|
|
Crypt::SEAL2 - The SEAL stream cipher, version 2.0
use Crypt::SEAL2;
$cipher = new Crypt::SEAL2 $key;
$ciphertext = $cipher->encrypt($plaintext);
$cipher->reset();
$ks = $cipher->keysize();
$plaintext = $cipher->decrypt($ciphertext);
$cipher->repos($position);
SEAL2 is the second version of the stream cipher, SEAL, designed by Don Coppersmith and Phillip Rogaway.
This module supports the following functions:
Creates a pseudorandom string (PRS), using a user-supplied key as a seed to the pseudorandom generator of SEAL2. A PRS pointer initially points at the beginning of the PRS.
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 position for every byte of $data that is encrypted.
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 position for every byte of $data that is decrypted.
decrypt($data) is exactly the same as encrypt($data).
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).
Re-positions the PRS pointer at byte position $position
Returns the size (in bytes) of the key used (20, in this case)
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. Mathematically,
P xor R = C
C xor R = P
#!/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";
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 (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.