#!/usr/bin/perl -T

# keytrans: this is an RSA key translation utility; it is capable of
# transforming RSA keys (both public keys and secret keys) between
# several popular representations, including OpenPGP, PEM-encoded
# PKCS#1 DER, and OpenSSH-style public key lines.

# How it behaves depends on the name under which it is invoked.  The
# two implementations currently are: pem2openpgp and openpgp2ssh.



# pem2openpgp: take a PEM-encoded RSA private-key on standard input, a
# User ID as the first argument, and generate an OpenPGP secret key
# and certificate from it.

# WARNING: the secret key material *will* appear on stdout (albeit in
# OpenPGP form) -- if you redirect stdout to a file, make sure the
# permissions on that file are appropriately locked down!

# Usage:

# pem2openpgp 'ssh://'$(hostname -f) < /etc/ssh/ssh_host_rsa_key | gpg --import




# openpgp2ssh: take a stream of OpenPGP packets containing public or
# secret key material on standard input, and a Key ID (or fingerprint)
# as the first argument.  Find the matching key in the input stream,
# and emit it on stdout in an OpenSSH-compatible format.  If the input
# key is an OpenPGP public key (either primary or subkey), the output
# will be an OpenSSH single-line public key.  If the input key is an
# OpenPGP secret key, the output will be a PEM-encoded RSA key.

# Example usage:

# gpg --export-secret-subkeys --export-options export-reset-subkey-passwd $KEYID | \
#  openpgp2ssh $KEYID | ssh-add /dev/stdin


# Authors:
#  Jameson Rollins <jrollins@finestructure.net>
#  Daniel Kahn Gillmor <dkg@fifthhorseman.net>

# Started on: 2009-01-07 02:01:19-0500

# License: GPL v3 or later (we may need to adjust this given that this
# connects to OpenSSL via perl)

use strict;
use warnings;
use File::Basename;
use Crypt::OpenSSL::RSA;
use Crypt::OpenSSL::Bignum;
use Crypt::OpenSSL::Bignum::CTX;
use Digest::SHA;
use MIME::Base64;
use POSIX;

## make sure all length() and substr() calls use bytes only:
use bytes;

my $old_format_packet_lengths = { one => 0,
				  two => 1,
				  four => 2,
				  indeterminate => 3,
};

# see RFC 4880 section 9.1 (ignoring deprecated algorithms for now)
my $asym_algos = { rsa => 1,
		   elgamal => 16,
		   dsa => 17,
		   };

# see RFC 4880 section 9.2
my $ciphers = { plaintext => 0,
		idea => 1,
		tripledes => 2,
		cast5 => 3,
		blowfish => 4,
		aes128 => 7,
		aes192 => 8,
		aes256 => 9,
		twofish => 10,
	      };

# see RFC 4880 section 9.3
my $zips = { uncompressed => 0,
	     zip => 1,
	     zlib => 2,
	     bzip2 => 3,
	   };

# see RFC 4880 section 9.4
my $digests = { md5 => 1,
		sha1 => 2,
		ripemd160 => 3,
		sha256 => 8,
		sha384 => 9,
		sha512 => 10,
		sha224 => 11,
	      };

# see RFC 4880 section 5.2.3.21
my $usage_flags = { certify => 0x01,
		    sign => 0x02,
		    encrypt_comms => 0x04,
		    encrypt_storage => 0x08,
		    encrypt => 0x0c, ## both comms and storage
		    split => 0x10, # the private key is split via secret sharing
		    authenticate => 0x20,
		    shared => 0x80, # more than one person holds the entire private key
		  };

# see RFC 4880 section 4.3
my $packet_types = { pubkey_enc_session => 1,
		     sig => 2,
		     symkey_enc_session => 3,
		     onepass_sig => 4,
		     seckey => 5,
		     pubkey => 6,
		     sec_subkey => 7,
		     compressed_data => 8,
		     symenc_data => 9,
		     marker => 10,
		     literal => 11,
		     trust => 12,
		     uid => 13,
		     pub_subkey => 14,
		     uat => 17,
		     symenc_w_integrity => 18,
		     mdc => 19,
		   };

# see RFC 4880 section 5.2.1
my $sig_types = { binary_doc => 0x00,
		  text_doc => 0x01,
		  standalone => 0x02,
		  generic_certification => 0x10,
		  persona_certification => 0x11,
		  casual_certification => 0x12,
		  positive_certification => 0x13,
		  subkey_binding => 0x18,
		  primary_key_binding => 0x19,
		  key_signature => 0x1f,
		  key_revocation => 0x20,
		  subkey_revocation => 0x28,
		  certification_revocation => 0x30,
		  timestamp => 0x40,
		  thirdparty => 0x50,
		};


