#!/usr/bin/perl

# Author: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
# Copyright: 2011
# License: GPL-3+

# WARNING: This is very rough code!  the interface WILL change
# dramatically.  The only thing I can commit to keeping stable are the
# OIDs.

# Use this code to take an OpenPGP certificate (pubkey) and emit a
# corresponding OpenPGP-validated X.509 certificate.

# Usage: openpgp2x509 ssh://lair.fifthhorseman.net

use strict;
use warnings;
use Crypt::X509 0.50;
use Math::BigInt;
use GnuPG::Interface 0.43;
use Regexp::Common qw /net/;
use MIME::Base64;

my $cert = Crypt::X509::_init('Certificate');
$cert->configure('encode' => { 'time' => 'raw' } );
my $pgpe = Crypt::X509::_init('PGPExtension');
$pgpe->configure('encode' => { 'time' => 'raw' } );
my $rsapubkeyinfo = Crypt::X509::_init('RSAPubKeyInfo');

my $dntypes = { 'CN' => '2.5.4.3', # common name
                'emailAddress' => '1.2.840.113549.1.9.1', # e-mail address
                'C' => '2.5.4.6', # country
                'ST' => '2.5.4.8', # state
                'L' => '2.5.4.7', # locality
                'O' => '2.5.4.10', # organization
                'OU' => '2.5.4.11', # organization unit (often used as a comment)
            };

my $algos = {
             'RSA' => '1.2.840.113549.1.1.1',
             'RSAwithMD2' => '1.2.840.113549.1.1.2',
             'RSAwithMD4' => '1.2.840.113549.1.1.3',
             'RSAwithMD5' => '1.2.840.113549.1.1.4',
             'RSAwithSHA1' => '1.2.840.113549.1.1.5',
             'OAEP' => '1.2.840.113549.1.1.6',
             'RSAwithSHA256' => '1.2.840.113549.1.1.11',
             'RSAwithSHA384' => '1.2.840.113549.1.1.12',
             'RSAwithSHA512' => '1.2.840.113549.1.1.13',
             'RSAwithSHA224' => '1.2.840.113549.1.1.14',
             'NullSignatureUseOpenPGP' => '1.3.6.1.4.1.37210.1.1',
             'OpenPGPCertificateEmbedded' => '1.3.6.1.4.1.37210.1.2',

            };

# NullSignatureUseOpenPGP: this X509 certificate is not
# self-verifiable.  It must be verified by fetching certificate
# material from OpenPGP keyservers or from the user's private OpenPGP
# keyring.

# The identity material and usage in the OpenPGP keyservers SHOULD be
# tested against the context in which the certificate is being used.
# If no context information is explicitly available to the
# implementation checking the certificate's validity, the
# implementation MUST assume that the context is the full set of
# possible contexts asserted by the X.509 material itself (is this
# doable?)

# 0) certificate validity ambiguity -- X.509 certificates are
#    generally considered to be entirely valid or entirely invalid.
#    OpenPGP certificates can have some User IDs that are valid, and
#    others that are not.  If an implementation is asked to return a
#    simple boolean response to a validity inquiry, without knowing
#    the context in which the certificate was proposed for use, it
#    MUST validate the full conjunction of all assertions made in the
#    X.509 certificate itself in order to return "true".



# OpenPGPCertificateEmbedded: the "signature" material in the X.509
# certificate is actually a set of OpenPGP packets corresponding to a
# complete "transferable public key" as specified in
# https://tools.ietf.org/html/rfc4880#section-11.1 , in "raw"
# (non-ascii-armored) form.

# If it were implemented, it would be the same as
# NullSignatureUseOpenPGP, but with the OpenPGP material transported
# in-band in addition.

## NOTE: There is no implementation of the OpenPGPCertificateEmbedded,
## and maybe there never will be.  Another approach would be to
## transmitting OpenPGP signature packets in the TLS channel itself,
## with an extension comparable to OCSP stapling.

# the OpenPGPCertificateEmbedded concept has a few downsides:

# 1) data duplication -- the X.509 Subject Public Key material is
#    repeated (either in the primary key packet, or in one of the
#    subkey packets).  The X.509 Subject material (and any
#    subjectAltNames) are also duplicated in the User ID packets.
#    This increases the size of the certificate.  It also creates
#    potential inconsistencies.  If the X.509 Subject Public Key
#    material is not found found in the OpenPGP Transferable Public
#    Key (either as a primary key or as a subkey), conforming
#    implementations MUST reject the certificate.

