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 9use of X.509 certificates and uses raw keys. This approach has some 10benefits relating to simplicity of configuration and minimisation 11of attack surface, but it does not support the important use-cases 12of centrally managed, passwordless authentication and centrally 13certified host keys. 14 15These protocol extensions build on the simple public key authentication 16system already in SSH to allow certificate-based authentication. 17The certificates used are not traditional X.509 certificates, with 18numerous options and complex encoding rules, but something rather 19more minimal: a key, some identity information and usage options 20that have been signed with some other trusted key. 21 22A sshd server may be configured to allow authentication via certified 23keys, by extending the existing ~/.ssh/authorized_keys mechanism 24to allow specification of certification authority keys in addition 25to raw user keys. The ssh client will support automatic verification 26of acceptance of certified host keys, by adding a similar ability 27to specify CA keys in ~/.ssh/known_hosts. 28 29Certified keys are represented using two new key types: 30ssh-rsa-cert-v01@openssh.com and ssh-dss-cert-v01@openssh.com that 31include certification information along with the public key that is used 32to sign challenges. ssh-keygen performs the CA signing operation. 33 34Protocol extensions 35------------------- 36 37The SSH wire protocol includes several extensibility mechanisms. 38These modifications shall take advantage of namespaced public key 39algorithm names to add support for certificate authentication without 40breaking the protocol - implementations that do not support the 41extensions will simply ignore them. 42 43Authentication using the new key formats described below proceeds 44using the existing SSH "publickey" authentication method described 45in RFC4252 section 7. 46 47New public key formats 48---------------------- 49 50The ssh-rsa-cert-v01@openssh.com and ssh-dss-cert-v01@openssh.com key 51types take a similar high-level format (note: data types and 52encoding are as per RFC4251 section 5). The serialised wire encoding of 53these certificates is also used for storing them on disk. 54 55#define SSH_CERT_TYPE_USER 1 56#define SSH_CERT_TYPE_HOST 2 57 58RSA certificate 59 60 string "ssh-rsa-cert-v01@openssh.com" 61 string nonce 62 mpint e 63 mpint n 64 uint64 serial 65 uint32 type 66 string key id 67 string valid principals 68 uint64 valid after 69 uint64 valid before 70 string critical options 71 string extensions 72 string reserved 73 string signature key 74 string signature 75 76DSA certificate 77 78 string "ssh-dss-cert-v01@openssh.com" 79 string nonce 80 mpint p 81 mpint q 82 mpint g 83 mpint y 84 uint64 serial 85 uint32 type 86 string key id 87 string valid principals 88 uint64 valid after 89 uint64 valid before 90 string critical options 91 string extensions 92 string reserved 93 string signature key 94 string signature 95 96The nonce field is a CA-provided random bitstring of arbitrary length 97(but typically 16 or 32 bytes) included to make attacks that depend on 98inducing collisions in the signature hash infeasible. 99 100e and n are the RSA exponent and public modulus respectively. 101 102p, q, g, y are the DSA parameters as described in FIPS-186-2. 103 104serial is an optional certificate serial number set by the CA to 105provide an abbreviated way to refer to certificates from that CA. 106If a CA does not wish to number its certificates it must set this 107field to zero. 108 109type specifies whether this certificate is for identification of a user 110or a host using a SSH_CERT_TYPE_... value. 111 112key id is a free-form text field that is filled in by the CA at the time 113of signing; the intention is that the contents of this field are used to 114identify the identity principal in log messages. 115 116"valid principals" is a string containing zero or more principals as 117strings packed inside it. These principals list the names for which this 118certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and 119usernames for SSH_CERT_TYPE_USER certificates. As a special case, a 120zero-length "valid principals" field means the certificate is valid for 121any principal of the specified type. XXX DNS wildcards? 122 123"valid after" and "valid before" specify a validity period for the 124certificate. Each represents a time in seconds since 1970-01-01 12500:00:00. A certificate is considered valid if: 126 valid after <= current time < valid before 127 128criticial options is a set of zero or more key options encoded as 129below. All such options are "critical" in the sense that an implementation 130must refuse to authorise a key that has an unrecognised option. 131 132extensions is a set of zero or more optional extensions. These extensions 133are not critical, and an implementation that encounters one that it does 134not recognise may safely ignore it. 135 136The reserved field is currently unused and is ignored in this version of 137the protocol. 138 139signature key contains the CA key used to sign the certificate. 140The valid key types for CA keys are ssh-rsa and ssh-dss. "Chained" 141certificates, where the signature key type is a certificate type itself 142are NOT supported. Note that it is possible for a RSA certificate key to 143be signed by a DSS CA key and vice-versa. 144 145signature is computed over all preceding fields from the initial string 146up to, and including the signature key. Signatures are computed and 147encoded according to the rules defined for the CA's public key algorithm 148(RFC4253 section 6.6 for ssh-rsa and ssh-dss). 149 150Critical options 151---------------- 152 153The critical options section of the certificate specifies zero or more 154options on the certificates validity. The format of this field 155is a sequence of zero or more tuples: 156 157 string name 158 string data 159 160Options must be lexically ordered by "name" if they appear in the 161sequence. 162 163The name field identifies the option and the data field encodes 164option-specific information (see below). All options are 165"critical", if an implementation does not recognise a option 166then the validating party should refuse to accept the certificate. 167 168The supported options and the contents and structure of their 169data fields are: 170 171Name Format Description 172----------------------------------------------------------------------------- 173force-command string Specifies a command that is executed 174 (replacing any the user specified on the 175 ssh command-line) whenever this key is 176 used for authentication. 177 178source-address string Comma-separated list of source addresses 179 from which this certificate is accepted 180 for authentication. Addresses are 181 specified in CIDR format (nn.nn.nn.nn/nn 182 or hhhh::hhhh/nn). 183 If this option is not present then 184 certificates may be presented from any 185 source address. 186 187Extensions 188---------- 189 190The extensions section of the certificate specifies zero or more 191non-critical certificate extensions. The encoding and ordering of 192extensions in this field is identical to that of the critical options. 193If an implementation does not recognise an extension, then it should 194ignore it. 195 196The supported extensions and the contents and structure of their data 197fields are: 198 199Name Format Description 200----------------------------------------------------------------------------- 201permit-X11-forwarding empty Flag indicating that X11 forwarding 202 should be permitted. X11 forwarding will 203 be refused if this option is absent. 204 205permit-agent-forwarding empty Flag indicating that agent forwarding 206 should be allowed. Agent forwarding 207 must not be permitted unless this 208 option is present. 209 210permit-port-forwarding empty Flag indicating that port-forwarding 211 should be allowed. If this option is 212 not present then no port forwarding will 213 be allowed. 214 215permit-pty empty Flag indicating that PTY allocation 216 should be permitted. In the absence of 217 this option PTY allocation will be 218 disabled. 219 220permit-user-rc empty Flag indicating that execution of 221 ~/.ssh/rc should be permitted. Execution 222 of this script will not be permitted if 223 this option is not present. 224 225$OpenBSD: PROTOCOL.certkeys,v 1.7 2010/08/04 05:40:39 djm Exp $ 226