# see RFC 4880 section 5.2.3.23
my $revocation_reasons = { no_reason_specified => 0,
			   key_superseded => 1,
			   key_compromised => 2,
			   key_retired => 3,
			   user_id_no_longer_valid => 32,
			 };

# see RFC 4880 section 5.2.3.1
my $subpacket_types = { sig_creation_time => 2,
			sig_expiration_time => 3,
			exportable => 4,
			trust_sig => 5,
			regex => 6,
			revocable => 7,
			key_expiration_time => 9,
			preferred_cipher => 11,
			revocation_key => 12,
			issuer => 16,
			notation => 20,
			preferred_digest => 21,
			preferred_compression => 22,
			keyserver_prefs => 23,
			preferred_keyserver => 24,
			primary_uid => 25,
			policy_uri => 26,
			usage_flags => 27,
			signers_uid => 28,
			revocation_reason => 29,
			features => 30,
			signature_target => 31,
			embedded_signature => 32,
		       };

# bitstring (see RFC 4880 section 5.2.3.24)
my $features = { mdc => 0x01
	       };

# bitstring (see RFC 4880 5.2.3.17)
my $keyserver_prefs = { nomodify => 0x80
		      };

###### end lookup tables ######

# FIXME: if we want to be able to interpret openpgp data as well as
# produce it, we need to produce key/value-swapped lookup tables as well.


########### Math/Utility Functions ##############


# see the bottom of page 44 of RFC 4880 (http://tools.ietf.org/html/rfc4880#page-44)
sub simple_checksum {
  my $bytes = shift;

  return unpack("%16C*",$bytes);
}

# calculate the multiplicative inverse of a mod b this is euclid's
# extended algorithm.  For more information see:
# http://en.wikipedia.org/wiki/Extended_Euclidean_algorithm the
# arguments here should be Crypt::OpenSSL::Bignum objects.  $a should
# be the larger of the two values, and the two values should be
# coprime.

sub modular_multi_inverse {
  my $a = shift;
  my $b = shift;


  my $origdivisor = $b->copy();

  my $ctx = Crypt::OpenSSL::Bignum::CTX->new();
  my $x = Crypt::OpenSSL::Bignum->zero();
  my $y = Crypt::OpenSSL::Bignum->one();
  my $lastx = Crypt::OpenSSL::Bignum->one();
  my $lasty = Crypt::OpenSSL::Bignum->zero();

  my $finalquotient;
  my $finalremainder;

  while (! $b->is_zero()) {
    my ($quotient, $remainder) = $a->div($b, $ctx);

    $a = $b;
    $b = $remainder;

    my $temp = $x;
    $x = $lastx->sub($quotient->mul($x, $ctx));
    $lastx = $temp;

    $temp = $y;
    $y = $lasty->sub($quotient->mul($y, $ctx));
    $lasty = $temp;
  }

  if (!$a->is_one()) {
    die "did this math wrong.\n";
  }

  # let's make sure that we return a positive value because RFC 4880,
  # section 3.2 only allows unsigned values:

  ($finalquotient, $finalremainder) = $lastx->add($origdivisor)->div($origdivisor, $ctx);

  return $finalremainder;
}


############ OpenPGP formatting functions ############

# make an old-style packet out of the given packet type and body.
# old-style  (see RFC 4880 section 4.2)
sub make_packet {
  my $type = shift;
  my $body = shift;
  my $options = shift;

  my $len = length($body);
  my $pseudolen = $len;

  # if the caller wants to use at least N octets of packet length,
  # pretend that we're using that many.
  if (defined $options && defined $options->{'packet_length'}) {
      $pseudolen = 2**($options->{'packet_length'} * 8) - 1;
  }
  if ($pseudolen < $len) {
      $pseudolen = $len;
  }

  my $lenbytes;
  my $lencode;

  if ($pseudolen < 2**8) {
    $lenbytes = $old_format_packet_lengths->{one};
    $lencode = 'C';
  } elsif ($pseudolen < 2**16) {
    $lenbytes = $old_format_packet_lengths->{two};
    $lencode = 'n';
  } elsif ($pseudolen < 2**31) {
    ## not testing against full 32 bits because i don't want to deal
    ## with potential overflow.
    $lenbytes = $old_format_packet_lengths->{four};
    $lencode = 'N';
  } else {
    ## what the hell do we do here?
    $lenbytes = $old_format_packet_lengths->{indeterminate};
    $lencode = '';
  }

  return pack('C'.$lencode, 0x80 + ($type * 4) + $lenbytes, $len).
    $body;
}


# takes a Crypt::OpenSSL::Bignum, returns it formatted as OpenPGP MPI
# (RFC 4880 section 3.2)
sub mpi_pack {
  my $num = shift;

  my $val = $num->to_bin();
  my $mpilen = length($val)*8;

