1This document describes a simple public-key certificate authentication
2system for use by SSH.
3
4Background
5----------
6
7The SSH protocol currently supports a simple public key authentication
8mechanism. Unlike other public key implementations, SSH eschews the use
9of X.509 certificates and uses raw keys. This approach has some benefits
10relating to simplicity of configuration and minimisation of attack
11surface, but it does not support the important use-cases of centrally
12managed, passwordless authentication and centrally certified host keys.
13
14These protocol extensions build on the simple public key authentication
15system already in SSH to allow certificate-based authentication. The
16certificates used are not traditional X.509 certificates, with numerous
17options and complex encoding rules, but something rather more minimal: a
18key, some identity information and usage options that have been signed
19with some other trusted key.
20
21A sshd server may be configured to allow authentication via certified
22keys, by extending the existing ~/.ssh/authorized_keys mechanism to
23allow specification of certification authority keys in addition to
24raw user keys. The ssh client will support automatic verification of
25acceptance of certified host keys, by adding a similar ability to
26specify CA keys in ~/.ssh/known_hosts.
27
28Certified keys are represented using new key types:
29
30    ssh-rsa-cert-v01@openssh.com
31    ssh-dss-cert-v01@openssh.com
32    ecdsa-sha2-nistp256-cert-v01@openssh.com
33    ecdsa-sha2-nistp384-cert-v01@openssh.com
34    ecdsa-sha2-nistp521-cert-v01@openssh.com
35
36These include certification information along with the public key
37that is used to sign challenges. ssh-keygen performs the CA signing
38operation.
39
40Protocol extensions
41-------------------
42
43The SSH wire protocol includes several extensibility mechanisms.
44These modifications shall take advantage of namespaced public key
45algorithm names to add support for certificate authentication without
46breaking the protocol - implementations that do not support the
47extensions will simply ignore them.
48
49Authentication using the new key formats described below proceeds
50using the existing SSH "publickey" authentication method described
51in RFC4252 section 7.
52
53New public key formats
54----------------------
55
56The certificate key types take a similar high-level format (note: data
57types and encoding are as per RFC4251 section 5). The serialised wire
58encoding of these certificates is also used for storing them on disk.
59
60#define SSH_CERT_TYPE_USER    1
61#define SSH_CERT_TYPE_HOST    2
62
63RSA certificate
64
65    string    "ssh-rsa-cert-v01@openssh.com"
66    string    nonce
67    mpint     e
68    mpint     n
69    uint64    serial
70    uint32    type
71    string    key id
72    string    valid principals
73    uint64    valid after
74    uint64    valid before
75    string    critical options
76    string    extensions
77    string    reserved
78    string    signature key
79    string    signature
80
81DSA certificate
82
83    string    "ssh-dss-cert-v01@openssh.com"
84    string    nonce
85    mpint     p
86    mpint     q
87    mpint     g
88    mpint     y
89    uint64    serial
90    uint32    type
91    string    key id
92    string    valid principals
93    uint64    valid after
94    uint64    valid before
95    string    critical options
96    string    extensions
97    string    reserved
98    string    signature key
99    string    signature
100
101ECDSA certificate
102
103    string    "ecdsa-sha2-nistp256@openssh.com" |
104              "ecdsa-sha2-nistp384@openssh.com" |
105              "ecdsa-sha2-nistp521@openssh.com"
106    string    nonce
107    string    curve
108    string    public_key
109    uint64    serial
110    uint32    type
111    string    key id
112    string    valid principals
113    uint64    valid after
114    uint64    valid before
115    string    critical options
116    string    extensions
117    string    reserved
118    string    signature key
119    string    signature
120
121The nonce field is a CA-provided random bitstring of arbitrary length
122(but typically 16 or 32 bytes) included to make attacks that depend on
123inducing collisions in the signature hash infeasible.
124
125e and n are the RSA exponent and public modulus respectively.
126
127p, q, g, y are the DSA parameters as described in FIPS-186-2.
128
129curve and public key are respectively the ECDSA "[identifier]" and "Q"
130defined in section 3.1 of RFC5656.
131
132serial is an optional certificate serial number set by the CA to
133provide an abbreviated way to refer to certificates from that CA.
134If a CA does not wish to number its certificates it must set this
135field to zero.
136
137type specifies whether this certificate is for identification of a user
138or a host using a SSH_CERT_TYPE_... value.
