SEAL2(3) User Contributed Perl Documentation SEAL2(3)NAMECrypt::SEAL2 - The SEAL stream cipher, version 2.0
SYNOPSIS
use Crypt::SEAL2;
$cipher = new Crypt::SEAL2 $key;
$ciphertext = $cipher->encrypt($plaintext);
$cipher->reset();
$ks = $cipher->keysize();
$plaintext = $cipher->decrypt($ciphertext);
$cipher->repos($position);
DESCRIPTION
SEAL2 is the second version of the stream cipher, SEAL, designed by Don
Coppersmith and Phillip Rogaway.
This module supports the following functions:
new()
Creates a pseudorandom string (PRS), using a user-supplied key as a
seed to the pseudorandom generator of SEAL2. A PRS pointer ini-
tially points at the beginning of the PRS.
encrypt($data)
Encrypts the data stream $data by XOR-ing it with the PRS, starting
at the position being pointed to by the PRS pointer, and returns
the resulting ciphertext. The PRS pointer is advanced 1 byte posi-
tion for every byte of $data that is encrypted.
decrypt($data)
Decrypts the data stream $data by XOR-ing it with the PRS, starting
at the position being pointed to by the PRS pointer, and returns
the resulting plaintext. The PRS pointer is advanced 1 byte posi-
tion for every byte of $data that is decrypted.
decrypt($data) is exactly the same as encrypt($data).
reset()
Every time a call to either encrypt() or decrypt() is performed,
the PRS pointer is advanced. Therefore, it is necessary to reset()
the pointer in order to encrypt/decrypt the data stream correctly.
Alternatively, you may use repos() to manually re-position the PRS
pointer to where the encryption/decryption will start (see next
function).
repos($position)
Re-positions the PRS pointer at byte position $position
keysize()
Returns the size (in bytes) of the key used (20, in this case)
Note
Since the pseudorandom sequence generated by SEAL2 is XOR-ed with the
data stream, a call to encrypt is the same as a call to decrypt. Math-
ematically,
P xor R = C
C xor R = P
EXAMPLE
#!/usr/local/bin/perl
use diagnostics;
use strict;
use warnings;
use Crypt::SEAL2;
my $key = pack "H40", "00112233445566778899aabbccddeeff00112233";
my $cipher = new Crypt::SEAL2 $key;
my $plaintext1 = pack "H16", "0123456789abcdef";
print "old plaintext1 : ", unpack("H*", $plaintext1), "\n";
my $ciphertext1 = $cipher->encrypt($plaintext1);
print "ciphertext1 : ", unpack("H*", $ciphertext1), "\n";
$cipher->reset();
my $decrypted1 = $cipher->decrypt($ciphertext1);
print "new plaintext1 : ", unpack("H*", $decrypted1), "\n";
print "\n";
my $plaintext2 = pack "H40", "fedcba98765432100123456789abcdef01234567";
print "old plaintext2 : ", unpack("H*", $plaintext2), "\n";
$cipher->reset();
my $ciphertext2 = $cipher->encrypt($plaintext2);
print "ciphertext2 : ", unpack("H*", $ciphertext2), "\n";
$cipher->reset();
my $decrypted2 = $cipher->decrypt($ciphertext2);
print "new plaintext2 : ", unpack("H*", $decrypted2), "\n";
CAVEAT
SEAL2 is designed to generate up to 2^48 bytes of output per seed. In
1997, Handschuh and Gilbert showed, however, that the output stream can
be distinguished from a random sequence after only seeing roughly 2^34
bytes of output. Thus, it is prudent to avoid using the same seed for
more than 2^34 bytes of output.
COPYRIGHT AND LICENSE
Copyright (C) 2003 Julius C. Duque. Please read contact.html that comes
with this distribution for details on how to contact the author.
This library is free software; you can redistribute it and/or modify it
under the same terms as the GNU General Public License.
perl v5.8.8 2003-07-14 SEAL2(3)