# this is a kludgy way to get the number of significant bits in the
# first byte:
  my $bitsinfirstbyte = length(sprintf("%b", ord($val)));

  $mpilen -= (8 - $bitsinfirstbyte);

  return pack('n', $mpilen).$val;
}

# takes a Crypt::OpenSSL::Bignum, returns an MPI packed in preparation
# for an OpenSSH-style public key format.  see:
# http://marc.info/?l=openssh-unix-dev&m=121866301718839&w=2
sub openssh_mpi_pack {
  my $num = shift;

  my $val = $num->to_bin();
  my $mpilen = length($val);

  my $ret = pack('N', $mpilen);

  # if the first bit of the leading byte is high, we should include a
  # 0 byte:
  if (ord($val) & 0x80) {
    $ret = pack('NC', $mpilen+1, 0);
  }

  return $ret.$val;
}

sub openssh_pubkey_pack {
  my $key = shift;

  my ($modulus, $exponent) = $key->get_key_parameters();

  return openssh_mpi_pack(Crypt::OpenSSL::Bignum->new_from_bin("ssh-rsa")).
      openssh_mpi_pack($exponent).
	openssh_mpi_pack($modulus);
}

# pull an OpenPGP-specified MPI off of a given stream, returning it as
# a Crypt::OpenSSL::Bignum.
sub read_mpi {
  my $instr = shift;
  my $readtally = shift;

  my $bitlen;
  read($instr, $bitlen, 2) or die "could not read MPI length.\n";
  $bitlen = unpack('n', $bitlen);
  $$readtally += 2;

  my $bytestoread = POSIX::floor(($bitlen + 7)/8);
  my $ret;
  read($instr, $ret, $bytestoread) or die "could not read MPI body.\n";
  $$readtally += $bytestoread;
  return Crypt::OpenSSL::Bignum->new_from_bin($ret);
}


# FIXME: genericize these to accept either RSA or DSA keys:
sub make_rsa_pub_key_body {
  my $key = shift;
  my $key_timestamp = shift;

  my ($n, $e) = $key->get_key_parameters();

  return
    pack('CN', 4, $key_timestamp).
      pack('C', $asym_algos->{rsa}).
	mpi_pack($n).
	  mpi_pack($e);
}

sub make_rsa_sec_key_body {
  my $key = shift;
  my $key_timestamp = shift;

  # we're not using $a and $b, but we need them to get to $c.
  my ($n, $e, $d, $p, $q) = $key->get_key_parameters();

  my $c3 = modular_multi_inverse($p, $q);

  my $secret_material = mpi_pack($d).
    mpi_pack($p).
      mpi_pack($q).
	mpi_pack($c3);

  # according to Crypt::OpenSSL::RSA, the closest value we can get out
  # of get_key_parameters is 1/q mod p; but according to sec 5.5.3 of
  # RFC 4880, we're actually looking for u, the multiplicative inverse
  # of p, mod q.  This is why we're calculating the value directly
  # with modular_multi_inverse.

  return
    pack('CN', 4, $key_timestamp).
      pack('C', $asym_algos->{rsa}).
	mpi_pack($n).
	  mpi_pack($e).
	    pack('C', 0). # seckey material is not encrypted -- see RFC 4880 sec 5.5.3
	      $secret_material.
		pack('n', simple_checksum($secret_material));
}

# expects an RSA key (public or private) and a timestamp
sub fingerprint {
  my $key = shift;
  my $key_timestamp = shift;

  my $rsabody = make_rsa_pub_key_body($key, $key_timestamp);

  return Digest::SHA::sha1(pack('Cn', 0x99, length($rsabody)).$rsabody);
}


# FIXME: handle DSA keys as well!
sub makeselfsig {
  my $rsa = shift;
  my $uid = shift;
  my $args = shift;

  # strong assertion of identity is the default (for a self-sig):
  if (! defined $args->{certification_type}) {
    $args->{certification_type} = $sig_types->{positive_certification};
  }

  if (! defined $args->{sig_timestamp}) {
    $args->{sig_timestamp} = time();
  }
  my $key_timestamp = $args->{key_timestamp} + 0;

