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1 This document describes a simple public-key certificate authentication
2 system for use by SSH.
4 Background
5 ----------
7 The SSH protocol currently supports a simple public key authentication
8 mechanism. Unlike other public key implementations, SSH eschews the use
9 of X.509 certificates and uses raw keys. This approach has some benefits
10 relating to simplicity of configuration and minimisation of attack
11 surface, but it does not support the important use-cases of centrally
12 managed, passwordless authentication and centrally certified host keys.
14 These protocol extensions build on the simple public key authentication
15 system already in SSH to allow certificate-based authentication. The
16 certificates used are not traditional X.509 certificates, with numerous
17 options and complex encoding rules, but something rather more minimal: a
18 key, some identity information and usage options that have been signed
19 with some other trusted key.
21 A sshd server may be configured to allow authentication via certified
22 keys, by extending the existing ~/.ssh/authorized_keys mechanism to
23 allow specification of certification authority keys in addition to
24 raw user keys. The ssh client will support automatic verification of
25 acceptance of certified host keys, by adding a similar ability to
26 specify CA keys in ~/.ssh/known_hosts.
28 All certificate types include certification information along with the
29 public key that is used to sign challenges. In OpenSSH, ssh-keygen
30 performs the CA signing operation.
32 Certified keys are represented using new key types:
34 ssh-rsa-cert-v01@openssh.com
35 ssh-dss-cert-v01@openssh.com
36 ecdsa-sha2-nistp256-cert-v01@openssh.com
37 ecdsa-sha2-nistp384-cert-v01@openssh.com
38 ecdsa-sha2-nistp521-cert-v01@openssh.com
39 ssh-ed25519-cert-v01@openssh.com
41 Two additional types exist for RSA certificates to force use of
42 SHA-2 signatures (SHA-256 and SHA-512 respectively):
44 rsa-sha2-256-cert-v01@openssh.com
45 rsa-sha2-512-cert-v01@openssh.com
47 These RSA/SHA-2 types should not appear in keys at rest or transmitted
48 on the wire, but do appear in a SSH_MSG_KEXINIT's host-key algorithms
49 field or in the "public key algorithm name" field of a "publickey"
50 SSH_USERAUTH_REQUEST to indicate that the signature will use the
51 specified algorithm.
53 Protocol extensions
54 -------------------
56 The SSH wire protocol includes several extensibility mechanisms.
57 These modifications shall take advantage of namespaced public key
58 algorithm names to add support for certificate authentication without
59 breaking the protocol - implementations that do not support the
60 extensions will simply ignore them.
62 Authentication using the new key formats described below proceeds
63 using the existing SSH "publickey" authentication method described
64 in RFC4252 section 7.
66 New public key formats
67 ----------------------
69 The certificate key types take a similar high-level format (note: data
70 types and encoding are as per RFC4251 section 5). The serialised wire
71 encoding of these certificates is also used for storing them on disk.
73 #define SSH_CERT_TYPE_USER 1
74 #define SSH_CERT_TYPE_HOST 2
76 RSA certificate
78 string "ssh-rsa-cert-v01@openssh.com"
79 string nonce
80 mpint e
81 mpint n
82 uint64 serial
83 uint32 type
84 string key id
85 string valid principals
86 uint64 valid after
87 uint64 valid before
88 string critical options
89 string extensions
90 string reserved
91 string signature key
92 string signature
94 DSA certificate
96 string "ssh-dss-cert-v01@openssh.com"
97 string nonce
98 mpint p
99 mpint q
100 mpint g
101 mpint y
102 uint64 serial
103 uint32 type
104 string key id
105 string valid principals
106 uint64 valid after
107 uint64 valid before
108 string critical options
109 string extensions
110 string reserved
111 string signature key
112 string signature
114 ECDSA certificate
116 string "ecdsa-sha2-nistp256-cert-v01@openssh.com" |
117 "ecdsa-sha2-nistp384-cert-v01@openssh.com" |
118 "ecdsa-sha2-nistp521-cert-v01@openssh.com"
119 string nonce
120 string curve
121 string public_key
122 uint64 serial
123 uint32 type
124 string key id
125 string valid principals
126 uint64 valid after
127 uint64 valid before
128 string critical options
129 string extensions
130 string reserved
131 string signature key
132 string signature
134 ED25519 certificate
136 string "ssh-ed25519-cert-v01@openssh.com"
137 string nonce
138 string pk
139 uint64 serial
140 uint32 type
141 string key id
142 string valid principals
143 uint64 valid after
144 uint64 valid before
145 string critical options
146 string extensions
147 string reserved
148 string signature key
149 string signature
151 The nonce field is a CA-provided random bitstring of arbitrary length
152 (but typically 16 or 32 bytes) included to make attacks that depend on
153 inducing collisions in the signature hash infeasible.
155 e and n are the RSA exponent and public modulus respectively.
157 p, q, g, y are the DSA parameters as described in FIPS-186-2.
159 curve and public key are respectively the ECDSA "[identifier]" and "Q"
160 defined in section 3.1 of RFC5656.
162 pk is the encoded Ed25519 public key as defined by RFC8032.
164 serial is an optional certificate serial number set by the CA to
165 provide an abbreviated way to refer to certificates from that CA.
166 If a CA does not wish to number its certificates, it must set this
167 field to zero.
