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version 1.1, 2010/02/26 20:29:54 version 1.18, 2021/06/04 04:02:21
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 ----------  ----------
   
 The SSH protocol currently supports a simple public key authentication  The SSH protocol currently supports a simple public key authentication
 mechanism. Unlike other public key implementations, SSH eschews the  mechanism. Unlike other public key implementations, SSH eschews the use
 use of X.509 certificates and uses raw keys. This approach has some  of X.509 certificates and uses raw keys. This approach has some benefits
 benefits relating to simplicity of configuration and minimisation  relating to simplicity of configuration and minimisation of attack
 of attack surface, but it does not support the important use-cases  surface, but it does not support the important use-cases of centrally
 of centrally managed, passwordless authentication and centrally  managed, passwordless authentication and centrally certified host keys.
 certified host keys.  
   
 These protocol extensions build on the simple public key authentication  These protocol extensions build on the simple public key authentication
 system already in SSH to allow certificate-based authentication.  system already in SSH to allow certificate-based authentication. The
 The certificates used are not traditional X.509 certificates, with  certificates used are not traditional X.509 certificates, with numerous
 numerous options and complex encoding rules, but something rather  options and complex encoding rules, but something rather more minimal: a
 more minimal: a key, some identity information and usage constraints  key, some identity information and usage options that have been signed
 that have been signed with some other trusted key.  with some other trusted key.
   
 A sshd server may be configured to allow authentication via certified  A sshd server may be configured to allow authentication via certified
 keys, by extending the existing ~/.ssh/authorized_keys mechanism  keys, by extending the existing ~/.ssh/authorized_keys mechanism to
 to allow specification of certification authority keys in addition  allow specification of certification authority keys in addition to
 to raw user keys. The ssh client will support automatic verification  raw user keys. The ssh client will support automatic verification of
 of acceptance of certified host keys, by adding a similar ability  acceptance of certified host keys, by adding a similar ability to
 to specify CA keys in ~/.ssh/known_hosts.  specify CA keys in ~/.ssh/known_hosts.
   
 Certified keys are represented using two new key types:  All certificate types include certification information along with the
 ssh-rsa-cert-v00@openssh.com and ssh-dss-cert-v00@openssh.com that  public key that is used to sign challenges. In OpenSSH, ssh-keygen
 include certification information along with the public key that is used  performs the CA signing operation.
 to sign challenges. ssh-keygen performs the CA signing operation.  
   
   Certified keys are represented using new key types:
   
       ssh-rsa-cert-v01@openssh.com
       ssh-dss-cert-v01@openssh.com
       ecdsa-sha2-nistp256-cert-v01@openssh.com
       ecdsa-sha2-nistp384-cert-v01@openssh.com
       ecdsa-sha2-nistp521-cert-v01@openssh.com
       ssh-ed25519-cert-v01@openssh.com
   
   Two additional types exist for RSA certificates to force use of
   SHA-2 signatures (SHA-256 and SHA-512 respectively):
   
       rsa-sha2-256-cert-v01@openssh.com
       rsa-sha2-512-cert-v01@openssh.com
   
   These RSA/SHA-2 types should not appear in keys at rest or transmitted
   on their wire, but do appear in a SSH_MSG_KEXINIT's host-key algorithms
   field or in the "public key algorithm name" field of a "publickey"
   SSH_USERAUTH_REQUEST to indicate that the signature will use the
   specified algorithm.
   
 Protocol extensions  Protocol extensions
 -------------------  -------------------
   
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 New public key formats  New public key formats
 ----------------------  ----------------------
   
 The ssh-rsa-cert-v00@openssh.com and ssh-dss-cert-v00@openssh.com key  The certificate key types take a similar high-level format (note: data
 types take a similar same high-level format (note: data types and  types and encoding are as per RFC4251 section 5). The serialised wire
 encoding are as per RFC4251 section 5). The serialised wire encoding of  encoding of these certificates is also used for storing them on disk.
 these certificates is also used for storing them on disk.  
   
 #define SSH_CERT_TYPE_USER    1  #define SSH_CERT_TYPE_USER    1
 #define SSH_CERT_TYPE_HOST    2  #define SSH_CERT_TYPE_HOST    2
   
 RSA certificate  RSA certificate
   
     string    "ssh-rsa-cert-v00@openssh.com"      string    "ssh-rsa-cert-v01@openssh.com"
       string    nonce
     mpint     e      mpint     e
     mpint     n      mpint     n
       uint64    serial
     uint32    type      uint32    type
     string    key id      string    key id
     string    valid principals      string    valid principals
     uint64    valid after      uint64    valid after
     uint64    valid before      uint64    valid before
     string    constraints      string    critical options
     string    nonce      string    extensions
     string    reserved      string    reserved
     string    signature key      string    signature key
     string    signature      string    signature
   