  # generate and aggregate subpackets:

  # key usage flags:
  my $flags = 0;
  if (! defined $args->{usage_flags}) {
    $flags = $usage_flags->{certify};
  } else {
    my @ff = split(",", $args->{usage_flags});
    foreach my $f (@ff) {
      if (! defined $usage_flags->{$f}) {
	die "No such flag $f";
      }
      $flags |= $usage_flags->{$f};
    }
  }
  my $usage_subpacket = pack('CCC', 2, $subpacket_types->{usage_flags}, $flags);

  # how should we determine how far off to set the expiration date?
  # default is no expiration.  Specify the timestamp in seconds from the
  # key creation.
  my $expiration_subpacket = '';
  if (defined $args->{expiration}) {
    my $expires_in = $args->{expiration} + 0;
    $expiration_subpacket = pack('CCN', 5, $subpacket_types->{key_expiration_time}, $expires_in);
  }


  # prefer AES-256, AES-192, AES-128, CAST5, 3DES:
  my $pref_sym_algos = pack('CCCCCCC', 6, $subpacket_types->{preferred_cipher},
			    $ciphers->{aes256},
			    $ciphers->{aes192},
			    $ciphers->{aes128},
			    $ciphers->{cast5},
			    $ciphers->{tripledes}
			   );

  # prefer SHA-512, SHA-384, SHA-256, SHA-224, RIPE-MD/160, SHA-1
  my $pref_hash_algos = pack('CCCCCCCC', 7, $subpacket_types->{preferred_digest},
			     $digests->{sha512},
			     $digests->{sha384},
			     $digests->{sha256},
			     $digests->{sha224},
			     $digests->{ripemd160},
			     $digests->{sha1}
			    );

  # prefer ZLIB, BZip2, ZIP
  my $pref_zip_algos = pack('CCCCC', 4, $subpacket_types->{preferred_compression},
			    $zips->{zlib},
			    $zips->{bzip2},
			    $zips->{zip}
			   );

  # we support the MDC feature:
  my $feature_subpacket = pack('CCC', 2, $subpacket_types->{features},
			       $features->{mdc});

  # keyserver preference: only owner modify (???):
  my $keyserver_pref = pack('CCC', 2, $subpacket_types->{keyserver_prefs},
			    $keyserver_prefs->{nomodify});


  $args->{hashed_subpackets} =
      $usage_subpacket.
	$expiration_subpacket.
	  $pref_sym_algos.
	    $pref_hash_algos.
	      $pref_zip_algos.
		$feature_subpacket.
		  $keyserver_pref;

  return gensig($rsa, $uid, $args);
}

# FIXME: handle non-RSA keys

# FIXME: this currently only makes self-sigs -- we should parameterize
# it to make certifications over keys other than the issuer.
sub gensig {
  my $rsa = shift;
  my $uid = shift;
  my $args = shift;

  # FIXME: allow signature creation using digests other than SHA256
  $rsa->use_sha256_hash();

  # see page 22 of RFC 4880 for why i think this is the right padding
  # choice to use:
  $rsa->use_pkcs1_padding();

  if (! $rsa->check_key()) {
    die "key does not check\n";
  }

  my $certtype = $args->{certification_type} + 0;

  my $version = pack('C', 4);
  my $sigtype = pack('C', $certtype);
  # RSA
  my $pubkey_algo = pack('C', $asym_algos->{rsa});
  # SHA256 FIXME: allow signature creation using digests other than SHA256
  my $hash_algo = pack('C', $digests->{sha256});

  # FIXME: i'm worried about generating a bazillion new OpenPGP
  # certificates from the same key, which could easily happen if you run
  # this script more than once against the same key (because the
  # timestamps will differ).  How can we prevent this?

