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package Crypt::DSA;
use 5.006;
use strict;
use Digest::SHA1 qw( sha1 );
use Carp qw( croak );
use Crypt::DSA::KeyChain;
use Crypt::DSA::Key;
use Crypt::DSA::Signature;
use Crypt::DSA::Util qw( bitsize bin2mp mod_inverse mod_exp makerandom );
use vars qw( $VERSION );
BEGIN {
$VERSION = '1.17';
}
sub new {
my $class = shift;
my $dsa = bless { @_ }, $class;
$dsa->{_keychain} = Crypt::DSA::KeyChain->new(@_);
$dsa;
}
sub keygen {
my $dsa = shift;
my $key = $dsa->{_keychain}->generate_params(@_);
$dsa->{_keychain}->generate_keys($key);
$key;
}
sub sign {
my $dsa = shift;
my %param = @_;
my($key, $dgst);
croak __PACKAGE__, "->sign: Need a Key" unless $key = $param{Key};
unless ($dgst = $param{Digest}) {
croak __PACKAGE__, "->sign: Need either Message or Digest"
unless $param{Message};
$dgst = sha1($param{Message});
}
my $dlen = length $dgst;
my $i = bitsize($key->q) / 8;
croak "Data too large for key size"
if $dlen > $i || $dlen > 50;
$dsa->_sign_setup($key)
unless $key->kinv && $key->r;
my $m = bin2mp($dgst);
my $xr = ($key->priv_key * $key->r) % $key->q;
my $s = $xr + $m;
$s -= $key->q if $s > $key->q;
$s = ($s * $key->kinv) % $key->q;
my $sig = Crypt::DSA::Signature->new;
$sig->r($key->r);
$sig->s($s);
$sig;
}
sub _sign_setup {
my $dsa = shift;
my $key = shift;
my($k, $r);
{
$k = makerandom(Size => bitsize($key->q));
$k -= $key->q if $k >= $key->q;
redo if $k == 0;
}
$r = mod_exp($key->g, $k, $key->p);
$r %= $key->q;
my $kinv = mod_inverse($k, $key->q);
$key->r($r);
$key->kinv($kinv);
}
sub verify {
my $dsa = shift;
my %param = @_;
my($key, $dgst, $sig);
croak __PACKAGE__, "->verify: Need a Key" unless $key = $param{Key};
unless ($dgst = $param{Digest}) {
croak __PACKAGE__, "->verify: Need either Message or Digest"
unless $param{Message};
$dgst = sha1($param{Message});
}
croak __PACKAGE__, "->verify: Need a Signature"
unless $sig = $param{Signature};
my $u2 = mod_inverse($sig->s, $key->q);
my $u1 = bin2mp($dgst);
$u1 = ($u1 * $u2) % $key->q;
$u2 = ($sig->r * $u2) % $key->q;
my $t1 = mod_exp($key->g, $u1, $key->p);
my $t2 = mod_exp($key->pub_key, $u2, $key->p);
$u1 = ($t1 * $t2) % $key->p;
$u1 %= $key->q;
$u1 == $sig->r;
}
1;
__END__
=pod
=head1 NAME
Crypt::DSA - DSA Signatures and Key Generation
=head1 SYNOPSIS
use Crypt::DSA;
my $dsa = Crypt::DSA->new;
my $key = $dsa->keygen(
Size => 512,
Seed => $seed,
Verbosity => 1
);
my $sig = $dsa->sign(
Message => "foo bar",
Key => $key
);
my $verified = $dsa->verify(
Message => "foo bar",
Signature => $sig,
Key => $key,
);
=head1 DESCRIPTION
I<Crypt::DSA> is an implementation of the DSA (Digital Signature
Algorithm) signature verification system. The implementation
itself is pure Perl, although the heavy-duty mathematics underneath
are provided by the I<Math::Pari> library.
This package provides DSA signing, signature verification, and key
generation.
=head1 USAGE
The I<Crypt::DSA> public interface is similar to that of
I<Crypt::RSA>. This was done intentionally.
=head2 Crypt::DSA->new
Constructs a new I<Crypt::DSA> object. At the moment this isn't
particularly useful in itself, other than being the object you
need to do much else in the system.
Returns the new object.
=head2 $key = $dsa->keygen(%arg)
Generates a new set of DSA keys, including both the public and
private portions of the key.
I<%arg> can contain:
=over 4
=item * Size
The size in bits of the I<p> value to generate. The I<q> and
I<g> values are always 160 bits each.
This argument is mandatory.
=item * Seed
A seed with which I<q> generation will begin. If this seed does
not lead to a suitable prime, it will be discarded, and a new
random seed chosen in its place, until a suitable prime can be
found.
This is entirely optional, and if not provided a random seed will
be generated automatically.
=item * Verbosity
Should be either 0 or 1. A value of 1 will give you a progress
meter during I<p> and I<q> generation--this can be useful, since
the process can be relatively long.
The default is 0.
=back
=head2 $signature = $dsa->sign(%arg)
Signs a message (or the digest of a message) using the private
portion of the DSA key and returns the signature.
The return value--the signature--is a I<Crypt::DSA::Signature>
object.
I<%arg> can include:
=over 4
=item * Digest
A digest to be signed. The digest should be 20 bytes in length
or less.
You must provide either this argument or I<Message> (see below).
=item * Key
The I<Crypt::DSA::Key> object with which the signature will be
generated. Should contain a private key attribute (I<priv_key>).
This argument is required.
=item * Message
A plaintext message to be signed. If you provide this argument,
I<sign> will first produce a SHA1 digest of the plaintext, then
use that as the digest to sign. Thus writing
my $sign = $dsa->sign(Message => $message, ... );
is a shorter way of writing
use Digest::SHA1 qw( sha1 );
my $sig = $dsa->sign(Digest => sha1( $message ), ... );
=back
=head2 $verified = $dsa->verify(%arg)
Verifies a signature generated with I<sign>. Returns a true
value on success and false on failure.
I<%arg> can contain:
=over 4
=item * Key
Key of the signer of the message; a I<Crypt::DSA::Key> object.
The public portion of the key is used to verify the signature.
This argument is required.
=item * Signature
The signature itself. Should be in the same format as returned
from I<sign>, a I<Crypt::DSA::Signature> object.
This argument is required.
=item * Digest
The original signed digest whose length is less than or equal to
20 bytes.
Either this argument or I<Message> (see below) must be present.
=item * Message
As above in I<sign>, the plaintext message that was signed, a
string of arbitrary length. A SHA1 digest of this message will
be created and used in the verification process.
=back
=head1 TODO
Add ability to munge format of keys. For example, read/write keys
from/to key files (SSH key files, etc.), and also write them in
other formats.
=head1 SUPPORT
Bugs should be reported via the CPAN bug tracker at
L<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Crypt-DSA>
For other issues, contact the author.
=head1 AUTHOR
Benjamin Trott E<lt>ben@sixapart.comE<gt>
=head1 COPYRIGHT
Except where otherwise noted,
Crypt::DSA is Copyright 2006 - 2011 Benjamin Trott.
Crypt::DSA is free software; you may redistribute it
and/or modify it under the same terms as Perl itself.
=cut
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