139
140key id is a free-form text field that is filled in by the CA at the time
141of signing; the intention is that the contents of this field are used to
142identify the identity principal in log messages.
143
144"valid principals" is a string containing zero or more principals as
145strings packed inside it. These principals list the names for which this
146certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and
147usernames for SSH_CERT_TYPE_USER certificates. As a special case, a
148zero-length "valid principals" field means the certificate is valid for
149any principal of the specified type. XXX DNS wildcards?
150
151"valid after" and "valid before" specify a validity period for the
152certificate. Each represents a time in seconds since 1970-01-01
15300:00:00. A certificate is considered valid if:
154
155    valid after <= current time < valid before
156
157criticial options is a set of zero or more key options encoded as
158below. All such options are "critical" in the sense that an implementation
159must refuse to authorise a key that has an unrecognised option.
160
161extensions is a set of zero or more optional extensions. These extensions
162are not critical, and an implementation that encounters one that it does
163not recognise may safely ignore it.
164
165Generally, critical options are used to control features that restrict
166access where extensions are used to enable features that grant access.
167This ensures that certificates containing unknown restrictions do not
168inadvertently grant access while allowing new protocol features to be
169enabled via extensions without breaking certificates' backwards
170compatibility.
171
172The reserved field is currently unused and is ignored in this version of
173the protocol.
174
175signature key contains the CA key used to sign the certificate.
176The valid key types for CA keys are ssh-rsa, ssh-dss and the ECDSA types
177ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained"
178certificates, where the signature key type is a certificate type itself
179are NOT supported. Note that it is possible for a RSA certificate key to
180be signed by a DSS or ECDSA CA key and vice-versa.
181
182signature is computed over all preceding fields from the initial string
183up to, and including the signature key. Signatures are computed and
184encoded according to the rules defined for the CA's public key algorithm
185(RFC4253 section 6.6 for ssh-rsa and ssh-dss, RFC5656 for the ECDSA
186types).
187
188Critical options
189----------------
190
191The critical options section of the certificate specifies zero or more
192options on the certificates validity. The format of this field
193is a sequence of zero or more tuples:
194
195    string       name
196    string       data
197
198Options must be lexically ordered by "name" if they appear in the
199sequence. Each named option may only appear once in a certificate.
200
201The name field identifies the option and the data field encodes
202option-specific information (see below). All options are
203"critical", if an implementation does not recognise a option
204then the validating party should refuse to accept the certificate.
205
206The supported options and the contents and structure of their
207data fields are:
208
209Name                    Format        Description
210-----------------------------------------------------------------------------
211force-command           string        Specifies a command that is executed
212                                      (replacing any the user specified on the
213                                      ssh command-line) whenever this key is
214                                      used for authentication.
215
216source-address          string        Comma-separated list of source addresses
217                                      from which this certificate is accepted
218                                      for authentication. Addresses are
219                                      specified in CIDR format (nn.nn.nn.nn/nn
220                                      or hhhh::hhhh/nn).
221                                      If this option is not present then
222                                      certificates may be presented from any
223                                      source address.
224
225Extensions
226----------
227
228The extensions section of the certificate specifies zero or more
229non-critical certificate extensions. The encoding and ordering of
230extensions in this field is identical to that of the critical options,
231as is the requirement that each name appear only once.
232
233If an implementation does not recognise an extension, then it should
234ignore it.
235
236The supported extensions and the contents and structure of their data
237fields are:
238
239Name                    Format        Description
240-----------------------------------------------------------------------------
241permit-X11-forwarding   empty         Flag indicating that X11 forwarding
242                                      should be permitted. X11 forwarding will
243                                      be refused if this option is absent.
244
245permit-agent-forwarding empty         Flag indicating that agent forwarding
246                                      should be allowed. Agent forwarding
247                                      must not be permitted unless this
248                                      option is present.
249
250permit-port-forwarding  empty         Flag indicating that port-forwarding
251                                      should be allowed. If this option is
252                                      not present then no port forwarding will
253                                      be allowed.
254
255permit-pty              empty         Flag indicating that PTY allocation
256                                      should be permitted. In the absence of
257                                      this option PTY allocation will be
258                                      disabled.
259
260permit-user-rc          empty         Flag indicating that execution of
261                                      ~/.ssh/rc should be permitted. Execution
262                                      of this script will not be permitted if
263                                      this option is not present.
264
265$OpenBSD: PROTOCOL.certkeys,v 1.9 2012/03/28 07:23:22 djm Exp $
266