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 28Certified keys are represented using new key types: 29 30 ssh-rsa-cert-v01@openssh.com 31 ssh-dss-cert-v01@openssh.com 32 ecdsa-sha2-nistp256-cert-v01@openssh.com 33 ecdsa-sha2-nistp384-cert-v01@openssh.com 34 ecdsa-sha2-nistp521-cert-v01@openssh.com 35 36These include certification information along with the public key 37that is used to sign challenges. ssh-keygen performs the CA signing 38operation. 39 40Protocol extensions 41------------------- 42 43The SSH wire protocol includes several extensibility mechanisms. 44These modifications shall take advantage of namespaced public key 45algorithm names to add support for certificate authentication without 46breaking the protocol - implementations that do not support the 47extensions will simply ignore them. 48 49Authentication using the new key formats described below proceeds 50using the existing SSH "publickey" authentication method described 51in RFC4252 section 7. 52 53New public key formats 54---------------------- 55 56The certificate key types take a similar high-level format (note: data 57types and encoding are as per RFC4251 section 5). The serialised wire 58encoding 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 63RSA certificate 64 65 string "ssh-rsa-cert-v01@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 81DSA certificate 82 83 string "ssh-dss-cert-v01@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 101ECDSA certificate 102 103 string "ecdsa-sha2-nistp256-v01@openssh.com" | 104 "ecdsa-sha2-nistp384-v01@openssh.com" | 105 "ecdsa-sha2-nistp521-v01@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 121ED25519 certificate 122 123 string "ssh-ed25519-cert-v01@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 138The 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 140inducing collisions in the signature hash infeasible. 141 142e and n are the RSA exponent and public modulus respectively. 143 144p, q, g, y are the DSA parameters as described in FIPS-186-2. 145 146curve and public key are respectively the ECDSA "[identifier]" and "Q" 147defined in section 3.1 of RFC5656. 148 149pk is the encoded Ed25519 public key as defined by 150draft-josefsson-eddsa-ed25519-03. 151 152serial is an optional certificate serial number set by the CA to 153provide an abbreviated way to refer to certificates from that CA. 154If a CA does not wish to number its certificates it must set this 155field to zero. 156 157type specifies whether this certificate is for identification of a user 158or a host using a SSH_CERT_TYPE_... value. 159 160key id is a free-form text field that is filled in by the CA at the time 161of signing; the intention is that the contents of this field are used to 162identify the identity principal in log messages. 163 164"valid principals" is a string containing zero or more principals as 165strings packed inside it. These principals list the names for which this 166certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and 167usernames for SSH_CERT_TYPE_USER certificates. As a special case, a 168zero-length "valid principals" field means the certificate is valid for 169any principal of the specified type. 170 171"valid after" and "valid before" specify a validity period for the 172certificate. Each represents a time in seconds since 1970-01-01 17300:00:00. A certificate is considered valid if: 174 175 valid after <= current time < valid before 176 177criticial options is a set of zero or more key options encoded as 178below. All such options are "critical" in the sense that an implementation 179must refuse to authorise a key that has an unrecognised option. 180 181extensions is a set of zero or more optional extensions. These extensions 182are not critical, and an implementation that encounters one that it does 183not recognise may safely ignore it. 184 185Generally, critical options are used to control features that restrict 186access where extensions are used to enable features that grant access. 187This ensures that certificates containing unknown restrictions do not 188inadvertently grant access while allowing new protocol features to be 189enabled via extensions without breaking certificates' backwards 190compatibility. 191 192The reserved field is currently unused and is ignored in this version of 193the protocol. 194 195signature key contains the CA key used to sign the certificate. 196The valid key types for CA keys are ssh-rsa, ssh-dss and the ECDSA types 197ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained" 198certificates, where the signature key type is a certificate type itself 199are NOT supported. Note that it is possible for a RSA certificate key to 200be signed by a DSS or ECDSA CA key and vice-versa. 201 202signature is computed over all preceding fields from the initial string 203up to, and including the signature key. Signatures are computed and 204encoded 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 206types), and draft-josefsson-eddsa-ed25519-03 for Ed25519. 207 208Critical options 209---------------- 210 211The critical options section of the certificate specifies zero or more 212options on the certificates validity. The format of this field 213is a sequence of zero or more tuples: 214 215 string name 216 string data 217 218Options must be lexically ordered by "name" if they appear in the 219sequence. Each named option may only appear once in a certificate. 220 221The name field identifies the option and the data field encodes 222option-specific information (see below). All options are 223"critical", if an implementation does not recognise a option 224then the validating party should refuse to accept the certificate. 225 226No critical options are defined for host certificates at present. The 227supported user certificate options and the contents and structure of 228their data fields are: 229 230Name Format Description 231----------------------------------------------------------------------------- 232force-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 237source-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 246Extensions 247---------- 248 249The extensions section of the certificate specifies zero or more 250non-critical certificate extensions. The encoding and ordering of 251extensions in this field is identical to that of the critical options, 252as is the requirement that each name appear only once. 253 254If an implementation does not recognise an extension, then it should 255ignore it. 256 257No extensions are defined for host certificates at present. The 258supported user certificate extensions and the contents and structure of 259their data fields are: 260 261Name Format Description 262----------------------------------------------------------------------------- 263permit-X11-forwarding empty Flag indicating that X11 forwarding 264 should be permitted. X11 forwarding will 265 be refused if this option is absent. 266 267permit-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 272permit-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 277permit-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 282permit-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