=head1 NAME

Net::FCP::Key::CHK - manage CHK keys.

=head1 SYNOPSIS

 use Net::FCP::Key::CHK;

 my $key = new Net::FCP::Key::CHK;
 my $key = new_from_uri Net::FCP::Key::CHK $uri;
 my $key = new_from_data Net::FCP::Key::CHK $data, $metadata;
 ... more to come


=head1 DESCRIPTION

=back

=head2 THE Net::FCP::Key::CHK CLASS

=over 4

=cut

package Net::FCP::Key::CHK;

use Carp;
use Digest::SHA1;
use Crypt::Twofish;
use MIME::Base64;

use Net::FCP::Util;

no warnings;

=item my $key = new Net::FCP::Key::CHK;

Don't use :)

=cut

sub new {
   my $class = shift;

   bless { }, $class;
}

=item my $key = new_from_data Net::FCP::Key::CHK $metadata, $data;

Generate a CHK from the given data and metadata strings.

=cut

sub new_from_data {
   my ($class, $metadata, $data) = @_;

   $class->new->set_data ($metadata, $data);
}

sub rolling_hashpad($$) {
   my $d = Digest::SHA1->new->add ($_[0]);
   my $pad = "";

   while ($_[1] > length $_[0]) {
      my $dig = $d->digest_noreset;
      $d->add ($dig);
      $pad .= $dig;
      $_[0] .= $pad;
   }

   substr $_[0], $_[1], length $_[0], "";
}

sub encode_number($) {
   my $num = pack "N", $_[0];
   $num =~ s/^\x00+//;
   pack "n a*", length $num, $num;
}

sub set_data {
   my ($self, $metadata, $data) = @_;

   my $total_len = (length $metadata) + (length $data);
   my $padded_len = 1 << Net::FCP::Util::log2 $total_len, 10;

   my $plaintext = "$metadata$data";

   my $d = new Digest::SHA1;
   $d->add ($plaintext);
   $d = $d->digest;

   # only works for 128 bit keys
   my $k = new Digest::SHA1;
   $k->add ("\x00" x 1);
   $k->add ($d);

   my $hash = substr +(my $x=$k->digest), 0, 16; # extract leading 128 bit

   my $buf = "";

   $buf .= pack "n a20", 20, Digest::SHA1::sha1 $hash;

   $buf .= encode_number $total_len;
   $buf .= encode_number length $metadata;

   $buf .= "\x00\x00";

   rolling_hashpad $buf, 1 << Net::FCP::Util::log2 length $buf;

   my $pcfbc = new Crypt::Twofish $hash;
   my $pcfbr = "\x00" x 16;
   my $pcfb = 16;

   my $pcfb_enc = sub  {
      if ($pcfb == 16) {
         $pcfb = 0;
         $pcfbr = $pcfbc->encrypt ($pcfbr);
      }
      (substr $pcfbr, $pcfb++, 1) ^= $_[0];
   };

   $buf = join "", map $pcfb_enc->($_), split //, $buf;

   my $senc = unpack "H*", $buf;

   rolling_hashpad $plaintext, $padded_len;

   $plaintext = join "", map $pcfb_enc->($_), split //, $plaintext;

   my $partsize = $padded_len > 16384 ? $padded_len >> 7 : $padded_len;
   $partsize = 16384 if $partsize < 16384; # up to 4MB

   my $dig = "";
   for (my $ofs = ($padded_len-1) - ($padded_len-1) % $partsize; $ofs >= 0; $ofs -= $partsize) {
      $dig = Digest::SHA1::sha1 substr ($plaintext, $ofs, $partsize) . $dig;
   }

   my $sini = unpack "H*", $dig;

   my $s = sprintf
           "Document-header\xfe%s\xff"
         . "Initial-digest\xfe%s\xff"
         . "Part-size\xfe%x\xff"
         . "Symmetric-cipher\xfe%s\xff",
           $senc, $sini, $partsize, "Twofish";

   warn "$s\n";

   $s = Net::FCP::Util::encode_base64 +(Digest::SHA1::sha1 $s) . (pack "C", Net::FCP::Util::log2 $padded_len) . "\x03\x02";
   print "$s\n";


   $self;
}

=item $size = $key->size

Returns the size of the data (in bytes).

=cut

=item $digest = $key->digest

Return the store digest/hash.

=cut

=item $keynum = $key->keynumber

Returns the keynumber (version?)

=cut

=item $chk = $key->chk

Return the full CHK.

=cut

=back

=head1 SEE ALSO

L<Net::FCP>.

=head1 BUGS

Not heavily tested.

=head1 AUTHOR

 Marc Lehmann <pcg@goof.com>
 http://www.goof.com/pcg/marc/

=cut

my $data = do { local $/; <> };
my $metadata = substr $data, 0, 0x3f, "";
#my $metadata = substr $data, 0, 0x28, "";

my $key = Net::FCP::Key::CHK->new_from_data ($metadata, $data);
#use Net::FCP; my $fcp = new Net::FCP; warn $fcp->generate_chk ($metadata, $data);

1;