1This document describes a simple public-key certificate authentication 2system for use by SSH. 3 4Background 5---------- 6 7The SSH protocol currently supports a simple public key authentication 8mechanism. Unlike other public key implementations, SSH eschews the use 9of X.509 certificates and uses raw keys. This approach has some benefits 10relating to simplicity of configuration and minimisation of attack 11surface, but it does not support the important use-cases of centrally 12managed, passwordless authentication and centrally certified host keys. 13 14These protocol extensions build on the simple public key authentication 15system already in SSH to allow certificate-based authentication. The 16certificates used are not traditional X.509 certificates, with numerous 17options and complex encoding rules, but something rather more minimal: a 18key, some identity information and usage options that have been signed 19with some other trusted key. 20 21A sshd server may be configured to allow authentication via certified 22keys, by extending the existing ~/.ssh/authorized_keys mechanism to 23allow specification of certification authority keys in addition to 24raw user keys. The ssh client will support automatic verification of 25acceptance of certified host keys, by adding a similar ability to 26specify CA keys in ~/.ssh/known_hosts. 27 28All certificate types include certification information along with the 29public key that is used to sign challenges. In OpenSSH, ssh-keygen 30performs the CA signing operation. 31 32Certified keys are represented using new key types: 33 34 ssh-rsa-cert-v01@openssh.com 35 ssh-dss-cert-v01@openssh.com 36 ecdsa-sha2-nistp256-cert-v01@openssh.com 37 ecdsa-sha2-nistp384-cert-v01@openssh.com 38 ecdsa-sha2-nistp521-cert-v01@openssh.com 39 ssh-ed25519-cert-v01@openssh.com 40 41Two additional types exist for RSA certificates to force use of 42SHA-2 signatures (SHA-256 and SHA-512 respectively): 43 44 rsa-sha2-256-cert-v01@openssh.com 45 rsa-sha2-512-cert-v01@openssh.com 46 47These RSA/SHA-2 types should not appear in keys at rest or transmitted 48on the wire, but do appear in a SSH_MSG_KEXINIT's host-key algorithms 49field or in the "public key algorithm name" field of a "publickey" 50SSH_USERAUTH_REQUEST to indicate that the signature will use the 51specified algorithm. 52 53Protocol extensions 54------------------- 55 56The SSH wire protocol includes several extensibility mechanisms. 57These modifications shall take advantage of namespaced public key 58algorithm names to add support for certificate authentication without 59breaking the protocol - implementations that do not support the 60extensions will simply ignore them. 61 62Authentication using the new key formats described below proceeds 63using the existing SSH "publickey" authentication method described 64in RFC4252 section 7. 65 66New public key formats 67---------------------- 68 69The certificate key types take a similar high-level format (note: data 70types and encoding are as per RFC4251 section 5). The serialised wire 71encoding of these certificates is also used for storing them on disk. 72 73#define SSH_CERT_TYPE_USER 1 74#define SSH_CERT_TYPE_HOST 2 75 76RSA certificate 77 78 string "ssh-rsa-cert-v01@openssh.com" 79 string nonce 80 mpint e 81 mpint n 82 uint64 serial 83 uint32 type 84 string key id 85 string valid principals 86 uint64 valid after 87 uint64 valid before 88 string critical options 89 string extensions 90 string reserved 91 string signature key 92 string signature 93 94DSA certificate 95 96 string "ssh-dss-cert-v01@openssh.com" 97 string nonce 98 mpint p 99 mpint q 100 mpint g 101 mpint y 102 uint64 serial 103 uint32 type 104 string key id 105 string valid principals 106 uint64 valid after 107 uint64 valid before 108 string critical options 109 string extensions 110 string reserved 111 string signature key 112 string signature 113 114ECDSA certificate 115 116 string "ecdsa-sha2-nistp256-cert-v01@openssh.com" | 117 "ecdsa-sha2-nistp384-cert-v01@openssh.com" | 118 "ecdsa-sha2-nistp521-cert-v01@openssh.com" 119 string nonce 120 string curve 121 string public_key 122 uint64 serial 123 uint32 type 124 string key id 125 string valid principals 126 uint64 valid after 127 uint64 valid before 128 string critical options 129 string extensions 130 string reserved 131 string signature key 132 string signature 133 134ED25519 certificate 135 136 string "ssh-ed25519-cert-v01@openssh.com" 137 string nonce 138 string pk 139 uint64 serial 140 uint32 type 141 string key id 142 string valid principals 143 uint64 valid after 144 uint64 valid before 145 string critical options 146 string extensions 147 string reserved 148 string signature key 149 string signature 150 151The nonce field is a CA-provided random bitstring of arbitrary length 152(but typically 16 or 32 bytes) included to make attacks that depend on 153inducing collisions in the signature hash infeasible. 154 155e and n are the RSA exponent and public modulus respectively. 156 157p, q, g, y are the DSA parameters as described in FIPS-186-2. 158 159curve and public key are respectively the ECDSA "[identifier]" and "Q" 160defined in section 3.1 of RFC5656. 161 162pk is the encoded Ed25519 public key as defined by RFC8032. 163 164serial is an optional certificate serial number set by the CA to 165provide an abbreviated way to refer to certificates from that CA. 166If a CA does not wish to number its certificates, it must set this 167field to zero. 168 169type specifies whether this certificate is for identification of a user 170or a host using a SSH_CERT_TYPE_... value. 171 172key id is a free-form text field that is filled in by the CA at the time 173of signing; the intention is that the contents of this field are used to 174identify the identity principal in log messages. 