| 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. |
| |
| 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 |
| |
| These 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 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@openssh.com" | |
| "ecdsa-sha2-nistp384@openssh.com" | |
| "ecdsa-sha2-nistp521@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 |
| |
| 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. |
| |
| 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. |
| |
| 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 |
| types). |
| |
| 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 |
| 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. |
| |
| 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. |
| |
| 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. |
| |
| 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 |
| 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 1.9 2012/03/28 07:23:22 djm Exp $ |