Annotation of src/usr.bin/ssh/PROTOCOL.certkeys, Revision 1.1
1.1 ! djm 1: This document describes a simple public-key certificate authentication
! 2: system for use by SSH.
! 3:
! 4: Background
! 5: ----------
! 6:
! 7: The SSH protocol currently supports a simple public key authentication
! 8: mechanism. Unlike other public key implementations, SSH eschews the
! 9: use of X.509 certificates and uses raw keys. This approach has some
! 10: benefits relating to simplicity of configuration and minimisation
! 11: of attack surface, but it does not support the important use-cases
! 12: of centrally managed, passwordless authentication and centrally
! 13: certified host keys.
! 14:
! 15: These protocol extensions build on the simple public key authentication
! 16: system already in SSH to allow certificate-based authentication.
! 17: The certificates used are not traditional X.509 certificates, with
! 18: numerous options and complex encoding rules, but something rather
! 19: more minimal: a key, some identity information and usage constraints
! 20: that have been signed with some other trusted key.
! 21:
! 22: A sshd server may be configured to allow authentication via certified
! 23: keys, by extending the existing ~/.ssh/authorized_keys mechanism
! 24: to allow specification of certification authority keys in addition
! 25: to raw user keys. The ssh client will support automatic verification
! 26: of acceptance of certified host keys, by adding a similar ability
! 27: to specify CA keys in ~/.ssh/known_hosts.
! 28:
! 29: Certified keys are represented using two new key types:
! 30: ssh-rsa-cert-v00@openssh.com and ssh-dss-cert-v00@openssh.com that
! 31: include certification information along with the public key that is used
! 32: to sign challenges. ssh-keygen performs the CA signing operation.
! 33:
! 34: Protocol extensions
! 35: -------------------
! 36:
! 37: The SSH wire protocol includes several extensibility mechanisms.
! 38: These modifications shall take advantage of namespaced public key
! 39: algorithm names to add support for certificate authentication without
! 40: breaking the protocol - implementations that do not support the
! 41: extensions will simply ignore them.
! 42:
! 43: Authentication using the new key formats described below proceeds
! 44: using the existing SSH "publickey" authentication method described
! 45: in RFC4252 section 7.
! 46:
! 47: New public key formats
! 48: ----------------------
! 49:
! 50: The ssh-rsa-cert-v00@openssh.com and ssh-dss-cert-v00@openssh.com key
! 51: types take a similar same high-level format (note: data types and
! 52: encoding are as per RFC4251 section 5). The serialised wire encoding of
! 53: these certificates is also used for storing them on disk.
! 54:
! 55: #define SSH_CERT_TYPE_USER 1
! 56: #define SSH_CERT_TYPE_HOST 2
! 57:
! 58: RSA certificate
! 59:
! 60: string "ssh-rsa-cert-v00@openssh.com"
! 61: mpint e
! 62: mpint n
! 63: uint32 type
! 64: string key id
! 65: string valid principals
! 66: uint64 valid after
! 67: uint64 valid before
! 68: string constraints
! 69: string nonce
! 70: string reserved
! 71: string signature key
! 72: string signature
! 73:
! 74: DSA certificate
! 75:
! 76: string "ssh-dss-cert-v00@openssh.com"
! 77: mpint p
! 78: mpint q
! 79: mpint g
! 80: mpint y
! 81: uint32 type
! 82: string key id
! 83: string valid principals
! 84: uint64 valid after
! 85: uint64 valid before
! 86: string constraints
! 87: string nonce
! 88: string reserved
! 89: string signature key
! 90: string signature
! 91:
! 92: e and n are the RSA exponent and public modulus respectively.
! 93:
! 94: p, q, g, y are the DSA parameters as described in FIPS-186-2.
! 95:
! 96: type specifies whether this certificate is for identification of a user
! 97: or a host using a SSH_CERT_TYPE_... value.