  # this argument (if set) overrides the current time, to
  # be able to create a standard key.  If we read the key from a file
  # instead of stdin, should we use the creation time on the file?
  my $sig_timestamp = ($args->{sig_timestamp} + 0);
  my $key_timestamp = ($args->{key_timestamp} + 0);

  if ($key_timestamp > $sig_timestamp) {
    die "key timestamp must not be later than signature timestamp\n";
  }

  my $creation_time_packet = pack('CCN', 5, $subpacket_types->{sig_creation_time}, $sig_timestamp);

  my $hashed_subs = $creation_time_packet.$args->{hashed_subpackets};

  my $subpacket_octets = pack('n', length($hashed_subs));

  my $sig_data_to_be_hashed =
    $version.
      $sigtype.
	$pubkey_algo.
	  $hash_algo.
	    $subpacket_octets.
	      $hashed_subs;

  my $pubkey = make_rsa_pub_key_body($rsa, $key_timestamp);

  # this is for signing.  it needs to be an old-style header with a
  # 2-packet octet count.

  my $key_data = make_packet($packet_types->{pubkey}, $pubkey, {'packet_length'=>2});

  # take the last 8 bytes of the fingerprint as the keyid:
  my $keyid = substr(fingerprint($rsa, $key_timestamp), 20 - 8, 8);

  # the v4 signature trailer is:

  # version number, literal 0xff, and then a 4-byte count of the
  # signature data itself.
  my $trailer = pack('CCN', 4, 0xff, length($sig_data_to_be_hashed));

  my $uid_data =
    pack('CN', 0xb4, length($uid)).
      $uid;

  my $datatosign =
    $key_data.
      $uid_data.
	$sig_data_to_be_hashed.
	  $trailer;

  # FIXME: handle signatures over digests other than SHA256:
  my $data_hash = Digest::SHA::sha256_hex($datatosign);

  my $issuer_packet = pack('CCa8', 9, $subpacket_types->{issuer}, $keyid);

  my $sig = Crypt::OpenSSL::Bignum->new_from_bin($rsa->sign($datatosign));

  my $sig_body =
    $sig_data_to_be_hashed.
      pack('n', length($issuer_packet)).
	$issuer_packet.
	  pack('n', hex(substr($data_hash, 0, 4))).
	    mpi_pack($sig);

  return make_packet($packet_types->{sig}, $sig_body);
}

# FIXME: switch to passing the whole packet as the arg, instead of the
# input stream.

# FIXME: think about native perl representation of the packets instead.

# Put a user ID into the $data
sub finduid {
  my $data = shift;
  my $instr = shift;
  my $tag = shift;
  my $packetlen = shift;

  my $dummy;
  ($tag == $packet_types->{uid}) or die "This should not be called on anything but a User ID packet\n";

  read($instr, $dummy, $packetlen);
  $data->{uid}->{$dummy} = {};
  $data->{current}->{uid} = $dummy;
}


# find signatures associated with the given fingerprint and user ID.
sub findsig {
  my $data = shift;
  my $instr = shift;
  my $tag = shift;
  my $packetlen = shift;

  ($tag == $packet_types->{sig}) or die "No calling findsig on anything other than a signature packet.\n";

  my $dummy;
  my $readbytes = 0;

  read($instr, $dummy, $packetlen - $readbytes) or die "Could not read in this packet.\n";

  if ((! defined $data->{key}) ||
      (! defined $data->{uid}) ||
      (! defined $data->{uid}->{$data->{target}->{uid}})) {
    # the user ID we are looking for has not been found yet.
    return;
  }

  # FIXME: if we get two primary keys on stdin, both with the same
  # targetd user ID, we'll store signatures from both keys, which is
  # probably wrong.

  # the current ID is not what we're looking for:
  return if ($data->{current}->{uid} ne $data->{target}->{uid});

  # just storing the raw signatures for the moment:
  push @{$data->{sigs}}, make_packet($packet_types->{sig}, $dummy);
  return;

}

# given an input stream and data, store the found key in data and
# consume the rest of the stream corresponding to the packet.
# data contains: (fpr: fingerprint to find, key: current best guess at key)
sub findkey {
  my $data = shift;
  my $instr = shift;
  my $tag = shift;
  my $packetlen = shift;

  my $dummy;
  my $ver;
  my $readbytes = 0;

  read($instr, $ver, 1) or die "could not read key version\n";
  $readbytes += 1;
  $ver = ord($ver);

  if ($ver != 4) {
    printf(STDERR "We only work with version 4 keys.  This key appears to be version %s.\n", $ver);
    read($instr, $dummy, $packetlen - $readbytes) or die "Could not skip past this packet.\n";
    return;
  }

  my $key_timestamp;
  read($instr, $key_timestamp, 4) or die "could not read key timestamp.\n";
  $readbytes += 4;
  $key_timestamp = unpack('N', $key_timestamp);

  my $algo;
  read($instr, $algo, 1) or die "could not read key algorithm.\n";
  $readbytes += 1;
  $algo = ord($algo);
  if ($algo != $asym_algos->{rsa}) {
    printf(STDERR "We only support RSA keys (this key used algorithm %d).\n", $algo);
    read($instr, $dummy, $packetlen - $readbytes) or die "Could not skip past this packet.\n";
    return;
  }

