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 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 1.3 2010/03/03 22:50:40 djm Exp $