| 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 constraints |
| 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 two new key types: |
| ssh-rsa-cert-v00@openssh.com and ssh-dss-cert-v00@openssh.com that |
| include certification information along with the public key that is used |
| to sign challenges. ssh-keygen performs the CA signing operation. |
| |
| 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 ssh-rsa-cert-v00@openssh.com and ssh-dss-cert-v00@openssh.com key |
| types take a similar same 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-v00@openssh.com" |
| mpint e |
| mpint n |
| uint32 type |
| string key id |
| string valid principals |
| uint64 valid after |
| uint64 valid before |
| string constraints |
| string nonce |
| string reserved |
| string signature key |
| string signature |
| |
| DSA certificate |
| |
| string "ssh-dss-cert-v00@openssh.com" |
| mpint p |
| mpint q |
| mpint g |
| mpint y |
| uint32 type |
| string key id |
| string valid principals |
| uint64 valid after |
| uint64 valid before |
| string constraints |
| string nonce |
| string reserved |
| string signature key |
| string signature |
| |
| 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. |
| |
| 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 |
| |
| constraints is a set of zero or more key constraints encoded as below. |
| |
| 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. |
| |
| The reserved field is current 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 and ssh-dss. "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 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). |
| |
| Constraints |
| ----------- |
| |
| The constraints section of the certificate specifies zero or more |
| constraints on the certificates validity. The format of this field |
| is a sequence of zero or more tuples: |
| |
| string name |
| string data |
| |
| The name field identifies the constraint and the data field encodes |
| constraint-specific information (see below). All constraints are |
| "critical", if an implementation does not recognise a constraint |
| then the validating party should refuse to accept the certificate. |
| |
| The supported constraints 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. |
| |
| permit-X11-forwarding empty Flag indicating that X11 forwarding |
| should be permitted. X11 forwarding will |
| be refused if this constraint is absent. |
| |
| permit-agent-forwarding empty Flag indicating that agent forwarding |
| should be allowed. Agent forwarding |
| must not be permitted unless this |
| constraint is present. |
| |
| permit-port-forwarding empty Flag indicating that port-forwarding |
| should be allowed. If this constraint 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 constraint 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 constraint is not present. |
| |
| 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 constraint is not present then |
| certificates may be presented from any |
| source address. |
| |
| $OpenBSD: PROTOCOL.certkeys,v 1.2 2010/03/02 23:22:44 djm Exp $ |