This document describes a simple public-key certificate authentication
system for use by SSH.

Background
----------

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.

All certificate types include certification information along with the
public key that is used to sign challenges. In OpenSSH, ssh-keygen
performs the CA signing operation.

Certified keys are represented using new key types:

    ssh-rsa-cert-v01@openssh.com
    ssh-dss-cert-v01@openssh.com
    ecdsa-sha2-nistp256-cert-v01@openssh.com
    ecdsa-sha2-nistp384-cert-v01@openssh.com
    ecdsa-sha2-nistp521-cert-v01@openssh.com
    ssh-ed25519-cert-v01@openssh.com

Two additional types exist for RSA certificates to force use of
SHA-2 signatures (SHA-256 and SHA-512 respectively):

    rsa-sha2-256-cert-v01@openssh.com
    rsa-sha2-512-cert-v01@openssh.com

These RSA/SHA-2 types should not appear in keys at rest or transmitted
on the wire, but do appear in a SSH_MSG_KEXINIT's host-key algorithms
field or in the "public key algorithm name" field of a "publickey"
SSH_USERAUTH_REQUEST to indicate that the signature will use the
specified algorithm.

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.

#define SSH_CERT_TYPE_USER    1
#define SSH_CERT_TYPE_HOST    2

RSA certificate

    string    "ssh-rsa-cert-v01@openssh.com"
    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    "ssh-dss-cert-v01@openssh.com"
    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    "ecdsa-sha2-nistp256-cert-v01@openssh.com" |
              "ecdsa-sha2-nistp384-cert-v01@openssh.com" |
              "ecdsa-sha2-nistp521-cert-v01@openssh.com"
    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

ED25519 certificate

    string    "ssh-ed25519-cert-v01@openssh.com"
    string    nonce
    string    pk
    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.

pk is the encoded Ed25519 public key as defined by RFC8032.

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.

"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

critical 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.

Generally, critical options are used to control features that restrict
access where extensions are used to enable features that grant access.
This ensures that certificates containing unknown restrictions do not
inadvertently grant access while allowing new protocol features to be
enabled via extensions without breaking certificates' backwards
compatibility.

The reserved field is currently unused and is ignored in this version of
the protocol.

The signature key field contains the CA key used to sign the
certificate. The valid key types for CA keys are ssh-rsa,
ssh-dss, ssh-ed25519 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
Ed25519 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
types, and RFC8032 for Ed25519).

Critical options
----------------

The critical options section of the certificate specifies zero or more
options on the certificate's 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
sequence. Each named option may only appear once in a certificate.

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.

Custom options should append the originating author or organisation's
domain name to the option name, e.g. "my-option@example.com".

No critical options are defined for host certificates at present. The
supported user certificate 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.

verify-required         empty         Flag indicating that signatures made
                                      with this certificate must assert FIDO
                                      user verification (e.g. PIN or
                                      biometric). This option only makes sense
                                      for the U2F/FIDO security key types that
                                      support this feature in their signature
                                      formats.

Extensions
----------

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,
as is the requirement that each name appear only once.

If an implementation does not recognise an extension, then it should
ignore it.

Custom options should append the originating author or organisation's
domain name to the option name, e.g. "my-option@example.com".

No extensions are defined for host certificates at present. The
supported user certificate extensions and the contents and structure of
their data fields are:

Name                    Format        Description
-----------------------------------------------------------------------------
no-touch-required       empty         Flag indicating that signatures made
                                      with this certificate need not assert
                                      FIDO user presence. This option only
                                      makes sense for the U2F/FIDO security
                                      key types that support this feature in
                                      their signature formats.

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
                                      disabled.

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 1.19 2021/06/05 13:47:00 naddy Exp $