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 40Two additional types exist for RSA certificates to force use of 41SHA-2 signatures (SHA-256 and SHA-512 respectively): 42 43 rsa-sha2-256-cert-v01@openssh.com 44 rsa-sha2-512-cert-v01@openssh.com 45 46These RSA/SHA-2 types should not appear in keys at rest or transmitted 47on their wire, but do appear in a SSH_MSG_KEXINIT's host-key algorithms 48field or in the "public key algorithm name" field of a "publickey" 49SSH_USERAUTH_REQUEST to indicate that the signature will use the 50specified algorithm. 51 52Protocol extensions 53------------------- 54 55The SSH wire protocol includes several extensibility mechanisms. 56These modifications shall take advantage of namespaced public key 57algorithm names to add support for certificate authentication without 58breaking the protocol - implementations that do not support the 59extensions will simply ignore them. 60 61Authentication using the new key formats described below proceeds 62using the existing SSH "publickey" authentication method described 63in RFC4252 section 7. 64 65New public key formats 66---------------------- 67 68The certificate key types take a similar high-level format (note: data 69types and encoding are as per RFC4251 section 5). The serialised wire 70encoding of these certificates is also used for storing them on disk. 71 72#define SSH_CERT_TYPE_USER 1 73#define SSH_CERT_TYPE_HOST 2 74 75RSA certificate 76 77 string "ssh-rsa-cert-v01@openssh.com" 78 string nonce 79 mpint e 80 mpint n 81 uint64 serial 82 uint32 type 83 string key id 84 string valid principals 85 uint64 valid after 86 uint64 valid before 87 string critical options 88 string extensions 89 string reserved 90 string signature key 91 string signature 92 93DSA certificate 94 95 string "ssh-dss-cert-v01@openssh.com" 96 string nonce 97 mpint p 98 mpint q 99 mpint g 100 mpint y 101 uint64 serial 102 uint32 type 103 string key id 104 string valid principals 105 uint64 valid after 106 uint64 valid before 107 string critical options 108 string extensions 109 string reserved 110 string signature key 111 string signature 112 113ECDSA certificate 114 115 string "ecdsa-sha2-nistp256-cert-v01@openssh.com" | 116 "ecdsa-sha2-nistp384-cert-v01@openssh.com" | 117 "ecdsa-sha2-nistp521-cert-v01@openssh.com" 118 string nonce 119 string curve 120 string public_key 121 uint64 serial 122 uint32 type 123 string key id 124 string valid principals 125 uint64 valid after 126 uint64 valid before 127 string critical options 128 string extensions 129 string reserved 130 string signature key 131 string signature 132 133ED25519 certificate 134 135 string "ssh-ed25519-cert-v01@openssh.com" 136 string nonce 137 string pk 138 uint64 serial 139 uint32 type 140 string key id 141 string valid principals 142 uint64 valid after 143 uint64 valid before 144 string critical options 145 string extensions 146 string reserved 147 string signature key 148 string signature 149 150The nonce field is a CA-provided random bitstring of arbitrary length 151(but typically 16 or 32 bytes) included to make attacks that depend on 152inducing collisions in the signature hash infeasible. 153 154e and n are the RSA exponent and public modulus respectively. 155 156p, q, g, y are the DSA parameters as described in FIPS-186-2. 157 158curve and public key are respectively the ECDSA "[identifier]" and "Q" 159defined in section 3.1 of RFC5656. 160 161pk is the encoded Ed25519 public key as defined by 162draft-josefsson-eddsa-ed25519-03. 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 draft-josefsson-eddsa-ed25519-03 for Ed25519. 220 221Critical options 222---------------- 223 224The critical options section of the certificate specifies zero or more 225options on the certificates 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 and the data field encodes 235option-specific information (see below). All options are 236"critical", if an implementation does not recognise a option 237then the validating party should refuse to accept the certificate. 238 239Custom options should append the originating author or organisation's 240domain name to the option name, e.g. "my-option@example.com". 241 242No critical options are defined for host certificates at present. The 243supported user certificate options and the contents and structure of 244their data fields are: 245 246Name Format Description 247----------------------------------------------------------------------------- 248force-command string Specifies a command that is executed 249 (replacing any the user specified on the 250 ssh command-line) whenever this key is 251 used for authentication. 252 253source-address string Comma-separated list of source addresses 254 from which this certificate is accepted 255 for authentication. Addresses are 256 specified in CIDR format (nn.nn.nn.nn/nn 257 or hhhh::hhhh/nn). 258 If this option is not present then 259 certificates may be presented from any 260 source address. 261 262Extensions 263---------- 264 265The extensions section of the certificate specifies zero or more 266non-critical certificate extensions. The encoding and ordering of 267extensions in this field is identical to that of the critical options, 268as is the requirement that each name appear only once. 269 270If an implementation does not recognise an extension, then it should 271ignore it. 272 273Custom options should append the originating author or organisation's 274domain name to the option name, e.g. "my-option@example.com". 275 276No extensions are defined for host certificates at present. The 277supported user certificate extensions and the contents and structure of 278their data fields are: 279 280Name Format Description 281----------------------------------------------------------------------------- 282permit-X11-forwarding empty Flag indicating that X11 forwarding 283 should be permitted. X11 forwarding will 284 be refused if this option is absent. 285 286permit-agent-forwarding empty Flag indicating that agent forwarding 287 should be allowed. Agent forwarding 288 must not be permitted unless this 289 option is present. 290 291permit-port-forwarding empty Flag indicating that port-forwarding 292 should be allowed. If this option is 293 not present then no port forwarding will 294 be allowed. 295 296permit-pty empty Flag indicating that PTY allocation 297 should be permitted. In the absence of 298 this option PTY allocation will be 299 disabled. 300 301permit-user-rc empty Flag indicating that execution of 302 ~/.ssh/rc should be permitted. Execution 303 of this script will not be permitted if 304 this option is not present. 305 306$OpenBSD: PROTOCOL.certkeys,v 1.15 2018/07/03 11:39:54 djm Exp $ 307