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
40 Two additional types exist for RSA certificates to force use of
41 SHA-2 signatures (SHA-256 and SHA-512 respectively):
43 rsa-sha2-256-cert-v01@openssh.com
44 rsa-sha2-512-cert-v01@openssh.com
46 These RSA/SHA-2 types should not appear in keys at rest or transmitted
47 on their wire, but do appear in a SSH_MSG_KEXINIT's host-key algorithms
48 field or in the "public key algorithm name" field of a "publickey"
49 SSH_USERAUTH_REQUEST to indicate that the signature will use the
55 The SSH wire protocol includes several extensibility mechanisms.
56 These modifications shall take advantage of namespaced public key
57 algorithm names to add support for certificate authentication without
58 breaking the protocol - implementations that do not support the
59 extensions will simply ignore them.
61 Authentication using the new key formats described below proceeds
62 using the existing SSH "publickey" authentication method described
65 New public key formats
66 ----------------------
68 The certificate key types take a similar high-level format (note: data
69 types and encoding are as per RFC4251 section 5). The serialised wire
70 encoding of these certificates is also used for storing them on disk.
72 #define SSH_CERT_TYPE_USER 1
73 #define SSH_CERT_TYPE_HOST 2
77 string "ssh-rsa-cert-v01@openssh.com"
84 string valid principals
87 string critical options
95 string "ssh-dss-cert-v01@openssh.com"
104 string valid principals
107 string critical options
115 string "ecdsa-sha2-nistp256-cert-v01@openssh.com" |
116 "ecdsa-sha2-nistp384-cert-v01@openssh.com" |
117 "ecdsa-sha2-nistp521-cert-v01@openssh.com"
124 string valid principals
127 string critical options
135 string "ssh-ed25519-cert-v01@openssh.com"
141 string valid principals
144 string critical options
150 The nonce field is a CA-provided random bitstring of arbitrary length
151 (but typically 16 or 32 bytes) included to make attacks that depend on
152 inducing collisions in the signature hash infeasible.
154 e and n are the RSA exponent and public modulus respectively.
156 p, q, g, y are the DSA parameters as described in FIPS-186-2.
158 curve and public key are respectively the ECDSA "[identifier]" and "Q"
159 defined in section 3.1 of RFC5656.
161 pk is the encoded Ed25519 public key as defined by
162 draft-josefsson-eddsa-ed25519-03.
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
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
204 The reserved field is currently unused and is ignored in this version of
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 draft-josefsson-eddsa-ed25519-03 for Ed25519.
224 The critical options section of the certificate specifies zero or more
225 options on the certificates validity. The format of this field
226 is a sequence of zero or more tuples:
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
258 If this option is not present then
259 certificates may be presented from any
265 The extensions section of the certificate specifies zero or more
266 non-critical certificate extensions. The encoding and ordering of
267 extensions in this field is identical to that of the critical options,
268 as is the requirement that each name appear only once.
270 If an implementation does not recognise an extension, then it should
273 Custom options should append the originating author or organisation's
274 domain name to the option name, e.g. "my-option@example.com".
276 No extensions are defined for host certificates at present. The
277 supported user certificate extensions and the contents and structure of
278 their data fields are:
280 Name Format Description
281 -----------------------------------------------------------------------------
282 permit-X11-forwarding empty Flag indicating that X11 forwarding
283 should be permitted. X11 forwarding will
284 be refused if this option is absent.
286 permit-agent-forwarding empty Flag indicating that agent forwarding
287 should be allowed. Agent forwarding
288 must not be permitted unless this
291 permit-port-forwarding empty Flag indicating that port-forwarding
292 should be allowed. If this option is
293 not present then no port forwarding will
296 permit-pty empty Flag indicating that PTY allocation
297 should be permitted. In the absence of
298 this option PTY allocation will be
301 permit-user-rc empty Flag indicating that execution of
302 ~/.ssh/rc should be permitted. Execution
303 of this script will not be permitted if
304 this option is not present.
306 $OpenBSD: PROTOCOL.certkeys,v 1.15 2018/07/03 11:39:54 djm Exp $