# 2) the requirement for out-of-band verification is not entirely
#    removed, since conformant implementations may want to check the
#    public keyservers for things like revocation certificates.




# this is a 5 followed by a 0.  it fits into the "Parameters" section
# of an ASN.1 algorithmIdentifier object. what does this mean?
# I think it means the NULL type.
my $noparams = sprintf('%c%c', 5, 0);

my $extensions = { 'PGPExtension' => '1.3.6.1.4.1.3401.8.1.1' };

my $gnupg = GnuPG::Interface::->new();
$gnupg->options->quiet(1);
$gnupg->options->batch(1);

sub err {
  printf STDERR @_;
}


sub ts2Time {
  my $ts = shift;

  if (!defined($ts)) {
    # see https://tools.ietf.org/html/rfc5280#section-4.1.2.5
    return {'generalTime' => '99991231235959Z' };
  } else {
    my ($sec,$min,$hour,$mday,$mon,$year) = gmtime($ts);
    $year += 1900;
    if (($year < 1950) ||
        ($year >= 2050)) {
      return {'generalTime' => sprintf('%04d%02d%02d%02d%02d%02dZ', $year, $mon+1, $mday, $hour, $min, $sec) };
    } else {
      return {'utcTime' => sprintf('%02d%02d%02d%02d%02d%02dZ', ($year%100), $mon+1, $mday, $hour, $min, $sec) };
    }
  }
}

sub ts2ISO8601 {
  my $ts = shift;
  $ts = time()
    if (!defined($ts));
  my ($sec,$min,$hour,$mday,$mon,$year) = gmtime($ts);
  $year += 1900;
  return sprintf('%04d-%02d-%02dT%02d:%02d:%02dZ', $year, $mon+1, $mday, $hour, $min, $sec);
};

sub makeX509CertForUserID {
  my $userid = shift;
  my $hostname;
  my $protocol;
  my $emailaddress;
  my $humanname;
  my $subject;
  my $ret = [];

  if ($userid =~ /^\s+/) {
    err("We will not process User IDs with leading whitespace\n");
    return $ret;
  }
  if ($userid =~ /\s+$/) {
    err("We will not process User IDs with trailing whitespace\n");
    return $ret;
  }
  if ($userid =~ /\n/) {
    err("We will not process User IDs containing newlines\n");
    return $ret;
  }
  # FIXME: do we want to rule out any other forms of User ID?


  if ($userid =~ /^(.*)\s+<([^><@\s]+\@$RE{net}{domain})>$/ ) {
    # this is a typical/expected OpenPGP User ID.
    $humanname = $1;
    $emailaddress = $2;
    $subject = [
                [ {
                   'type' => $dntypes->{'CN'},
                   'value' => {
                               'printableString' => $humanname,
                              },
                  } ],
                [ {
                   'type' => $dntypes->{'emailAddress'},
                   'value' => {
                               'ia5String' => $emailaddress,
                              },
                  } ],
               ];
  } elsif ($userid =~ /^(https|ssh|smtps?|ike|postgresql|imaps?|submission):\/\/($RE{net}{domain})$/) {
    $protocol = $1;
    $hostname = $2;
    $subject = [ [ {
                    'type' => $dntypes->{'CN'},
                    'value' => {
                                'printableString' => $hostname
                               },
                   } ] ];
  } else {
    # what should we do here?  Maybe we just assume this is a bare Human Name?
    err("Assuming '%s' is a bare human name.\n", $userid);
    $humanname = $userid;
  }

  foreach my $gpgkey ($gnupg->get_public_keys('='.$userid)) {
    my $validity = '-';
    my @sans;
    foreach my $tryuid ($gpgkey->user_ids) {
      if ($tryuid->as_string eq $userid) {
        $validity = $tryuid->validity;
      }

      if (defined($protocol) &&
          ($tryuid->validity =~ /^[fu]$/) &&
          ($tryuid =~ /^$protocol\:\/\/($RE{net}{domain})/ )) {
        push(@sans, $2);
      }
    }
    if ($validity !~ /^[fu]$/) {
      err("key 0x%s only has validity %s for User ID '%s' (needs full or ultimate validity)\n", $gpgkey->fingerprint->as_hex_string, $validity, $userid);
      next;
    }

