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@openssh.com" | 104 "ecdsa-sha2-nistp384@openssh.com" | 105 "ecdsa-sha2-nistp521@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 121The 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 123inducing collisions in the signature hash infeasible. 124 125e and n are the RSA exponent and public modulus respectively. 126 127p, q, g, y are the DSA parameters as described in FIPS-186-2. 128 129curve and public key are respectively the ECDSA "[identifier]" and "Q" 130defined in section 3.1 of RFC5656. 131 132serial is an optional certificate serial number set by the CA to 133provide an abbreviated way to refer to certificates from that CA. 134If a CA does not wish to number its certificates it must set this 135field to zero. 136 137type specifies whether this certificate is for identification of a user 138or a host using a SSH_CERT_TYPE_... value. 139 140key id is a free-form text field that is filled in by the CA at the time 141of signing; the intention is that the contents of this field are used to 142identify the identity principal in log messages. 143 144"valid principals" is a string containing zero or more principals as 145strings packed inside it. These principals list the names for which this 146certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and 147usernames for SSH_CERT_TYPE_USER certificates. As a special case, a 148zero-length "valid principals" field means the certificate is valid for 149any principal of the specified type. XXX DNS wildcards? 150 151"valid after" and "valid before" specify a validity period for the 152certificate. Each represents a time in seconds since 1970-01-01 15300:00:00. A certificate is considered valid if: 154 155 valid after <= current time < valid before 156 157criticial options is a set of zero or more key options encoded as 158below. All such options are "critical" in the sense that an implementation 159must refuse to authorise a key that has an unrecognised option. 160 161extensions is a set of zero or more optional extensions. These extensions 162are not critical, and an implementation that encounters one that it does 163not recognise may safely ignore it. 164 165Generally, critical options are used to control features that restrict 166access where extensions are used to enable features that grant access. 167This ensures that certificates containing unknown restrictions do not 168inadvertently grant access while allowing new protocol features to be 169enabled via extensions without breaking certificates' backwards 170compatibility. 171 172The reserved field is currently unused and is ignored in this version of 173the protocol. 174 175signature key contains the CA key used to sign the certificate. 176The valid key types for CA keys are ssh-rsa, ssh-dss and the ECDSA types 177ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained" 178certificates, where the signature key type is a certificate type itself 179are NOT supported. Note that it is possible for a RSA certificate key to 180be signed by a DSS or ECDSA CA key and vice-versa. 181 182signature is computed over all preceding fields from the initial string 183up to, and including the signature key. Signatures are computed and 184encoded 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 186types). 187 188Critical options 189---------------- 190 191The critical options section of the certificate specifies zero or more 192options on the certificates validity. The format of this field 193is a sequence of zero or more tuples: 194 195 string name 196 string data 197 198Options must be lexically ordered by "name" if they appear in the 199sequence. Each named option may only appear once in a certificate. 200 201The name field identifies the option and the data field encodes 202option-specific information (see below). All options are 203"critical", if an implementation does not recognise a option 204then the validating party should refuse to accept the certificate. 205 206The supported options and the contents and structure of their 207data fields are: 208 209Name Format Description 210----------------------------------------------------------------------------- 211force-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 216source-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 225Extensions 226---------- 227 228The extensions section of the certificate specifies zero or more 229non-critical certificate extensions. The encoding and ordering of 230extensions in this field is identical to that of the critical options, 231as is the requirement that each name appear only once. 232 233If an implementation does not recognise an extension, then it should 234ignore it. 235 236The supported extensions and the contents and structure of their data 237fields are: 238 239Name Format Description 240----------------------------------------------------------------------------- 241permit-X11-forwarding empty Flag indicating that X11 forwarding 242 should be permitted. X11 forwarding will 243 be refused if this option is absent. 244 245permit-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 250permit-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 255permit-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 260permit-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