#!/usr/bin/perl -sw
##
## Crypt::RSA::SSA:PSS
##
## Copyright (c) 2001, Vipul Ved Prakash.  All rights reserved.
## This code is free software; you can redistribute it and/or modify
## it under the same terms as Perl itself.
##
## $Id: PSS.pm,v 1.11 2001/03/12 16:17:34 vipul Exp $

use lib "/home/vipul/PERL/crypto/rsa/lib";
package Crypt::RSA::SSA::PSS;
use strict;
use vars qw(@ISA);
use Crypt::RSA::Errorhandler; 
use Crypt::RSA::DataFormat qw(bitsize os2ip i2osp octet_xor generate_random_octet mgf1);
use Crypt::RSA::Primitives;
use Crypt::RSA::Debug qw(debug);
use Digest::SHA1 qw(sha1);
use Math::Pari qw(floor);
@ISA = qw(Crypt::RSA::Errorhandler);


sub new { 
    return bless { primitives => new Crypt::RSA::Primitives, 
                   hlen       => 20,
                 }, shift 
}


sub sign { 
    my ($self, %params) = @_; 
    my $key = $params{Key}; my $M = $params{Message};
    my $k = int(floor(bitsize($key->n)/8));  
    my $salt = generate_random_octet ($self->{hlen});
    my $em = $self->encode ($M, $salt, $k-1);
    my $m = os2ip ($em);
    my $sig = $self->{primitives}->core_sign (Key => $key, Message => $m);
    my $S = i2osp ($sig, $k);
    return ($S, $salt) if wantarray;
    return $S;
}    


sub verify_with_salt { 
    my ($self, %params) = @_;
    my $key = $params{Key}; my $M = $params{Message}; 
    my $S = $params{Signature}; my $salt = $params{Salt};
    my $k = int(floor(bitsize($key->n)/8));  
    my $em; 
    unless ($em = $self->encode ($M, $salt, $k-1)) { 
        return if $self->errstr eq "Message too long.";
        return $self->error ("Modulus too short.", \$M, \$S, \$key, \%params) if 
        $self->errstr eq "Intended encoded message length too short."; 
    }
    return $self->error ("Invalid signature.", \$M, \$S, $key, \%params) if length($S) < $k;
    my $s = os2ip ($S);
    my $m = $self->{primitives}->core_verify (Key => $key, Signature => $s) || 
        $self->error ("Invalid signature.", \$M, \$S, $key, \%params);
    my $em1 = i2osp ($m, $k-1) || 
        return $self->error ("Invalid signature.", \$M, \$S, $key, \%params);
    return 1 if $em eq $em1;
    return $self->error ("Invalid signature.", \$M, \$S, $key, \%params);
}


sub verify { 
    my ($self, %params) = @_;
    my $key = $params{Key}; my $M = $params{Message}; my $S = $params{Signature}; 
    my $k = int(floor(bitsize($key->n)/8));  
    my $s = os2ip ($S);
    my $m = $self->{primitives}->core_verify (Key => $key, Signature => $s) || 
        $self->error ("Invalid signature.", \$M, \$S, $key, \%params);
    my $em1 = i2osp ($m, $k-1) || 
        return $self->error ("Invalid signature.", \$M, \$S, $key, \%params);
    return 1 if $self->verify_with_salt_recovery ($M, $em1);
    print $self->errstr; 
    return $self->error ("Invalid signature.", \$M, \$S, $key, \%params);
}


sub encode { 
    my ($self, $M, $salt, $emlen) = @_;  
    return $self->error ("Intended encoded message length too short.", \$M, \$salt ) 
        if $emlen < 2 * $self->{hlen}; 
    my $H = $self->hash ("$salt$M");
    my $padlength = $emlen - (2*$$self{hlen}); 
    my $PS = chr(0)x$padlength;
    my $db = $salt . $PS; 
    my $dbmask = $self->mgf ($H, $emlen - $self->{hlen});
    my $maskeddb = octet_xor ($db, $dbmask);
    my $em = $H . $maskeddb; 
    return $em;
}


