found, the agent will
280: return SSH_AGENT_FAILURE.
281:
282: 2.4.3 Removing keys loaded from a smartcard
283:
284: A client may request that a server remove one or more smartcard-hosted
285: keys using this message:
286:
287: byte SSH_AGENTC_REMOVE_SMARTCARD_KEY
288: string reader_id
289: string pin
290:
291: "reader_id" the an identifier to a smartcard reader and "pin" is a PIN
292: or passphrase used to unlock the private key(s) on the device.
293:
294: When this message is received, and if the agent supports
295: smartcard-hosted keys, it will delete all keys that are hosted on the
296: specified smartcard that may be accessed with the given "pin".
297:
298: The agent will reply with a SSH_AGENT_SUCCESS if one or more keys have
299: been successfully removed or a SSH_AGENT_FAILURE if an error occurred.
300: The agent will also return SSH_AGENT_FAILURE if it does not support
301: smartcards.
302:
303: 2.5 Requesting a list of known keys
304:
305: An agent may be requested to list which keys it holds. Different
306: requests exist for protocol 1 and protocol 2 keys.
307:
308: 2.5.1 Requesting a list of protocol 1 keys
309:
310: To request a list of protocol 1 keys that are held in the agent, a
311: client may send the following message:
312:
313: byte SSH_AGENTC_REQUEST_RSA_IDENTITIES
314:
315: The agent will reply with the following message:
316:
317: byte SSH_AGENT_RSA_IDENTITIES_ANSWER
318: uint32 num_keys
319:
320: Followed by zero or more consecutive keys, encoded as:
321:
322: uint32 bits
323: mpint1 rsa_e
324: mpint1 rsa_n
325: string key_comment
326:
327: 2.5.2 Requesting a list of protocol 2 keys
328:
329: A client may send the following message to request a list of keys
330: protocol 2 keys that are stored in the agent:
331:
332: byte SSH2_AGENTC_REQUEST_IDENTITIES
333:
334: The agent will reply with the following message header:
335:
336: byte SSH2_AGENT_IDENTITIES_ANSWER
337: uint32 num_keys
338:
339: Followed by zero or more consecutive keys, encoded as:
340:
1.2 ! djm 341: string key_blob
1.1 djm 342: string key_comment
343:
344: Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key
345: Algorithms" for either of the supported key types: "ssh-dss" or
346: "ssh-rsa".
347:
348: 2.6 Private key operations
349:
350: The purpose of the agent is to perform private key operations, such as
351: signing and encryption without requiring a passphrase to unlock the
352: key and without allowing the private key itself to be exposed. There
353: are separate requests for the protocol 1 and protocol 2 private key
354: operations.
355:
356: 2.6.1 Protocol 1 private key challenge
357:
358: The private key operation used in version 1 of the SSH protocol is
359: decrypting a challenge that has been encrypted with a public key.
360: It may be requested using this message:
361:
362: byte SSH_AGENTC_RSA_CHALLENGE
363: uint32 ignored
364: mpint1 rsa_e
365: mpint1 rsa_n
366: mpint1 encrypted_challenge
367: byte[16] session_id
368: uint32 response_type /* must be 1 */
369:
370: "rsa_e" and "rsa_n" are used to identify which private key to use.
371: "encrypted_challenge" is a challenge blob that has (presumably)
372: been encrypted with the public key and must be in the range
373: 1 <= encrypted_challenge < 2^256. "session_id" is the SSH protocol 1
374: session ID (computed from the server host key, the server semi-ephemeral
375: key and the session cookie.)
376:
377: "ignored" and "response_type" exist for compatibility with legacy
378: implementations. "response_type" must be equal to 1; other response
379: types are not supported.
380:
381: On receiving this request, the server decrypts the "encrypted_challenge"
382: using private key matching the supplied (rsa_e, rsa_n) values. For
383: the response derivation, the decrypted challenge is represented as an
384: unsigned, big-endian integer encoded in a 32 byte buffer (i.e. values
385: smaller than 2^248 will have leading 0 bytes).
386:
387: The response value is then calculated as:
388:
389: response = MD5(decrypted_challenge || session_id)
390:
391: and returned in the following message
392:
393: byte SSH_AGENT_RSA_RESPONSE
394: byte[16] response
395:
396: If the agent cannot find the key specified by the supplied (rsa_e,
397: rsa_n) then it will return SSH_AGENT_FAILURE.
