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 use 9 of X.509 certificates and uses raw keys. This approach has some benefits 10 relating to simplicity of configuration and minimisation of attack 11 surface, but it does not support the important use-cases of centrally 12 managed, passwordless authentication and centrally certified host keys. 13 14 These protocol extensions build on the simple public key authentication 15 system already in SSH to allow certificate-based authentication. The 16 certificates used are not traditional X.509 certificates, with numerous 17 options and complex encoding rules, but something rather more minimal: a 18 key, some identity information and usage options that have been signed 19 with some other trusted key. 20 21 A sshd server may be configured to allow authentication via certified 22 keys, by extending the existing ~/.ssh/authorized_keys mechanism to 23 allow specification of certification authority keys in addition to 24 raw user keys. The ssh client will support automatic verification of 25 acceptance of certified host keys, by adding a similar ability to 26 specify CA keys in ~/.ssh/known_hosts. 27 28 Certified keys are represented using new key types: 29 30 ssh-rsa-cert-v01 (a] openssh.com 31 ssh-dss-cert-v01 (a] openssh.com 32 ecdsa-sha2-nistp256-cert-v01 (a] openssh.com 33 ecdsa-sha2-nistp384-cert-v01 (a] openssh.com 34 ecdsa-sha2-nistp521-cert-v01 (a] openssh.com 35 36 These include certification information along with the public key 37 that is used to sign challenges. ssh-keygen performs the CA signing 38 operation. 39 40 Protocol extensions 41 ------------------- 42 43 The SSH wire protocol includes several extensibility mechanisms. 44 These modifications shall take advantage of namespaced public key 45 algorithm names to add support for certificate authentication without 46 breaking the protocol - implementations that do not support the 47 extensions will simply ignore them. 48 49 Authentication using the new key formats described below proceeds 50 using the existing SSH "publickey" authentication method described 51 in RFC4252 section 7. 52 53 New public key formats 54 ---------------------- 55 56 The certificate key types take a similar high-level format (note: data 57 types and encoding are as per RFC4251 section 5). The serialised wire 58 encoding of these certificates is also used for storing them on disk. 59 60 #define SSH_CERT_TYPE_USER 1 61 #define SSH_CERT_TYPE_HOST 2 62 63 RSA certificate 64 65 string "ssh-rsa-cert-v01 (a] openssh.com" 66 string nonce 67 mpint e 68 mpint n 69 uint64 serial 70 uint32 type 71 string key id 72 string valid principals 73 uint64 valid after 74 uint64 valid before 75 string critical options 76 string extensions 77 string reserved 78 string signature key 79 string signature 80 81 DSA certificate 82 83 string "ssh-dss-cert-v01 (a] openssh.com" 84 string nonce 85 mpint p 86 mpint q 87 mpint g 88 mpint y 89 uint64 serial 90 uint32 type 91 string key id 92 string valid principals 93 uint64 valid after 94 uint64 valid before 95 string critical options 96 string extensions 97 string reserved 98 string signature key 99 string signature 100 101 ECDSA certificate 102 103 string "ecdsa-sha2-nistp256-v01 (a] openssh.com" | 104 "ecdsa-sha2-nistp384-v01 (a] openssh.com" | 105 "ecdsa-sha2-nistp521-v01 (a] openssh.com" 106 string nonce 107 string curve 108 string public_key 109 uint64 serial 110 uint32 type 111 string key id 112 string valid principals 113 uint64 valid after 114 uint64 valid before 115 string critical options 116 string extensions 117 string reserved 118 string signature key 119 string signature 120 121 ED25519 certificate 122 123 string "ssh-ed25519-cert-v01 (a] openssh.com" 124 string nonce 125 string pk 126 uint64 serial 127 uint32 type 128 string key id 129 string valid principals 130 uint64 valid after 131 uint64 valid before 132 string critical options 133 string extensions 134 string reserved 135 string signature key 136 string signature 137 138 The nonce field is a CA-provided random bitstring of arbitrary length 139 (but typically 16 or 32 bytes) included to make attacks that depend on 140 inducing collisions in the signature hash infeasible. 141 142 e and n are the RSA exponent and public modulus respectively. 143 144 p, q, g, y are the DSA parameters as described in FIPS-186-2. 145 146 curve and public key are respectively the ECDSA "[identifier]" and "Q" 147 defined in section 3.1 of RFC5656. 148 149 pk is the encoded Ed25519 public key as defined by 150 draft-josefsson-eddsa-ed25519-03. 151 152 serial is an optional certificate serial number set by the CA to 153 provide an abbreviated way to refer to certificates from that CA. 154 If a CA does not wish to number its certificates it must set this 155 field to zero. 156 157 type specifies whether this certificate is for identification of a user 158 or a host using a SSH_CERT_TYPE_... value. 159 160 key id is a free-form text field that is filled in by the CA at the time 161 of signing; the intention is that the contents of this field are used to 162 identify the identity principal in log messages. 