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This document describes a simple public-key certificate authentication
system for use by SSH.
The SSH protocol currently supports a simple public key authentication
mechanism. Unlike other public key implementations, SSH eschews the use
of X.509 certificates and uses raw keys. This approach has some benefits
relating to simplicity of configuration and minimisation of attack
surface, but it does not support the important use-cases of centrally
managed, passwordless authentication and centrally certified host keys.
These protocol extensions build on the simple public key authentication
system already in SSH to allow certificate-based authentication. The
certificates used are not traditional X.509 certificates, with numerous
options and complex encoding rules, but something rather more minimal: a
key, some identity information and usage options that have been signed
with some other trusted key.
A sshd server may be configured to allow authentication via certified
keys, by extending the existing ~/.ssh/authorized_keys mechanism to
allow specification of certification authority keys in addition to
raw user keys. The ssh client will support automatic verification of
acceptance of certified host keys, by adding a similar ability to
specify CA keys in ~/.ssh/known_hosts.
Certified keys are represented using new key types:
These include certification information along with the public key
that is used to sign challenges. ssh-keygen performs the CA signing
Protocol extensions
The SSH wire protocol includes several extensibility mechanisms.
These modifications shall take advantage of namespaced public key
algorithm names to add support for certificate authentication without
breaking the protocol - implementations that do not support the
extensions will simply ignore them.
Authentication using the new key formats described below proceeds
using the existing SSH "publickey" authentication method described
in RFC4252 section 7.
New public key formats
The certificate key types take a similar high-level format (note: data
types and encoding are as per RFC4251 section 5). The serialised wire
encoding of these certificates is also used for storing them on disk.
RSA certificate
string ""
string nonce
mpint e
mpint n
uint64 serial
uint32 type
string key id
string valid principals
uint64 valid after
uint64 valid before
string critical options
string extensions
string reserved
string signature key
string signature
DSA certificate
string ""
string nonce
mpint p
mpint q
mpint g
mpint y
uint64 serial
uint32 type
string key id
string valid principals
uint64 valid after
uint64 valid before
string critical options
string extensions
string reserved
string signature key
string signature
ECDSA certificate
string "" |
"" |
string nonce
string curve
string public_key
uint64 serial
uint32 type
string key id
string valid principals
uint64 valid after
uint64 valid before
string critical options
string extensions
string reserved
string signature key
string signature
The nonce field is a CA-provided random bitstring of arbitrary length
(but typically 16 or 32 bytes) included to make attacks that depend on
inducing collisions in the signature hash infeasible.
e and n are the RSA exponent and public modulus respectively.
p, q, g, y are the DSA parameters as described in FIPS-186-2.
curve and public key are respectively the ECDSA "[identifier]" and "Q"
defined in section 3.1 of RFC5656.
serial is an optional certificate serial number set by the CA to
provide an abbreviated way to refer to certificates from that CA.
If a CA does not wish to number its certificates it must set this
field to zero.
type specifies whether this certificate is for identification of a user
or a host using a SSH_CERT_TYPE_... value.
key id is a free-form text field that is filled in by the CA at the time
of signing; the intention is that the contents of this field are used to
identify the identity principal in log messages.
"valid principals" is a string containing zero or more principals as
strings packed inside it. These principals list the names for which this
certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and
usernames for SSH_CERT_TYPE_USER certificates. As a special case, a
zero-length "valid principals" field means the certificate is valid for
any principal of the specified type. XXX DNS wildcards?
"valid after" and "valid before" specify a validity period for the
certificate. Each represents a time in seconds since 1970-01-01
00:00:00. A certificate is considered valid if:
valid after <= current time < valid before
criticial options is a set of zero or more key options encoded as
below. All such options are "critical" in the sense that an implementation
must refuse to authorise a key that has an unrecognised option.
extensions is a set of zero or more optional extensions. These extensions
are not critical, and an implementation that encounters one that it does
not recognise may safely ignore it.
The reserved field is currently unused and is ignored in this version of
the protocol.
signature key contains the CA key used to sign the certificate.
The valid key types for CA keys are ssh-rsa, ssh-dss and the ECDSA types
ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained"
certificates, where the signature key type is a certificate type itself
are NOT supported. Note that it is possible for a RSA certificate key to
be signed by a DSS or ECDSA CA key and vice-versa.
signature is computed over all preceding fields from the initial string
up to, and including the signature key. Signatures are computed and
encoded according to the rules defined for the CA's public key algorithm
(RFC4253 section 6.6 for ssh-rsa and ssh-dss, RFC5656 for the ECDSA
Critical options
The critical options section of the certificate specifies zero or more
options on the certificates validity. The format of this field
is a sequence of zero or more tuples:
string name
string data
Options must be lexically ordered by "name" if they appear in the
The name field identifies the option and the data field encodes
option-specific information (see below). All options are
"critical", if an implementation does not recognise a option
then the validating party should refuse to accept the certificate.
The supported options and the contents and structure of their
data fields are:
Name Format Description
force-command string Specifies a command that is executed
(replacing any the user specified on the
ssh command-line) whenever this key is
used for authentication.
source-address string Comma-separated list of source addresses
from which this certificate is accepted
for authentication. Addresses are
specified in CIDR format (nn.nn.nn.nn/nn
or hhhh::hhhh/nn).
If this option is not present then
certificates may be presented from any
source address.
The extensions section of the certificate specifies zero or more
non-critical certificate extensions. The encoding and ordering of
extensions in this field is identical to that of the critical options.
If an implementation does not recognise an extension, then it should
ignore it.
The supported extensions and the contents and structure of their data
fields are:
Name Format Description
permit-X11-forwarding empty Flag indicating that X11 forwarding
should be permitted. X11 forwarding will
be refused if this option is absent.
permit-agent-forwarding empty Flag indicating that agent forwarding
should be allowed. Agent forwarding
must not be permitted unless this
option is present.
permit-port-forwarding empty Flag indicating that port-forwarding
should be allowed. If this option is
not present then no port forwarding will
be allowed.
permit-pty empty Flag indicating that PTY allocation
should be permitted. In the absence of
this option PTY allocation will be
permit-user-rc empty Flag indicating that execution of
~/.ssh/rc should be permitted. Execution
of this script will not be permitted if
this option is not present.
$OpenBSD: PROTOCOL.certkeys,v 1.8 2010/08/31 11:54:45 djm Exp $