#!/usr/bin/perl -w -T

# 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

# 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 Crypt::OpenSSL::RSA;
use Crypt::OpenSSL::Bignum;
use Crypt::OpenSSL::Bignum::CTX;
use Digest::SHA1;
use MIME::Base64;

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

my $uid = shift;

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



# 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 5.2.3.1
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 43 of RFC 4880
sub simple_checksum {
  my $bytes = shift;

  return unpack("%32W*",$bytes) % 65536;
}

# 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 $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();

  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";
  }

  return $lastx;
}


############ 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 $len = length($body);

  my $lenbytes;
  my $lencode;

  if ($len < 2**8) {
    $lenbytes = 0;
    $lencode = 'C';
  } elsif ($len < 2**16) {
    $lenbytes = 1;
    $lencode = 'n';
  } elsif ($len < 2**31) {
    ## not testing against full 32 bits because i don't want to deal
    ## with potential overflow.
    $lenbytes = 2;
    $lencode = 'N';
  } else {
    ## what the hell do we do here?
    $lenbytes = 3;
    $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;
}

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

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

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

sub make_rsa_sec_key_body {
  my $key = shift;
  my $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 $secret_material = mpi_pack($d).
    mpi_pack($p).
      mpi_pack($q).
	mpi_pack(modular_multi_inverse($p, $q));

  # 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, $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 $timestamp = shift;

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

  return Digest::SHA1::sha1(pack('Cn', 0x99, length($rsabody)).$rsabody);
}

# we're just not dealing with newline business right now.  slurp in
# the whole file.
undef $/;
my $buf = <STDIN>;


my $rsa = Crypt::OpenSSL::RSA->new_private_key($buf);

$rsa->use_sha1_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";
}

my $version = pack('C', 4);
# strong assertion of identity:
my $sigtype = pack('C', $sig_types->{positive_certification});
# RSA
my $pubkey_algo = pack('C', $asym_algos->{rsa});
# SHA1
my $hash_algo = pack('C', $digests->{sha1});

# 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?

# could an environment variable (if set) override 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 $timestamp = 0;
if (defined $ENV{PEM2OPENPGP_TIMESTAMP}) {
  $timestamp = ($ENV{PEM2OPENPGP_TIMESTAMP} + 0);
} else {
  $timestamp = time();
}

my $flags = 0;
if (! defined $ENV{PEM2OPENPGP_USAGE_FLAGS}) {
  $flags = $usage_flags->{authenticate};
} else {
  my @ff = split(",", $ENV{PEM2OPENPGP_USAGE_FLAGS});
  foreach $f (@ff) {
    if (! defined $usage_flags->{$f})
      die "No such flag $f";
    $flags |= $usage_flags->{$f};
  }
}

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


# FIXME: HARDCODED: what if someone wants to select a different set of
# usage flags?  For now, we do only authentication because that's what
# monkeysphere needs.
my $usage_packet = pack('CCC', 2, $subpacket_types->{usage_flags}, $usage_flags->{authenticate});


# FIXME: HARDCODED: how should we determine how far off to set the
# expiration date?  default is to expire in 2 days, which is insanely
# short (but good for testing).  The user ought to be able to decide
# this directly, rather than having to do "monkeysphere-server
# extend-key".
my $expires_in = 86400*2;
my $expiration_packet = 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-1, SHA-256, RIPE-MD/160
my $pref_hash_algos = pack('CCCCC', 4, $subpacket_types->{preferred_digest},
			   $digests->{sha1},
			   $digests->{sha256},
			   $digests->{ripemd160}
			  );

# 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});

my $subpackets_to_be_hashed =
  $creation_time_packet.
  $usage_packet.
  $expiration_packet.
  $pref_sym_algos.
  $pref_hash_algos.
  $pref_zip_algos.
  $feature_subpacket.
  $keyserver_pref;

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

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

my $pubkey = make_rsa_pub_key_body($rsa, $timestamp);
my $seckey = make_rsa_sec_key_body($rsa, $timestamp);

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

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

my $data_hash = Digest::SHA1::sha1_hex($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);

print
  make_packet($packet_types->{seckey}, $seckey).
  make_packet($packet_types->{uid}, $uid).
  make_packet($packet_types->{sig}, $sig_body);