163 164 "valid principals" is a string containing zero or more principals as 165 strings packed inside it. These principals list the names for which this 166 certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and 167 usernames for SSH_CERT_TYPE_USER certificates. As a special case, a 168 zero-length "valid principals" field means the certificate is valid for 169 any principal of the specified type. 170 171 "valid after" and "valid before" specify a validity period for the 172 certificate. Each represents a time in seconds since 1970-01-01 173 00:00:00. A certificate is considered valid if: 174 175 valid after <= current time < valid before 176 177 criticial options is a set of zero or more key options encoded as 178 below. All such options are "critical" in the sense that an implementation 179 must refuse to authorise a key that has an unrecognised option. 180 181 extensions is a set of zero or more optional extensions. These extensions 182 are not critical, and an implementation that encounters one that it does 183 not recognise may safely ignore it. 184 185 Generally, critical options are used to control features that restrict 186 access where extensions are used to enable features that grant access. 187 This ensures that certificates containing unknown restrictions do not 188 inadvertently grant access while allowing new protocol features to be 189 enabled via extensions without breaking certificates' backwards 190 compatibility. 191 192 The reserved field is currently unused and is ignored in this version of 193 the protocol. 194 195 signature key contains the CA key used to sign the certificate. 196 The valid key types for CA keys are ssh-rsa, ssh-dss and the ECDSA types 197 ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained" 198 certificates, where the signature key type is a certificate type itself 199 are NOT supported. Note that it is possible for a RSA certificate key to 200 be signed by a DSS or ECDSA CA key and vice-versa. 201 202 signature is computed over all preceding fields from the initial string 203 up to, and including the signature key. Signatures are computed and 204 encoded according to the rules defined for the CA's public key algorithm 205 (RFC4253 section 6.6 for ssh-rsa and ssh-dss, RFC5656 for the ECDSA 206 types), and draft-josefsson-eddsa-ed25519-03 for Ed25519. 207 208 Critical options 209 ---------------- 210 211 The critical options section of the certificate specifies zero or more 212 options on the certificates validity. The format of this field 213 is a sequence of zero or more tuples: 214 215 string name 216 string data 217 218 Options must be lexically ordered by "name" if they appear in the 219 sequence. Each named option may only appear once in a certificate. 220 221 The name field identifies the option and the data field encodes 222 option-specific information (see below). All options are 223 "critical", if an implementation does not recognise a option 224 then the validating party should refuse to accept the certificate. 225 226 No critical options are defined for host certificates at present. The 227 supported user certificate options and the contents and structure of 228 their data fields are: 229 230 Name Format Description 231 ----------------------------------------------------------------------------- 232 force-command string Specifies a command that is executed 233 (replacing any the user specified on the 234 ssh command-line) whenever this key is 235 used for authentication. 236 237 source-address string Comma-separated list of source addresses 238 from which this certificate is accepted 239 for authentication. Addresses are 240 specified in CIDR format (nn.nn.nn.nn/nn 241 or hhhh::hhhh/nn). 242 If this option is not present then 243 certificates may be presented from any 244 source address. 245 246 Extensions 247 ---------- 248 249 The extensions section of the certificate specifies zero or more 250 non-critical certificate extensions. The encoding and ordering of 251 extensions in this field is identical to that of the critical options, 252 as is the requirement that each name appear only once. 253 254 If an implementation does not recognise an extension, then it should 255 ignore it. 256 257 No extensions are defined for host certificates at present. The 258 supported user certificate extensions and the contents and structure of 259 their data fields are: 260 261 Name Format Description 262 ----------------------------------------------------------------------------- 263 permit-X11-forwarding empty Flag indicating that X11 forwarding 264 should be permitted. X11 forwarding will 265 be refused if this option is absent. 266 267 permit-agent-forwarding empty Flag indicating that agent forwarding 268 should be allowed. Agent forwarding 269 must not be permitted unless this 270 option is present. 271 272 permit-port-forwarding empty Flag indicating that port-forwarding 273 should be allowed. If this option is 274 not present then no port forwarding will 275 be allowed. 276 277 permit-pty empty Flag indicating that PTY allocation 278 should be permitted. In the absence of 279 this option PTY allocation will be 280 disabled. 281 282 permit-user-rc empty Flag indicating that execution of 283 ~/.ssh/rc should be permitted. Execution 284 of this script will not be permitted if 285 this option is not present. 286 287 $OpenBSD: PROTOCOL.certkeys,v 1.10 2016/05/03 10:27:59 djm Exp $ 288
