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
9use of X.509 certificates and uses raw keys. This approach has some
10benefits relating to simplicity of configuration and minimisation
11of attack surface, but it does not support the important use-cases
12of centrally managed, passwordless authentication and centrally
13certified host keys.
14
15These protocol extensions build on the simple public key authentication
16system already in SSH to allow certificate-based authentication.
17The certificates used are not traditional X.509 certificates, with
18numerous options and complex encoding rules, but something rather
19more minimal: a key, some identity information and usage options
20that have been signed with some other trusted key.
21
22A sshd server may be configured to allow authentication via certified
23keys, by extending the existing ~/.ssh/authorized_keys mechanism
24to allow specification of certification authority keys in addition
25to raw user keys. The ssh client will support automatic verification
26of acceptance of certified host keys, by adding a similar ability
27to specify CA keys in ~/.ssh/known_hosts.
28
29Certified keys are represented using two new key types:
30ssh-rsa-cert-v01@openssh.com and ssh-dss-cert-v01@openssh.com that
31include certification information along with the public key that is used
32to sign challenges. ssh-keygen performs the CA signing operation.
33
34Protocol extensions
35-------------------
36
37The SSH wire protocol includes several extensibility mechanisms.
38These modifications shall take advantage of namespaced public key
39algorithm names to add support for certificate authentication without
40breaking the protocol - implementations that do not support the
41extensions will simply ignore them.
42
43Authentication using the new key formats described below proceeds
44using the existing SSH "publickey" authentication method described
45in RFC4252 section 7.
46
47New public key formats
48----------------------
49
50The ssh-rsa-cert-v01@openssh.com and ssh-dss-cert-v01@openssh.com key
51types take a similar high-level format (note: data types and
52encoding are as per RFC4251 section 5). The serialised wire encoding of
53these certificates is also used for storing them on disk.
54
55#define SSH_CERT_TYPE_USER    1
56#define SSH_CERT_TYPE_HOST    2
57
58RSA certificate
59
60    string    "ssh-rsa-cert-v01@openssh.com"
61    string    nonce
62    mpint     e
63    mpint     n
64    uint64    serial
65    uint32    type
66    string    key id
67    string    valid principals
68    uint64    valid after
69    uint64    valid before
70    string    critical options
71    string    extensions
72    string    reserved
73    string    signature key
74    string    signature
75
76DSA certificate
77
78    string    "ssh-dss-cert-v01@openssh.com"
79    string    nonce
80    mpint     p
81    mpint     q
82    mpint     g
83    mpint     y
84    uint64    serial
85    uint32    type
86    string    key id
87    string    valid principals
88    uint64    valid after
89    uint64    valid before
90    string    critical options
91    string    extensions
92    string    reserved
93    string    signature key
94    string    signature
95
96The nonce field is a CA-provided random bitstring of arbitrary length
97(but typically 16 or 32 bytes) included to make attacks that depend on
98inducing collisions in the signature hash infeasible.
99
100e and n are the RSA exponent and public modulus respectively.
101
102p, q, g, y are the DSA parameters as described in FIPS-186-2.
103
104serial is an optional certificate serial number set by the CA to
105provide an abbreviated way to refer to certificates from that CA.
106If a CA does not wish to number its certificates it must set this
107field to zero.
108
109type specifies whether this certificate is for identification of a user
110or a host using a SSH_CERT_TYPE_... value.
111
112key id is a free-form text field that is filled in by the CA at the time
113of signing; the intention is that the contents of this field are used to
114identify the identity principal in log messages.
115
116"valid principals" is a string containing zero or more principals as
117strings packed inside it. These principals list the names for which this
118certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and
119usernames for SSH_CERT_TYPE_USER certificates. As a special case, a
120zero-length "valid principals" field means the certificate is valid for
121any principal of the specified type. XXX DNS wildcards?
122
123"valid after" and "valid before" specify a validity period for the
124certificate. Each represents a time in seconds since 1970-01-01
12500:00:00. A certificate is considered valid if:
126	 valid after <= current time < valid before
127
128criticial options is a set of zero or more key options encoded as
129below. All such options are "critical" in the sense that an implementation
130must refuse to authorise a key that has an unrecognised option.
131
132extensions is a set of zero or more optional extensions. These extensions
133are not critical, and an implementation that encounters one that it does
134not recognise may safely ignore it.
135
136The reserved field is currently unused and is ignored in this version of
137the protocol.
138
139signature key contains the CA key used to sign the certificate.
140The valid key types for CA keys are ssh-rsa and ssh-dss. "Chained"
141certificates, where the signature key type is a certificate type itself
142are NOT supported. Note that it is possible for a RSA certificate key to
143be signed by a DSS CA key and vice-versa.
144
145signature is computed over all preceding fields from the initial string
146up to, and including the signature key. Signatures are computed and
147encoded according to the rules defined for the CA's public key algorithm
148(RFC4253 section 6.6 for ssh-rsa and ssh-dss).
149
150Critical options
151----------------
152
153The critical options section of the certificate specifies zero or more
154options on the certificates validity. The format of this field
155is a sequence of zero or more tuples:
156
157    string       name
158    string       data
159
160Options must be lexically ordered by "name" if they appear in the
161sequence.
162
163The name field identifies the option and the data field encodes
164option-specific information (see below). All options are
165"critical", if an implementation does not recognise a option
166then the validating party should refuse to accept the certificate.
167
168The supported options and the contents and structure of their
169data fields are:
170
171Name                    Format        Description
172-----------------------------------------------------------------------------
173force-command           string        Specifies a command that is executed
174                                      (replacing any the user specified on the
175                                      ssh command-line) whenever this key is
176                                      used for authentication.
177
178source-address          string        Comma-separated list of source addresses
179                                      from which this certificate is accepted
180                                      for authentication. Addresses are
181                                      specified in CIDR format (nn.nn.nn.nn/nn
182                                      or hhhh::hhhh/nn).
183                                      If this option is not present then
184                                      certificates may be presented from any
185                                      source address.
186
187Extensions
188----------
189
190The extensions section of the certificate specifies zero or more
191non-critical certificate extensions. The encoding and ordering of
192extensions in this field is identical to that of the critical options.
193If an implementation does not recognise an extension, then it should
194ignore it.
195
196The supported extensions and the contents and structure of their data
197fields are:
198
199Name                    Format        Description
200-----------------------------------------------------------------------------
201permit-X11-forwarding   empty         Flag indicating that X11 forwarding
202                                      should be permitted. X11 forwarding will
203                                      be refused if this option is absent.
204
205permit-agent-forwarding empty         Flag indicating that agent forwarding
206                                      should be allowed. Agent forwarding
207                                      must not be permitted unless this
208                                      option is present.
209
210permit-port-forwarding  empty         Flag indicating that port-forwarding
211                                      should be allowed. If this option is
212                                      not present then no port forwarding will
213                                      be allowed.
214
215permit-pty              empty         Flag indicating that PTY allocation
216                                      should be permitted. In the absence of
217                                      this option PTY allocation will be
218                                      disabled.
219
220permit-user-rc          empty         Flag indicating that execution of
221                                      ~/.ssh/rc should be permitted. Execution
222                                      of this script will not be permitted if
223                                      this option is not present.
224
225$OpenBSD: PROTOCOL.certkeys,v 1.7 2010/08/04 05:40:39 djm Exp $
226