 DSA certificate  DSA certificate
   
     string    "ssh-dss-cert-v00@openssh.com"      string    "ssh-dss-cert-v01@openssh.com"
       string    nonce
     mpint     p      mpint     p
     mpint     q      mpint     q
     mpint     g      mpint     g
     mpint     y      mpint     y
       uint64    serial
     uint32    type      uint32    type
     string    key id      string    key id
     string    valid principals      string    valid principals
     uint64    valid after      uint64    valid after
     uint64    valid before      uint64    valid before
     string    constraints      string    critical options
       string    extensions
       string    reserved
       string    signature key
       string    signature
   
   ECDSA certificate
   
       string    "ecdsa-sha2-nistp256-cert-v01@openssh.com" |
                 "ecdsa-sha2-nistp384-cert-v01@openssh.com" |
                 "ecdsa-sha2-nistp521-cert-v01@openssh.com"
     string    nonce      string    nonce
       string    curve
       string    public_key
       uint64    serial
       uint32    type
       string    key id
       string    valid principals
       uint64    valid after
       uint64    valid before
       string    critical options
       string    extensions
     string    reserved      string    reserved
     string    signature key      string    signature key
     string    signature      string    signature
   
   ED25519 certificate
   
       string    "ssh-ed25519-cert-v01@openssh.com"
       string    nonce
       string    pk
       uint64    serial
       uint32    type
       string    key id
       string    valid principals
       uint64    valid after
       uint64    valid before
       string    critical options
       string    extensions
       string    reserved
       string    signature key
       string    signature
   
   The nonce field is a CA-provided random bitstring of arbitrary length
   (but typically 16 or 32 bytes) included to make attacks that depend on
   inducing collisions in the signature hash infeasible.
   
 e and n are the RSA exponent and public modulus respectively.  e and n are the RSA exponent and public modulus respectively.
   
 p, q, g, y are the DSA parameters as described in FIPS-186-2.  p, q, g, y are the DSA parameters as described in FIPS-186-2.
   
   curve and public key are respectively the ECDSA "[identifier]" and "Q"
   defined in section 3.1 of RFC5656.
   
   pk is the encoded Ed25519 public key as defined by
   draft-josefsson-eddsa-ed25519-03.
   
   serial is an optional certificate serial number set by the CA to
   provide an abbreviated way to refer to certificates from that CA.
   If a CA does not wish to number its certificates it must set this
   field to zero.
   
 type specifies whether this certificate is for identification of a user  type specifies whether this certificate is for identification of a user
 or a host using a SSH_CERT_TYPE_... value.  or a host using a SSH_CERT_TYPE_... value.
   
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 certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and  certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and
 usernames for SSH_CERT_TYPE_USER certificates. As a special case, a  usernames for SSH_CERT_TYPE_USER certificates. As a special case, a
 zero-length "valid principals" field means the certificate is valid for  zero-length "valid principals" field means the certificate is valid for
 any principal of the specified type. XXX DNS wildcards?  any principal of the specified type.
   
 "valid after" and "valid before" specify a validity period for the  "valid after" and "valid before" specify a validity period for the
 certificate. Each represents a time in seconds since 1970-01-01  certificate. Each represents a time in seconds since 1970-01-01
 00:00:00. A certificate is considered valid if:  00:00:00. A certificate is considered valid if:
          valid after <= current time < valid before  
   
 constraints is a set of zero or more key constraints encoded as below.      valid after <= current time < valid before
   
 The nonce field is a CA-provided random bitstring of arbitrary length  critical options is a set of zero or more key options encoded as
 (but typically 16 or 32 bytes) included to make attacks that depend on  below. All such options are "critical" in the sense that an implementation
 inducing collisions in the signature hash infeasible.  must refuse to authorise a key that has an unrecognised option.
   
 The reserved field is current unused and is ignored in this version of  extensions is a set of zero or more optional extensions. These extensions
   are not critical, and an implementation that encounters one that it does
   not recognise may safely ignore it.
   
   Generally, critical options are used to control features that restrict
   access where extensions are used to enable features that grant access.
   This ensures that certificates containing unknown restrictions do not
   inadvertently grant access while allowing new protocol features to be
   enabled via extensions without breaking certificates' backwards
   compatibility.
   
   The reserved field is currently unused and is ignored in this version of
 the protocol.  the protocol.
   
 signature key contains the CA key used to sign the certificate.  The signature key field contains the CA key used to sign the
 The valid key types for CA keys are ssh-rsa and ssh-dss. "Chained"  certificate. The valid key types for CA keys are ssh-rsa,
 certificates, where the signature key type is a certificate type itself  ssh-dss, ssh-ed25519 and the ECDSA types ecdsa-sha2-nistp256,
 are NOT supported. Note that it is possible for a RSA certificate key to  ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained" certificates, where
 be signed by a DSS CA key and vice-versa.  the signature key type is a certificate type itself are NOT supported.
   Note that it is possible for a RSA certificate key to be signed by a
   Ed25519 or ECDSA CA key and vice-versa.
   
 signature is computed over all preceding fields from the initial string  signature is computed over all preceding fields from the initial string
 up to, and including the signature key. Signatures are computed and  up to, and including the signature key. Signatures are computed and
 encoded according to the rules defined for the CA's public key algorithm  encoded according to the rules defined for the CA's public key algorithm
 (RFC4253 section 6.6 for ssh-rsa and ssh-dss).  (RFC4253 section 6.6 for ssh-rsa and ssh-dss, RFC5656 for the ECDSA
   types), and draft-josefsson-eddsa-ed25519-03 for Ed25519.
   