sub verify_with_salt_recovery { 
    my ($self, $M, $EM) = @_; 
    my $hlen = $$self{hlen}; 
    my $emlen = length ($EM); 
    return $self->error ("Encoded message too short.", \$M, \$EM) if $emlen < (2*$hlen); 
    my $H = substr $EM, 0, $hlen; 
    my $maskeddb = substr $EM, $hlen; 
    my $dbmask = $self->mgf ($H, $emlen-$hlen);
    my $db = octet_xor ($maskeddb, $dbmask); 
    my $salt = substr $db, 0, $hlen;
    my $PS = substr $db, $hlen; 
    my $check = chr(0) x ($emlen-(2*$hlen)); debug ("PS: $PS");
    return $self->error ("Inconsistent.") unless $check eq $PS; 
    my $H1 = $self->hash ("$salt$M");
    return 1 if $H eq $H1; 
    return $self->error ("Inconsistent.");
} 


sub hash { 
    my ($self, $data) = @_;
    return sha1 ($data);
}
    

sub mgf { 
    my ($self, @data) = @_;
    return mgf1 (@data);
}

1;

=head1 NAME

Crypt::RSA::SSA::PSS - Probablistic Signature Scheme based on RSA. 

=head1 SYNOPSIS

    my $pss = new Crypt::RSA::SSA::PSS; 

    my $signature = $pss->sign (
                        Message => $message,
                        Key     => $private, 
                    ) || die $pss->errstr;

    my $verify    = $pss->verify (
                        Message   => $message, 
                        Key       => $key, 
                        Signature => $signature, 
                    ) || die $pss->errstr;


=head1 DESCRIPTION

PSS (Probabilistic Signature Scheme) is a provably secure method of
creating digital signatures with RSA. "Provable" means that the
difficulty of forging signatures can be directly related to inverting
the RSA function. "Probablistic" alludes to the randomly generated salt
value included in the signature to enchance security. For more details
on PSS, see [4] & [13].

=head1 METHODS

=head2 B<new()>

Constructor. 

=head2 B<sign()>

Computes a PSS signature on a message with the private key of the signer.
In scalar context, sign() returns the computed signature. In array
context, it returns the signature and the random salt. The signature can
verified with verify() or verify_with_salt(). sign() takes a hash argument
with the following mandatory keys:

=over 4

=item B<Message> 

Message to be signed, a string of arbitary length.  

=item B<Key> 

Private key of the signer, a Crypt::RSA::Key::Private object.

=back

=head2 B<verify()>

Verfies a signature generated with sign(). The salt is recovered from the
signature and need not be passed. Returns a true value on success and
false on failure. $self->errstr is set to "Invalid signature." or
appropriate error on failure. verify() takes a hash argument with the
following mandatory keys:

=over 4

=item B<Key>

Public key of the signer, a Crypt::RSA::Key::Public object. 

=item B<Message>

The original signed message, a string of arbitary length. 

=item B<Signature>

Signature computed with sign(), a string. 

=head2 B<verify_with_salt()>

Verifies a signature given the salt. Takes the same arguments as verify()
in addition to B<Salt>, which is a 20-byte string returned by sign() in
array context.

=head1 ERROR HANDLING

See ERROR HANDLING in Crypt::RSA(3) manpage.

=head1 BIBLIOGRAPHY

See BIBLIOGRAPHY in Crypt::RSA(3) manpage.

=over

=head1 AUTHOR

Vipul Ved Prakash, E<lt>mail@vipul.netE<gt>

=head1 SEE ALSO 

Crypt::RSA(3), Crypt::RSA::Primitives(3), Crypt::RSA::Keys(3),
Crypt::RSA::EME::OAEP(3)

=cut


