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194 lines
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194 lines
8.0 KiB
Plaintext
This document describes a simple public-key certificate authentication
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system for use by SSH.
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Background
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----------
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The SSH protocol currently supports a simple public key authentication
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mechanism. Unlike other public key implementations, SSH eschews the
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use of X.509 certificates and uses raw keys. This approach has some
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benefits relating to simplicity of configuration and minimisation
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of attack surface, but it does not support the important use-cases
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of centrally managed, passwordless authentication and centrally
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certified host keys.
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These protocol extensions build on the simple public key authentication
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system already in SSH to allow certificate-based authentication.
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The certificates used are not traditional X.509 certificates, with
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numerous options and complex encoding rules, but something rather
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more minimal: a key, some identity information and usage constraints
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that have been signed with some other trusted key.
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A sshd server may be configured to allow authentication via certified
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keys, by extending the existing ~/.ssh/authorized_keys mechanism
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to allow specification of certification authority keys in addition
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to raw user keys. The ssh client will support automatic verification
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of acceptance of certified host keys, by adding a similar ability
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to specify CA keys in ~/.ssh/known_hosts.
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Certified keys are represented using two new key types:
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ssh-rsa-cert-v00@openssh.com and ssh-dss-cert-v00@openssh.com that
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include certification information along with the public key that is used
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to sign challenges. ssh-keygen performs the CA signing operation.
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Protocol extensions
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-------------------
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The SSH wire protocol includes several extensibility mechanisms.
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These modifications shall take advantage of namespaced public key
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algorithm names to add support for certificate authentication without
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breaking the protocol - implementations that do not support the
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extensions will simply ignore them.
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Authentication using the new key formats described below proceeds
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using the existing SSH "publickey" authentication method described
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in RFC4252 section 7.
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New public key formats
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----------------------
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The ssh-rsa-cert-v00@openssh.com and ssh-dss-cert-v00@openssh.com key
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types take a similar high-level format (note: data types and
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encoding are as per RFC4251 section 5). The serialised wire encoding of
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these certificates is also used for storing them on disk.
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#define SSH_CERT_TYPE_USER 1
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#define SSH_CERT_TYPE_HOST 2
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RSA certificate
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string "ssh-rsa-cert-v00@openssh.com"
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mpint e
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mpint n
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uint32 type
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string key id
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string valid principals
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uint64 valid after
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uint64 valid before
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string constraints
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string nonce
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string reserved
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string signature key
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string signature
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DSA certificate
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string "ssh-dss-cert-v00@openssh.com"
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mpint p
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mpint q
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mpint g
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mpint y
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uint32 type
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string key id
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string valid principals
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uint64 valid after
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uint64 valid before
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string constraints
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string nonce
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string reserved
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string signature key
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string signature
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e and n are the RSA exponent and public modulus respectively.
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p, q, g, y are the DSA parameters as described in FIPS-186-2.
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type specifies whether this certificate is for identification of a user
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or a host using a SSH_CERT_TYPE_... value.
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key id is a free-form text field that is filled in by the CA at the time
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of signing; the intention is that the contents of this field are used to
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identify the identity principal in log messages.
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"valid principals" is a string containing zero or more principals as
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strings packed inside it. These principals list the names for which this
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certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and
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usernames for SSH_CERT_TYPE_USER certificates. As a special case, a
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zero-length "valid principals" field means the certificate is valid for
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any principal of the specified type. XXX DNS wildcards?
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"valid after" and "valid before" specify a validity period for the
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certificate. Each represents a time in seconds since 1970-01-01
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00:00:00. A certificate is considered valid if:
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valid after <= current time < valid before
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constraints is a set of zero or more key constraints encoded as below.
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The nonce field is a CA-provided random bitstring of arbitrary length
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(but typically 16 or 32 bytes) included to make attacks that depend on
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inducing collisions in the signature hash infeasible.
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The reserved field is current unused and is ignored in this version of
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the protocol.
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signature key contains the CA key used to sign the certificate.
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The valid key types for CA keys are ssh-rsa and ssh-dss. "Chained"
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certificates, where the signature key type is a certificate type itself
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are NOT supported. Note that it is possible for a RSA certificate key to
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be signed by a DSS CA key and vice-versa.
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signature is computed over all preceding fields from the initial string
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up to, and including the signature key. Signatures are computed and
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encoded according to the rules defined for the CA's public key algorithm
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(RFC4253 section 6.6 for ssh-rsa and ssh-dss).
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Constraints
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-----------
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The constraints section of the certificate specifies zero or more
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constraints on the certificates validity. The format of this field
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is a sequence of zero or more tuples:
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string name
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string data
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The name field identifies the constraint and the data field encodes
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constraint-specific information (see below). All constraints are
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"critical", if an implementation does not recognise a constraint
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then the validating party should refuse to accept the certificate.
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The supported constraints and the contents and structure of their
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data fields are:
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Name Format Description
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-----------------------------------------------------------------------------
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force-command string Specifies a command that is executed
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(replacing any the user specified on the
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ssh command-line) whenever this key is
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used for authentication.
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permit-X11-forwarding empty Flag indicating that X11 forwarding
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should be permitted. X11 forwarding will
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be refused if this constraint is absent.
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permit-agent-forwarding empty Flag indicating that agent forwarding
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should be allowed. Agent forwarding
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must not be permitted unless this
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constraint is present.
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permit-port-forwarding empty Flag indicating that port-forwarding
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should be allowed. If this constraint is
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not present then no port forwarding will
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be allowed.
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permit-pty empty Flag indicating that PTY allocation
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should be permitted. In the absence of
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this constraint PTY allocation will be
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disabled.
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permit-user-rc empty Flag indicating that execution of
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~/.ssh/rc should be permitted. Execution
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of this script will not be permitted if
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this constraint is not present.
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source-address string Comma-separated list of source addresses
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from which this certificate is accepted
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for authentication. Addresses are
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specified in CIDR format (nn.nn.nn.nn/nn
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or hhhh::hhhh/nn).
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If this constraint is not present then
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certificates may be presented from any
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source address.
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$OpenBSD: PROTOCOL.certkeys,v 1.3 2010/03/03 22:50:40 djm Exp $
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