  ## we have an RSA key.
  my $modulus = read_mpi($instr, \$readbytes);
  my $exponent = read_mpi($instr, \$readbytes);

  my $pubkey = Crypt::OpenSSL::RSA->new_key_from_parameters($modulus, $exponent);
  my $foundfpr = fingerprint($pubkey, $key_timestamp);

  my $foundfprstr = Crypt::OpenSSL::Bignum->new_from_bin($foundfpr)->to_hex();
  # left-pad with 0's to bring up to full 40-char (160-bit) fingerprint:
  $foundfprstr = sprintf("%040s", $foundfprstr);

  # is this a match?
  if ((!defined($data->{target}->{fpr})) ||
      (substr($foundfprstr, -1 * length($data->{target}->{fpr})) eq $data->{target}->{fpr})) {
    if (defined($data->{key})) {
      die "Found two matching keys.\n";
    }
    $data->{key} = { 'rsa' => $pubkey,
		     'timestamp' => $key_timestamp };
  }

  if ($tag != $packet_types->{seckey} &&
      $tag != $packet_types->{sec_subkey}) {
    if ($readbytes < $packetlen) {
      read($instr, $dummy, $packetlen - $readbytes) or die "Could not skip past this packet.\n";
    }
    return;
  }
  if (!defined($data->{key})) {
    # we don't think the public part of this key matches
    if ($readbytes < $packetlen) {
      read($instr, $dummy, $packetlen - $readbytes) or die "Could not skip past this packet.\n";
    }
    return;
  }

  my $s2k;
  read($instr, $s2k, 1) or die "Could not read S2K octet.\n";
  $readbytes += 1;
  $s2k = ord($s2k);
  if ($s2k != 0) {
    printf(STDERR "We cannot handle encrypted secret keys.  Skipping!\n") ;
    read($instr, $dummy, $packetlen - $readbytes) or die "Could not skip past this packet.\n";
    return;
  }

  # secret material is unencrypted
  # see http://tools.ietf.org/html/rfc4880#section-5.5.3
  my $d = read_mpi($instr, \$readbytes);
  my $p = read_mpi($instr, \$readbytes);
  my $q = read_mpi($instr, \$readbytes);
  my $u = read_mpi($instr, \$readbytes);

  my $checksum;
  read($instr, $checksum, 2) or die "Could not read checksum of secret key material.\n";
  $readbytes += 2;
  $checksum = unpack('n', $checksum);

  # FIXME: compare with the checksum!  how?  the data is
  # gone into the Crypt::OpenSSL::Bignum

  $data->{key}->{rsa} = Crypt::OpenSSL::RSA->new_key_from_parameters($modulus,
								     $exponent,
								     $d,
								     $p,
								     $q);

  $data->{key}->{rsa}->check_key() or die "Secret key is not a valid RSA key.\n";

  if ($readbytes < $packetlen) {
    read($instr, $dummy, $packetlen - $readbytes) or die "Could not skip past this packet.\n";
  }
}

sub openpgp2rsa {
  my $instr = shift;
  my $fpr = shift;

  if (defined $fpr) {
    if (length($fpr) < 8) {
      die "We need at least 8 hex digits of fingerprint.\n";
    }
    $fpr = uc($fpr);
  }

  my $data = { 'fpr' => $fpr};
  my $subs = { $packet_types->{pubkey} => \&findkey,
	       $packet_types->{pub_subkey} => \&findkey,
	       $packet_types->{seckey} => \&findkey,
	       $packet_types->{sec_subkey} => \&findkey };

  packetwalk($instr, $subs, $data);

  return $data->{key}->{rsa};
}

sub adduserid {
  my $instr = shift;
  my $fpr = shift;
  my $uid = shift;
  my $args = shift;

  if ((! defined $fpr) ||
      (length($fpr) < 8)) {
    die "We need at least 8 hex digits of fingerprint.\n";
  }