 Constraints  Critical options
 -----------  ----------------
   
 The constraints section of the certificate specifies zero or more  The critical options section of the certificate specifies zero or more
 constraints on the certificates validity. The format of this field  options on the certificates validity. The format of this field
 is a sequence of zero or more tuples:  is a sequence of zero or more tuples:
   
     string       name      string       name
     string       data      string       data
   
 The name field identifies the constraint and the data field encodes  Options must be lexically ordered by "name" if they appear in the
 constraint-specific information (see below). All constraints are  sequence. Each named option may only appear once in a certificate.
 "critical", if an implementation does not recognise a constraint  
   The name field identifies the option and the data field encodes
   option-specific information (see below). All options are
   "critical", if an implementation does not recognise a option
 then the validating party should refuse to accept the certificate.  then the validating party should refuse to accept the certificate.
   
 The supported constraints and the contents and structure of their  Custom options should append the originating author or organisation's
 data fields are:  domain name to the option name, e.g. "my-option@example.com".
   
   No critical options are defined for host certificates at present. The
   supported user certificate options and the contents and structure of
   their data fields are:
   
 Name                    Format        Description  Name                    Format        Description
 -----------------------------------------------------------------------------  -----------------------------------------------------------------------------
 force-command           string        Specifies a command that is executed  force-command           string        Specifies a command that is executed
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                                       ssh command-line) whenever this key is                                        ssh command-line) whenever this key is
                                       used for authentication.                                        used for authentication.
   
   source-address          string        Comma-separated list of source addresses
                                         from which this certificate is accepted
                                         for authentication. Addresses are
                                         specified in CIDR format (nn.nn.nn.nn/nn
                                         or hhhh::hhhh/nn).
                                         If this option is not present then
                                         certificates may be presented from any
                                         source address.
   
   verify-required         empty         Flag indicating that signatures made
                                         with this certificate must assert FIDO
                                         user verification (e.g. PIN or
                                         biometric). This option only make sense
                                         for the U2F/FIDO security key types that
                                         support this feature in their signature
                                         formats.
   
   Extensions
   ----------
   
   The extensions section of the certificate specifies zero or more
   non-critical certificate extensions. The encoding and ordering of
   extensions in this field is identical to that of the critical options,
   as is the requirement that each name appear only once.
   
   If an implementation does not recognise an extension, then it should
   ignore it.
   
   Custom options should append the originating author or organisation's
   domain name to the option name, e.g. "my-option@example.com".
   
   No extensions are defined for host certificates at present. The
   supported user certificate extensions and the contents and structure of
   their data fields are:
   
   Name                    Format        Description
   -----------------------------------------------------------------------------
   no-touch-required       empty         Flag indicating that signatures made
                                         with this certificate need not assert
                                         FIDO user presence. This option only
                                         make sense for the U2F/FIDO security
                                         key types that support this feature in
                                         their signature formats.
   
 permit-X11-forwarding   empty         Flag indicating that X11 forwarding  permit-X11-forwarding   empty         Flag indicating that X11 forwarding
                                       should be permitted. X11 forwarding will                                        should be permitted. X11 forwarding will
                                       be refused if this constraint is absent.                                        be refused if this option is absent.
   
 permit-agent-forwarding empty         Flag indicating that agent forwarding  permit-agent-forwarding empty         Flag indicating that agent forwarding
                                       should be allowed. Agent forwarding                                        should be allowed. Agent forwarding
                                       must not be permitted unless this                                        must not be permitted unless this
                                       constraint is present.                                        option is present.
   
 permit-port-forwarding  empty         Flag indicating that port-forwarding  permit-port-forwarding  empty         Flag indicating that port-forwarding
                                       should be allowed. If this constraint is                                        should be allowed. If this option is
                                       not present then no port forwarding will                                        not present then no port forwarding will
                                       be allowed.                                        be allowed.
   
 permit-pty              empty         Flag indicating that PTY allocation  permit-pty              empty         Flag indicating that PTY allocation
                                       should be permitted. In the absence of                                        should be permitted. In the absence of
                                       this constraint PTY allocation will be                                        this option PTY allocation will be
                                       disabled.                                        disabled.
   
 permit-user-rc          empty         Flag indicating that execution of  permit-user-rc          empty         Flag indicating that execution of
                                       ~/.ssh/rc should be permitted. Execution                                        ~/.ssh/rc should be permitted. Execution
                                       of this script will not be permitted if                                        of this script will not be permitted if
                                       this constraint is not present.                                        this option is not present.
   
 source-address          string        Comma-separated list of source addresses  $OpenBSD$
                                       from which this certificate is accepted  
                                       for authentication. Addresses are  
                                       specified in CIDR format (nn.nn.nn.nn/nn  
                                       or hhhh::hhhh/nn).  
                                       If this constraint is not present then  
                                       certificates may be presented from any  
                                       source address.  
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