1 This document describes a simple public-key certificate authentication
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 their 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
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
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
78 string "ssh-rsa-cert-v01@openssh.com"
85 string valid principals
88 string critical options
96 string "ssh-dss-cert-v01@openssh.com"
105 string valid principals
108 string critical options
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"
125 string valid principals
128 string critical options
136 string "ssh-ed25519-cert-v01@openssh.com"
142 string valid principals
145 string critical options
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
163 draft-josefsson-eddsa-ed25519-03.
165 serial is an optional certificate serial number set by the CA to
166 provide an abbreviated way to refer to certificates from that CA.
167 If a CA does not wish to number its certificates it must set this
170 type specifies whether this certificate is for identification of a user
171 or a host using a SSH_CERT_TYPE_... value.
173 key id is a free-form text field that is filled in by the CA at the time
174 of signing; the intention is that the contents of this field are used to
175 identify the identity principal in log messages.
177 "valid principals" is a string containing zero or more principals as
178 strings packed inside it. These principals list the names for which this
179 certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and
180 usernames for SSH_CERT_TYPE_USER certificates. As a special case, a
181 zero-length "valid principals" field means the certificate is valid for
182 any principal of the specified type.
184 "valid after" and "valid before" specify a validity period for the
185 certificate. Each represents a time in seconds since 1970-01-01
186 00:00:00. A certificate is considered valid if:
188 valid after <= current time < valid before
190 critical options is a set of zero or more key options encoded as
191 below. All such options are "critical" in the sense that an implementation
192 must refuse to authorise a key that has an unrecognised option.
194 extensions is a set of zero or more optional extensions. These extensions
195 are not critical, and an implementation that encounters one that it does
196 not recognise may safely ignore it.
198 Generally, critical options are used to control features that restrict
199 access where extensions are used to enable features that grant access.
200 This ensures that certificates containing unknown restrictions do not
201 inadvertently grant access while allowing new protocol features to be
202 enabled via extensions without breaking certificates' backwards
205 The reserved field is currently unused and is ignored in this version of
208 The signature key field contains the CA key used to sign the
209 certificate. The valid key types for CA keys are ssh-rsa,
210 ssh-dss, ssh-ed25519 and the ECDSA types ecdsa-sha2-nistp256,
211 ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained" certificates, where
212 the signature key type is a certificate type itself are NOT supported.
213 Note that it is possible for a RSA certificate key to be signed by a
214 Ed25519 or ECDSA CA key and vice-versa.
216 signature is computed over all preceding fields from the initial string
217 up to, and including the signature key. Signatures are computed and
218 encoded according to the rules defined for the CA's public key algorithm
219 (RFC4253 section 6.6 for ssh-rsa and ssh-dss, RFC5656 for the ECDSA
220 types), and draft-josefsson-eddsa-ed25519-03 for Ed25519.
225 The critical options section of the certificate specifies zero or more
226 options on the certificates validity. The format of this field
227 is a sequence of zero or more tuples:
232 Options must be lexically ordered by "name" if they appear in the
233 sequence. Each named option may only appear once in a certificate.
235 The name field identifies the option and the data field encodes
236 option-specific information (see below). All options are
237 "critical", if an implementation does not recognise a option
238 then the validating party should refuse to accept the certificate.
240 Custom options should append the originating author or organisation's
241 domain name to the option name, e.g. "my-option@example.com".
243 No critical options are defined for host certificates at present. The
244 supported user certificate options and the contents and structure of
245 their data fields are:
247 Name Format Description
248 -----------------------------------------------------------------------------
249 force-command string Specifies a command that is executed
250 (replacing any the user specified on the
251 ssh command-line) whenever this key is
252 used for authentication.
254 source-address string Comma-separated list of source addresses
255 from which this certificate is accepted
256 for authentication. Addresses are
257 specified in CIDR format (nn.nn.nn.nn/nn
259 If this option is not present then
260 certificates may be presented from any
266 The extensions section of the certificate specifies zero or more
267 non-critical certificate extensions. The encoding and ordering of
268 extensions in this field is identical to that of the critical options,
269 as is the requirement that each name appear only once.
271 If an implementation does not recognise an extension, then it should
274 Custom options should append the originating author or organisation's
275 domain name to the option name, e.g. "my-option@example.com".
277 No extensions are defined for host certificates at present. The
278 supported user certificate extensions and the contents and structure of
279 their data fields are:
281 Name Format Description
282 -----------------------------------------------------------------------------
283 permit-X11-forwarding empty Flag indicating that X11 forwarding
284 should be permitted. X11 forwarding will
285 be refused if this option is absent.
287 permit-agent-forwarding empty Flag indicating that agent forwarding
288 should be allowed. Agent forwarding
289 must not be permitted unless this
292 permit-port-forwarding empty Flag indicating that port-forwarding
293 should be allowed. If this option is
294 not present then no port forwarding will
297 permit-pty empty Flag indicating that PTY allocation
298 should be permitted. In the absence of
299 this option PTY allocation will be
302 permit-user-rc empty Flag indicating that execution of
303 ~/.ssh/rc should be permitted. Execution
304 of this script will not be permitted if
305 this option is not present.
307 $OpenBSD: PROTOCOL.certkeys,v 1.16 2018/10/26 01:23:03 djm Exp $