169 type specifies whether this certificate is for identification of a user
170 or a host using a SSH_CERT_TYPE_... value.
172 key id is a free-form text field that is filled in by the CA at the time
173 of signing; the intention is that the contents of this field are used to
174 identify the identity principal in log messages.
176 "valid principals" is a string containing zero or more principals as
177 strings packed inside it. These principals list the names for which this
178 certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and
179 usernames for SSH_CERT_TYPE_USER certificates. As a special case, a
180 zero-length "valid principals" field means the certificate is valid for
181 any principal of the specified type.
183 "valid after" and "valid before" specify a validity period for the
184 certificate. Each represents a time in seconds since 1970-01-01
185 00:00:00. A certificate is considered valid if:
187 valid after <= current time < valid before
189 critical options is a set of zero or more key options encoded as
190 below. All such options are "critical" in the sense that an implementation
191 must refuse to authorise a key that has an unrecognised option.
193 extensions is a set of zero or more optional extensions. These extensions
194 are not critical, and an implementation that encounters one that it does
195 not recognise may safely ignore it.
197 Generally, critical options are used to control features that restrict
198 access where extensions are used to enable features that grant access.
199 This ensures that certificates containing unknown restrictions do not
200 inadvertently grant access while allowing new protocol features to be
201 enabled via extensions without breaking certificates' backwards
202 compatibility.
204 The reserved field is currently unused and is ignored in this version of
205 the protocol.
207 The signature key field contains the CA key used to sign the
208 certificate. The valid key types for CA keys are ssh-rsa,
209 ssh-dss, ssh-ed25519 and the ECDSA types ecdsa-sha2-nistp256,
210 ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained" certificates, where
211 the signature key type is a certificate type itself are NOT supported.
212 Note that it is possible for a RSA certificate key to be signed by a
213 Ed25519 or ECDSA CA key and vice-versa.
215 signature is computed over all preceding fields from the initial string
216 up to, and including the signature key. Signatures are computed and
217 encoded according to the rules defined for the CA's public key algorithm
218 (RFC4253 section 6.6 for ssh-rsa and ssh-dss, RFC5656 for the ECDSA
219 types, and RFC8032 for Ed25519).
221 Critical options
222 ----------------
224 The critical options section of the certificate specifies zero or more
225 options on the certificate's validity. The format of this field
226 is a sequence of zero or more tuples:
228 string name
229 string data
231 Options must be lexically ordered by "name" if they appear in the
232 sequence. Each named option may only appear once in a certificate.
234 The name field identifies the option and the data field encodes
235 option-specific information (see below). All options are
236 "critical"; if an implementation does not recognise a option,
237 then the validating party should refuse to accept the certificate.
239 Custom options should append the originating author or organisation's
240 domain name to the option name, e.g. "my-option@example.com".
242 No critical options are defined for host certificates at present. The
243 supported user certificate options and the contents and structure of
244 their data fields are:
246 Name Format Description
247 -----------------------------------------------------------------------------
248 force-command string Specifies a command that is executed
249 (replacing any the user specified on the
250 ssh command-line) whenever this key is
251 used for authentication.
253 source-address string Comma-separated list of source addresses
254 from which this certificate is accepted
255 for authentication. Addresses are
256 specified in CIDR format (nn.nn.nn.nn/nn
257 or hhhh::hhhh/nn).
258 If this option is not present, then
259 certificates may be presented from any
260 source address.
262 verify-required empty Flag indicating that signatures made
263 with this certificate must assert FIDO
264 user verification (e.g. PIN or
265 biometric). This option only makes sense
266 for the U2F/FIDO security key types that
267 support this feature in their signature
268 formats.
270 Extensions
271 ----------
273 The extensions section of the certificate specifies zero or more
274 non-critical certificate extensions. The encoding and ordering of
275 extensions in this field is identical to that of the critical options,
276 as is the requirement that each name appear only once.
278 If an implementation does not recognise an extension, then it should
279 ignore it.
281 Custom options should append the originating author or organisation's
282 domain name to the option name, e.g. "my-option@example.com".
284 No extensions are defined for host certificates at present. The
285 supported user certificate extensions and the contents and structure of
286 their data fields are:
288 Name Format Description
289 -----------------------------------------------------------------------------
290 no-touch-required empty Flag indicating that signatures made
291 with this certificate need not assert
292 FIDO user presence. This option only
293 makes sense for the U2F/FIDO security
294 key types that support this feature in
295 their signature formats.
297 permit-X11-forwarding empty Flag indicating that X11 forwarding
298 should be permitted. X11 forwarding will
299 be refused if this option is absent.
301 permit-agent-forwarding empty Flag indicating that agent forwarding
302 should be allowed. Agent forwarding
303 must not be permitted unless this
304 option is present.
306 permit-port-forwarding empty Flag indicating that port-forwarding
307 should be allowed. If this option is
308 not present, then no port forwarding will
309 be allowed.
311 permit-pty empty Flag indicating that PTY allocation
312 should be permitted. In the absence of
313 this option PTY allocation will be
314 disabled.
316 permit-user-rc empty Flag indicating that execution of
317 ~/.ssh/rc should be permitted. Execution
318 of this script will not be permitted if
319 this option is not present.
321 $OpenBSD: PROTOCOL.certkeys,v 1.19 2021/06/05 13:47:00 naddy Exp $