398:
399: 2.6.2 Protocol 2 private key signature request
400:
401: A client may use the following message to request signing of data using
402: a protocol 2 key:
403:
404: byte SSH2_AGENTC_SIGN_REQUEST
405: string key_blob
406: string data
407: uint32 flags
408:
409: Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key
410: Algorithms" for either of the supported key types: "ssh-dss" or
411: "ssh-rsa". "flags" is a bit-mask, but at present only one possible value
412: is defined (see below for its meaning):
413:
414: SSH_AGENT_OLD_SIGNATURE 1
415:
416: Upon receiving this request, the agent will look up the private key that
417: corresponds to the public key contained in key_blob. It will use this
418: private key to sign the "data" and produce a signature blob using the
419: key type-specific method described in RFC 4253 section 6.6 "Public Key
420: Algorithms".
421:
422: An exception to this is for "ssh-dss" keys where the "flags" word
423: contains the value SSH_AGENT_OLD_SIGNATURE. In this case, a legacy
424: signature encoding is used in lieu of the standard one. In this case,
425: the DSA signature blob is encoded as:
426:
427: byte[40] signature
428:
429: The signature will be returned in the response message:
430:
431: byte SSH2_AGENT_SIGN_RESPONSE
432: string signature_blob
433:
434: If the agent cannot find the key specified by the supplied key_blob then
435: it will return SSH_AGENT_FAILURE.
436:
437: 2.7 Locking or unlocking an agent
438:
439: The agent supports temporary locking with a passphrase to suspend
440: processing of sensitive operations until it has been unlocked with the
441: same passphrase. To lock an agent, a client send the following request:
442:
443: byte SSH_AGENTC_LOCK
444: string passphrase
445:
446: Upon receipt of this message and if the agent is not already locked,
447: it will suspend processing requests and return a SSH_AGENT_SUCCESS
448: reply. If the agent is already locked, it will return SSH_AGENT_FAILURE.
449:
450: While locked, the agent will refuse all requests except
451: SSH_AGENTC_UNLOCK, SSH_AGENTC_REQUEST_RSA_IDENTITIES and
452: SSH2_AGENTC_REQUEST_IDENTITIES. The "request identities" requests are
453: treated specially by a locked agent: it will always return an empty list
454: of keys.
455:
456: To unlock an agent, a client may request:
457:
458: byte SSH_AGENTC_UNLOCK
459: string passphrase
460:
461: If the passphrase matches and the agent is locked, then it will resume
462: processing all requests and return SSH_AGENT_SUCCESS. If the agent
463: is not locked or the passphrase does not match then it will return
464: SSH_AGENT_FAILURE.
465:
466: Locking and unlocking affects both protocol 1 and protocol 2 keys.
467:
468: 3. Protocol message numbers
469:
470: 3.1 Requests from client to agent for protocol 1 key operations
471:
472: SSH_AGENTC_REQUEST_RSA_IDENTITIES 1
473: SSH_AGENTC_RSA_CHALLENGE 3
474: SSH_AGENTC_ADD_RSA_IDENTITY 7
475: SSH_AGENTC_REMOVE_RSA_IDENTITY 8
476: SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES 9
477: SSH_AGENTC_ADD_RSA_ID_CONSTRAINED 24
478:
479: 3.2 Requests from client to agent for protocol 2 key operations
480:
481: SSH2_AGENTC_REQUEST_IDENTITIES 11
482: SSH2_AGENTC_SIGN_REQUEST 13
483: SSH2_AGENTC_ADD_IDENTITY 17
484: SSH2_AGENTC_REMOVE_IDENTITY 18
485: SSH2_AGENTC_REMOVE_ALL_IDENTITIES 19
486: SSH2_AGENTC_ADD_ID_CONSTRAINED 25
487:
488: 3.3 Key-type independent requests from client to agent
489:
490: SSH_AGENTC_ADD_SMARTCARD_KEY 20
491: SSH_AGENTC_REMOVE_SMARTCARD_KEY 21
492: SSH_AGENTC_LOCK 22
493: SSH_AGENTC_UNLOCK 23
494: SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED 26
495:
496: 3.4 Generic replies from agent to client
497:
498: SSH_AGENT_FAILURE 5
499: SSH_AGENT_SUCCESS 6
500:
501: 3.5 Replies from agent to client for protocol 1 key operations
502:
503: SSH_AGENT_RSA_IDENTITIES_ANSWER 2
504: SSH_AGENT_RSA_RESPONSE 4
505:
506: 3.6 Replies from agent to client for protocol 2 key operations
507:
508: SSH2_AGENT_IDENTITIES_ANSWER 12
509: SSH2_AGENT_SIGN_RESPONSE 14
510:
511: 3.7 Key constraint identifiers
512:
513: SSH_AGENT_CONSTRAIN_LIFETIME 1
514: SSH_AGENT_CONSTRAIN_CONFIRM 2
515:
1.2 ! djm 516: $OpenBSD: PROTOCOL.agent,v 1.1 2008/06/28 14:08:30 djm Exp $
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