! 98:
! 99: key id is a free-form text field that is filled in by the CA at the time
! 100: of signing; the intention is that the contents of this field are used to
! 101: identify the identity principal in log messages.
! 102:
! 103: "valid principals" is a string containing zero or more principals as
! 104: strings packed inside it. These principals list the names for which this
! 105: certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and
! 106: usernames for SSH_CERT_TYPE_USER certificates. As a special case, a
! 107: zero-length "valid principals" field means the certificate is valid for
! 108: any principal of the specified type. XXX DNS wildcards?
! 109:
! 110: "valid after" and "valid before" specify a validity period for the
! 111: certificate. Each represents a time in seconds since 1970-01-01
! 112: 00:00:00. A certificate is considered valid if:
! 113: valid after <= current time < valid before
! 114:
! 115: constraints is a set of zero or more key constraints encoded as below.
! 116:
! 117: The nonce field is a CA-provided random bitstring of arbitrary length
! 118: (but typically 16 or 32 bytes) included to make attacks that depend on
! 119: inducing collisions in the signature hash infeasible.
! 120:
! 121: The reserved field is current unused and is ignored in this version of
! 122: the protocol.
! 123:
! 124: signature key contains the CA key used to sign the certificate.
! 125: The valid key types for CA keys are ssh-rsa and ssh-dss. "Chained"
! 126: certificates, where the signature key type is a certificate type itself
! 127: are NOT supported. Note that it is possible for a RSA certificate key to
! 128: be signed by a DSS CA key and vice-versa.
! 129:
! 130: signature is computed over all preceding fields from the initial string
! 131: up to, and including the signature key. Signatures are computed and
! 132: encoded according to the rules defined for the CA's public key algorithm
! 133: (RFC4253 section 6.6 for ssh-rsa and ssh-dss).
! 134:
! 135: Constraints
! 136: -----------
! 137:
! 138: The constraints section of the certificate specifies zero or more
! 139: constraints on the certificates validity. The format of this field
! 140: is a sequence of zero or more tuples:
! 141:
! 142: string name
! 143: string data
! 144:
! 145: The name field identifies the constraint and the data field encodes
! 146: constraint-specific information (see below). All constraints are
! 147: "critical", if an implementation does not recognise a constraint
! 148: then the validating party should refuse to accept the certificate.
! 149:
! 150: The supported constraints and the contents and structure of their
! 151: data fields are:
! 152:
! 153: Name Format Description
! 154: -----------------------------------------------------------------------------
! 155: force-command string Specifies a command that is executed
! 156: (replacing any the user specified on the
! 157: ssh command-line) whenever this key is
! 158: used for authentication.
! 159:
! 160: permit-X11-forwarding empty Flag indicating that X11 forwarding
! 161: should be permitted. X11 forwarding will
! 162: be refused if this constraint is absent.
! 163:
! 164: permit-agent-forwarding empty Flag indicating that agent forwarding
! 165: should be allowed. Agent forwarding
! 166: must not be permitted unless this
! 167: constraint is present.
! 168:
! 169: permit-port-forwarding empty Flag indicating that port-forwarding
! 170: should be allowed. If this constraint is
! 171: not present then no port forwarding will
! 172: be allowed.
! 173:
! 174: permit-pty empty Flag indicating that PTY allocation
! 175: should be permitted. In the absence of
! 176: this constraint PTY allocation will be
! 177: disabled.
! 178:
! 179: permit-user-rc empty Flag indicating that execution of
! 180: ~/.ssh/rc should be permitted. Execution
! 181: of this script will not be permitted if
! 182: this constraint is not present.
! 183:
! 184: source-address string Comma-separated list of source addresses
! 185: from which this certificate is accepted
! 186: for authentication. Addresses are
! 187: specified in CIDR format (nn.nn.nn.nn/nn
! 188: or hhhh::hhhh/nn).
! 189: If this constraint is not present then
! 190: certificates may be presented from any
! 191: source address.
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