175 176"valid principals" is a string containing zero or more principals as 177strings packed inside it. These principals list the names for which this 178certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and 179usernames for SSH_CERT_TYPE_USER certificates. As a special case, a 180zero-length "valid principals" field means the certificate is valid for 181any principal of the specified type. 182 183"valid after" and "valid before" specify a validity period for the 184certificate. Each represents a time in seconds since 1970-01-01 18500:00:00. A certificate is considered valid if: 186 187 valid after <= current time < valid before 188 189critical options is a set of zero or more key options encoded as 190below. All such options are "critical" in the sense that an implementation 191must refuse to authorise a key that has an unrecognised option. 192 193extensions is a set of zero or more optional extensions. These extensions 194are not critical, and an implementation that encounters one that it does 195not recognise may safely ignore it. 196 197Generally, critical options are used to control features that restrict 198access where extensions are used to enable features that grant access. 199This ensures that certificates containing unknown restrictions do not 200inadvertently grant access while allowing new protocol features to be 201enabled via extensions without breaking certificates' backwards 202compatibility. 203 204The reserved field is currently unused and is ignored in this version of 205the protocol. 206 207The signature key field contains the CA key used to sign the 208certificate. The valid key types for CA keys are ssh-rsa, 209ssh-dss, ssh-ed25519 and the ECDSA types ecdsa-sha2-nistp256, 210ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained" certificates, where 211the signature key type is a certificate type itself are NOT supported. 212Note that it is possible for a RSA certificate key to be signed by a 213Ed25519 or ECDSA CA key and vice-versa. 214 215signature is computed over all preceding fields from the initial string 216up to, and including the signature key. Signatures are computed and 217encoded according to the rules defined for the CA's public key algorithm 218(RFC4253 section 6.6 for ssh-rsa and ssh-dss, RFC5656 for the ECDSA 219types, and RFC8032 for Ed25519). 220 221Critical options 222---------------- 223 224The critical options section of the certificate specifies zero or more 225options on the certificate's validity. The format of this field 226is a sequence of zero or more tuples: 227 228 string name 229 string data 230 231Options must be lexically ordered by "name" if they appear in the 232sequence. Each named option may only appear once in a certificate. 233 234The name field identifies the option. The data field contains 235option-specific information encoded as zero or more values inside 236the string. I.e. an empty data field would be encoded as a zero- 237length string (00 00 00 00), and data field that holds a single 238string value "a" would be encoded as (00 00 00 05 00 00 00 01 65). 239 240All options are "critical"; if an implementation does not recognise 241a option, then the validating party should refuse to accept the 242certificate. 243 244Custom options should append the originating author or organisation's 245domain name to the option name, e.g. "my-option@example.com". 246 247No critical options are defined for host certificates at present. The 248supported user certificate options and the contents and structure of 249their data fields are: 250 251Name Format Description 252----------------------------------------------------------------------------- 253force-command string Specifies a command that is executed 254 (replacing any the user specified on the 255 ssh command-line) whenever this key is 256 used for authentication. 257 258source-address string Comma-separated list of source addresses 259 from which this certificate is accepted 260 for authentication. Addresses are 261 specified in CIDR format (nn.nn.nn.nn/nn 262 or hhhh::hhhh/nn). 263 If this option is not present, then 264 certificates may be presented from any 265 source address. 266 267verify-required empty Flag indicating that signatures made 268 with this certificate must assert FIDO 269 user verification (e.g. PIN or 270 biometric). This option only makes sense 271 for the U2F/FIDO security key types that 272 support this feature in their signature 273 formats. 274 275Extensions 276---------- 277 278The extensions section of the certificate specifies zero or more 279non-critical certificate extensions. The encoding and ordering of 280extensions in this field is identical to that of the critical options, 281as is the requirement that each name appear only once. 282 283If an implementation does not recognise an extension, then it should 284ignore it. 285 286Custom options should append the originating author or organisation's 287domain name to the option name, e.g. "my-option@example.com". 288 289No extensions are defined for host certificates at present. The 290supported user certificate extensions and the contents and structure of 291their data fields are: 292 293Name Format Description 294----------------------------------------------------------------------------- 295no-touch-required empty Flag indicating that signatures made 296 with this certificate need not assert 297 FIDO user presence. This option only 298 makes sense for the U2F/FIDO security 299 key types that support this feature in 300 their signature formats. 301 302permit-X11-forwarding empty Flag indicating that X11 forwarding 303 should be permitted. X11 forwarding will 304 be refused if this option is absent. 305 306permit-agent-forwarding empty Flag indicating that agent forwarding 307 should be allowed. Agent forwarding 308 must not be permitted unless this 309 option is present. 310 311permit-port-forwarding empty Flag indicating that port-forwarding 312 should be allowed. If this option is 313 not present, then no port forwarding will 314 be allowed. 315 316permit-pty empty Flag indicating that PTY allocation 317 should be permitted. In the absence of 318 this option PTY allocation will be 319 disabled. 320 321permit-user-rc empty Flag indicating that execution of 322 ~/.ssh/rc should be permitted. Execution 323 of this script will not be permitted if 324 this option is not present. 325 326$OpenBSD: PROTOCOL.certkeys,v 1.20 2024/12/06 16:02:12 djm Exp $ 327