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 $