  $fpr = uc($fpr);

  if (! defined $uid) {
    die "No User ID defined.\n";
  }

  my $data = { target => { fpr => $fpr,
			   uid => $uid,
			 },
	     };
  my $subs = { $packet_types->{seckey} => \&findkey,
	       $packet_types->{uid} => \&finduid,
	       $packet_types->{sig} => \&findsig,
	     };

  packetwalk($instr, $subs, $data);

  if ((! defined $data->{key}) ||
      (! defined $data->{key}->{rsa}) ||
      (! defined $data->{key}->{timestamp})) {
    die "The key requested was not found.\n"
  }

  if (defined $data->{uid}->{$uid}) {
    die "The requested User ID '$uid' is already associated with this key.\n";
  }
  $args->{key_timestamp} = $data->{key}->{timestamp};

  return
    make_packet($packet_types->{pubkey}, make_rsa_pub_key_body($data->{key}->{rsa}, $data->{key}->{timestamp})).
      make_packet($packet_types->{uid}, $uid).
	makeselfsig($data->{key}->{rsa},
		    $uid,
		    $args);

}


sub revokeuserid {
  my $instr = shift;
  my $fpr = shift;
  my $uid = shift;
  my $sigtime = shift;

  if ((! defined $fpr) ||
      (length($fpr) < 8)) {
    die "We need at least 8 hex digits of fingerprint.\n";
  }

  $fpr = uc($fpr);

  if (! defined $uid) {
    die "No User ID defined.\n";
  }

  my $data = { target => { fpr => $fpr,
			   uid => $uid,
			 },
	     };
  my $subs = { $packet_types->{seckey} => \&findkey,
	       $packet_types->{uid} => \&finduid,
	       $packet_types->{sig} => \&findsig,
	     };

  packetwalk($instr, $subs, $data);

  if ((! defined $data->{uid}) ||
      (! defined $data->{uid}->{$uid})) {
    die "The User ID \"$uid\" is not associated with this key";
  }

  if ((! defined $data->{key}) ||
      (! defined $data->{key}->{rsa}) ||
      (! defined $data->{key}->{timestamp})) {
    die "The key requested was not found."
  }

  my $revocation_reason = 'No longer using this hostname';
  if (defined $data->{revocation_reason}) {
    $revocation_reason = $data->{revocation_reason};
  }

  my $rev_reason_subpkt = prefixsubpacket(pack('CC',
					       $subpacket_types->{revocation_reason},
					       $revocation_reasons->{user_id_no_longer_valid}).
					  $revocation_reason);

  if (! defined $sigtime) {
    $sigtime = time();
  }
  # what does a signature like this look like?
  my $args = { key_timestamp => $data->{key}->{timestamp},
	       sig_timestamp => $sigtime,
	       certification_type => $sig_types->{certification_revocation},
	       hashed_subpackets => $rev_reason_subpkt,
	     };

  return
    make_packet($packet_types->{pubkey}, make_rsa_pub_key_body($data->{key}->{rsa}, $data->{key}->{timestamp})).
      make_packet($packet_types->{uid}, $uid).
	join('', @{$data->{sigs}}).
	  gensig($data->{key}->{rsa}, $uid, $args);
}


# see 5.2.3.1 for tips on how to calculate the length of a subpacket:
sub prefixsubpacket {
  my $subpacket = shift;

  my $len = length($subpacket);
  my $prefix;
  use bytes;
  if ($len < 192) {
    # one byte:
    $prefix = pack('C', $len);
  } elsif ($len < 16576) {
    my $in = $len - 192;
    my $second = $in%256;
    my $first = ($in - $second)>>8;
    $prefix = pack('CC', $first + 192, $second)
  } else {
    $prefix = pack('CN', 255, $len);
  }
  return $prefix.$subpacket;
}



sub packetwalk {
  my $instr = shift;
  my $subs = shift;
  my $data = shift;

  my $packettag;
  my $dummy;
  my $tag;

  while (! eof($instr)) {
    read($instr, $packettag, 1);
    $packettag = ord($packettag);