    # treat primary keys just like subkeys:
    foreach my $subkey ($gpgkey, @{$gpgkey->subkeys}) {
      if ($subkey->{algo_num} != 1) {
        err("key 0x%s is algorithm %d (not RSA) -- we currently only handle RSA\n", $subkey->fingerprint->as_hex_string, $subkey->algo_num);
        next;
      }
      # FIXME: reject/skip over revoked/expired keys.

      my $pubkey = { 'modulus' => @{$subkey->pubkey_data}[0],
                     'exponent' => @{$subkey->pubkey_data}[1],
                   };
      my $vnotbefore = $subkey->creation_date;

      my $vnotafter = $subkey->expiration_date;
      # expiration date should be the minimum of the primary key and the subkey:
      if (!defined($vnotafter)) {
        $vnotafter = $gpgkey->expiration_date;
      } elsif (defined($gpgkey->expiration_date)) {
        $vnotafter = $gpgkey->expiration_date
          if ($gpgkey->expiration_date < $vnotafter);
      }

      my $cnotbefore = ts2Time($vnotbefore);
      my $cnotafter = ts2Time($vnotafter);

      my $pgpeval = $pgpe->encode({ 'version' => 0, 'keyCreation' => $cnotbefore });
      print $pgpe->{error}
        if (!defined($pgpeval));

      my $pubkeybitstring = $rsapubkeyinfo->encode($pubkey);
      print $rsapubkeyinfo->{error}
        if (!defined($pubkeybitstring));

      my @extensions;
      push(@extensions, { 'extnID' => $extensions->{'PGPExtension'},
                          'extnValue' => $pgpeval
                        });

      # FIXME: base some keyUsage extensions on the type of User ID
      # and on the usage flags of the key in question.

      # if 'a' is present
      # if protocol =~ /^http|ssh|smtps?|postgresql|imaps?|submission$/ then set TLS server eKU + ???
      # if protocol eq 'ike' then ??? (ask micah)
      # if protocol =~ /^smtps?$/ then set TLS client + ???
      # if defined($humanname) then set TLS client + ???

      # if 'e' is present:
      # ???

      # if 's' is present:
      # ???

      # if 'c' is present: I think we should never specify CA:TRUE or
      # CA:FALSE in these certificates, since (a) we do not expect
      # these keys to actually be making X.509-style certifications,
      # but (b) we also don't want to assert that they can't make
      # any certifications whatsoever.


      # FIXME: add subjectAltName that matches the type of information
      # we believe we're working with (see the cert-id draft).

      # FIXME: if @sans is present, add them as subjectAltNames (do we
      # want to do this? maybe this should be optional).


      my $newcert = {
                     'tbsCertificate' => {
                                          'version' => 2, # 0 == version 1, 1 == version 2, 2 == version 3
                                          # this is a convenient way to pass the fpr too.
                                          'serialNumber' => Math::BigInt->new('0x'.$subkey->fingerprint->as_hex_string),
                                          'subjectPublicKeyInfo' => {
                                                                     'algorithm' => {
                                                                                     'parameters' => $noparams,
                                                                                     'algorithm' => $algos->{'RSA'},
                                                                                    },
                                                                     'subjectPublicKey' => $pubkeybitstring,
                                                                    },
                                          'validity' => {
                                                         'notAfter' => $cnotafter,
                                                         'notBefore' => $cnotbefore,
                                                        },
                                          'signature' => { # maybe we should make up our own "signature algorithm" here?
                                                          'parameters' => $noparams,
                                                          'algorithm' => $algos->{'NullSignatureUseOpenPGP'}
                                                         },
                                          'subject' => {
                                                        'rdnSequence' => $subject,
                                                       },
                                          'issuer' => {
                                                       'rdnSequence' => [ [ {
                                                                           'type' => $dntypes->{'OU'},
                                                                           'value' => { 'printableString' => sprintf('Please check the OpenPGP keyservers for certification information. (certificate generated on %s)', ts2ISO8601(time())) },
                                                                          } ] ],
                                                      },
                                          'extensions' => \@extensions,
                                         },
                     'signature' => 'use OpenPGP',
                     'signatureAlgorithm' => {
                                              'parameters' => $noparams,
                                              'algorithm' => $algos->{'NullSignatureUseOpenPGP'}
                                             }
                    };

      my $dd = $cert->encode($newcert);

      push(@{$ret}, $dd);
    }
  }
  return $ret;
}


foreach $cert ( @{ makeX509CertForUserID($ARGV[0]) } ) {
  printf("-----BEGIN CERTIFICATE-----\n%s-----END CERTIFICATE-----\n", encode_base64($cert));
}