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 (a] openssh.com" | 104 "ecdsa-sha2-nistp384 (a] openssh.com" | 105 "ecdsa-sha2-nistp521 (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 The nonce field is a CA-provided random bitstring of arbitrary length 122 (but typically 16 or 32 bytes) included to make attacks that depend on 123 inducing collisions in the signature hash infeasible. 124 125 e and n are the RSA exponent and public modulus respectively. 126 127 p, q, g, y are the DSA parameters as described in FIPS-186-2. 128 129 curve and public key are respectively the ECDSA "[identifier]" and "Q" 130 defined in section 3.1 of RFC5656. 131 132 serial is an optional certificate serial number set by the CA to 133 provide an abbreviated way to refer to certificates from that CA. 134 If a CA does not wish to number its certificates it must set this 135 field to zero. 136 137 type specifies whether this certificate is for identification of a user 138 or a host using a SSH_CERT_TYPE_... value. 139 140 key id is a free-form text field that is filled in by the CA at the time 141 of signing; the intention is that the contents of this field are used to 142 identify the identity principal in log messages. 143 144 "valid principals" is a string containing zero or more principals as 145 strings packed inside it. These principals list the names for which this 146 certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and 147 usernames for SSH_CERT_TYPE_USER certificates. As a special case, a 148 zero-length "valid principals" field means the certificate is valid for 149 any principal of the specified type. XXX DNS wildcards? 150 151 "valid after" and "valid before" specify a validity period for the 152 certificate. Each represents a time in seconds since 1970-01-01 153 00:00:00. A certificate is considered valid if: 154 155 valid after <= current time < valid before 156 157 criticial options is a set of zero or more key options encoded as 158 below. All such options are "critical" in the sense that an implementation 159 must refuse to authorise a key that has an unrecognised option. 160 161 extensions is a set of zero or more optional extensions. These extensions 162 are not critical, and an implementation that encounters one that it does 163 not recognise may safely ignore it. 164 165 Generally, critical options are used to control features that restrict 166 access where extensions are used to enable features that grant access. 167 This ensures that certificates containing unknown restrictions do not 168 inadvertently grant access while allowing new protocol features to be 169 enabled via extensions without breaking certificates' backwards 170 compatibility. 171 172 The reserved field is currently unused and is ignored in this version of 173 the protocol. 174 175 signature key contains the CA key used to sign the certificate. 176 The valid key types for CA keys are ssh-rsa, ssh-dss and the ECDSA types 177 ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained" 178 certificates, where the signature key type is a certificate type itself 179 are NOT supported. Note that it is possible for a RSA certificate key to 180 be signed by a DSS or ECDSA CA key and vice-versa. 181 182 signature is computed over all preceding fields from the initial string 183 up to, and including the signature key. Signatures are computed and 184 encoded according to the rules defined for the CA's public key algorithm 185 (RFC4253 section 6.6 for ssh-rsa and ssh-dss, RFC5656 for the ECDSA 186 types). 187 188 Critical options 189 ---------------- 190 191 The critical options section of the certificate specifies zero or more 192 options on the certificates validity. The format of this field 193 is a sequence of zero or more tuples: 194 195 string name 196 string data 197 198 Options must be lexically ordered by "name" if they appear in the 199 sequence. Each named option may only appear once in a certificate. 200 201 The name field identifies the option and the data field encodes 202 option-specific information (see below). All options are 203 "critical", if an implementation does not recognise a option 204 then the validating party should refuse to accept the certificate. 205 206 The supported options and the contents and structure of their 207 data fields are: 208 209 Name Format Description 210 ----------------------------------------------------------------------------- 211 force-command string Specifies a command that is executed 212 (replacing any the user specified on the 213 ssh command-line) whenever this key is 214 used for authentication. 215 216 source-address string Comma-separated list of source addresses 217 from which this certificate is accepted 218 for authentication. Addresses are 219 specified in CIDR format (nn.nn.nn.nn/nn 220 or hhhh::hhhh/nn). 221 If this option is not present then 222 certificates may be presented from any 223 source address. 224 225 Extensions 226 ---------- 227 228 The extensions section of the certificate specifies zero or more 229 non-critical certificate extensions. The encoding and ordering of 230 extensions in this field is identical to that of the critical options, 231 as is the requirement that each name appear only once. 232 233 If an implementation does not recognise an extension, then it should 234 ignore it. 235 236 The supported extensions and the contents and structure of their data 237 fields are: 238 239 Name Format Description 240 ----------------------------------------------------------------------------- 241 permit-X11-forwarding empty Flag indicating that X11 forwarding 242 should be permitted. X11 forwarding will 243 be refused if this option is absent. 244 245 permit-agent-forwarding empty Flag indicating that agent forwarding 246 should be allowed. Agent forwarding 247 must not be permitted unless this 248 option is present. 249 250 permit-port-forwarding empty Flag indicating that port-forwarding 251 should be allowed. If this option is 252 not present then no port forwarding will 253 be allowed. 254 255 permit-pty empty Flag indicating that PTY allocation 256 should be permitted. In the absence of 257 this option PTY allocation will be 258 disabled. 259 260 permit-user-rc empty Flag indicating that execution of 261 ~/.ssh/rc should be permitted. Execution 262 of this script will not be permitted if 263 this option is not present. 264 265 $OpenBSD: PROTOCOL.certkeys,v 1.9 2012/03/28 07:23:22 djm Exp $ 266