    my $packetlen;
    if ( ! (0x80 & $packettag)) {
      die "This is not an OpenPGP packet\n";
    }
    if (0x40 & $packettag) {
      # this is a new-format packet.
      $tag = (0x3f & $packettag);
      my $nextlen = 0;
      read($instr, $nextlen, 1);
      $nextlen = ord($nextlen);
      if ($nextlen < 192) {
	$packetlen = $nextlen;
      } elsif ($nextlen < 224) {
	my $newoct;
	read($instr, $newoct, 1);
	$newoct = ord($newoct);
	$packetlen = (($nextlen - 192) << 8) + ($newoct) + 192;
      } elsif ($nextlen == 255) {
	read($instr, $nextlen, 4);
	$packetlen = unpack('N', $nextlen);
      } else {
	# packet length is undefined.
      }
    } else {
      # this is an old-format packet.
      my $lentype;
      $lentype = 0x03 & $packettag;
      $tag = ( 0x3c & $packettag ) >> 2;
      if ($lentype == 0) {
	read($instr, $packetlen, 1) or die "could not read packet length\n";
	$packetlen = unpack('C', $packetlen);
      } elsif ($lentype == 1) {
	read($instr, $packetlen, 2) or die "could not read packet length\n";
	$packetlen = unpack('n', $packetlen);
      } elsif ($lentype == 2) {
	read($instr, $packetlen, 4) or die "could not read packet length\n";
	$packetlen = unpack('N', $packetlen);
      } else {
	# packet length is undefined.
      }
    }

    if (! defined($packetlen)) {
      die "Undefined packet lengths are not supported.\n";
    }

    if (defined $subs->{$tag}) {
      $subs->{$tag}($data, $instr, $tag, $packetlen);
    } else {
      read($instr, $dummy, $packetlen) or die "Could not skip past this packet!\n";
    }
  }

  return $data->{key};
}


for (basename($0)) {
  if (/^pem2openpgp$/) {
    my $rsa;
    my $stdin;

    my $uid = shift;
    defined($uid) or die "You must specify a user ID string.\n";

    # FIXME: fail if there is no given user ID; or should we default to
    # hostname_long() from Sys::Hostname::Long ?

    if (defined $ENV{PEM2OPENPGP_NEWKEY}) {
      $rsa = Crypt::OpenSSL::RSA->generate_key($ENV{PEM2OPENPGP_NEWKEY});
    } else {
      $stdin = do {
	local $/; # slurp!
	<STDIN>;
      };

      $rsa = Crypt::OpenSSL::RSA->new_private_key($stdin);
    }

    my $key_timestamp = $ENV{PEM2OPENPGP_KEY_TIMESTAMP};
    my $sig_timestamp = $ENV{PEM2OPENPGP_TIMESTAMP};
    $sig_timestamp = time() if (!defined $sig_timestamp);
    $key_timestamp = $sig_timestamp if (!defined $key_timestamp);

    print
      make_packet($packet_types->{seckey}, make_rsa_sec_key_body($rsa, $key_timestamp)).
	make_packet($packet_types->{uid}, $uid).
	  makeselfsig($rsa,
		      $uid,
		      { sig_timestamp => $sig_timestamp,
			key_timestamp => $key_timestamp,
			expiration => $ENV{PEM2OPENPGP_EXPIRATION},
			usage_flags => $ENV{PEM2OPENPGP_USAGE_FLAGS},
		      }
		     );
  }
  elsif (/^openpgp2ssh$/) {
      my $fpr = shift;
      my $instream;
      open($instream,'-');
      binmode($instream, ":bytes");
      my $key = openpgp2rsa($instream, $fpr);
      if (defined($key)) {
	if ($key->is_private()) {
	  print $key->get_private_key_string();
	} else {
	  print "ssh-rsa ".encode_base64(openssh_pubkey_pack($key), '')."\n";
	}
      } else {
	die "No matching key found.\n";
      }
  }
  elsif (/^keytrans$/) {
    # subcommands when keytrans is invoked directly are UNSUPPORTED,
    # UNDOCUMENTED, and WILL NOT BE MAINTAINED.
    my $subcommand = shift;
    for ($subcommand) {
      if (/^revokeuserid$/) {
	my $fpr = shift;
	my $uid = shift;
	my $instream;
	open($instream,'-');
	binmode($instream, ":bytes");

	my $revcert = revokeuserid($instream, $fpr, $uid, $ENV{PEM2OPENPGP_TIMESTAMP});

	print $revcert;
      } elsif (/^adduserid$/) {
	my $fpr = shift;
	my $uid = shift;
	my $instream;
	open($instream,'-');
	binmode($instream, ":bytes");
	my $newuid = adduserid($instream, $fpr, $uid, 
			       { sig_timestamp => $ENV{PEM2OPENPGP_TIMESTAMP},
				 expiration => $ENV{PEM2OPENPGP_EXPIRATION},
				 usage_flags => $ENV{PEM2OPENPGP_USAGE_FLAGS},
			       });

	print $newuid;
      } else {
	die "Unrecognized subcomand.  keytrans subcommands are not a stable interface!\n";
      }
    }
  }
  else {
    die "Unrecognized keytrans call.\n";
  }
}