Crypt::Eksblowfish::SuUsereContributed Perl DocCrypt::Eksblowfish::Subkeyed(3)NAMECrypt::Eksblowfish::Subkeyed - Blowfish/Eksblowfish with access to
subkeys
SYNOPSIS
use Crypt::Eksblowfish::Subkeyed;
$block_size = Crypt::Eksblowfish::Subkeyed->blocksize;
$cipher = Crypt::Eksblowfish::Subkeyed
->new_from_subkeys(\@p_array, \@s_boxes);
$cipher = Crypt::Eksblowfish::Subkeyed->new_initial;
$block_size = $cipher->blocksize;
$ciphertext = $cipher->encrypt($plaintext);
$plaintext = $cipher->decrypt($ciphertext);
$p_array = $cipher->p_array;
$s_boxes = $cipher->s_boxes;
if($cipher->is_weak) { ...
DESCRIPTION
An object of this class encapsulates a keyed instance of the Blowfish
or Eksblowfish block cipher, ready to encrypt and decrypt. Normally
this class will not be used directly, but through subclasses such as
Crypt::Eksblowfish.
Eksblowfish is a variant of the Blowfish cipher with a modified key
setup algorithm. This class doesn't implement either form of key
setup, but only provides the actual encryption and decryption parts of
the ciphers. This part is shared between Blowfish and Eksblowfish, and
also any other cipher that uses the core of Blowfish but supplies its
own key setup. This class has "Eksblowfish" in its name rather than
"Blowfish" merely due to the historical accident that it is derived
from the encryption engine that was used to implement Eksblowfish.
The key setup phase of a block cipher, also known as the "key
schedule", produces a set of "subkeys", which are somewhat like
ordinary cryptographic keys (which are the input to the key setup
algorithm) but are much larger. In some block ciphers the subkeys also
have special interrelationships. In Blowfish the subkeys consist of a
"P-array" of 18 32-bit entries (one per encryption round plus two more)
and four "S-boxes" ("S" is for "substitution") each of which consists
of 256 32-bit entries. There is no special relationship between the
values of the subkeys.
Methods in this class allow a cipher object to be constructed from a
full set of subkeys, and for the subkeys to be extracted from a cipher
object. Normal users don't need to do either of these things. It's
mainly useful when devising a new key schedule to stick onto the
Blowfish core, or when performing cryptanalysis of the cipher
algorithm.
Generating subkeys directly by a strong random process, rather than by
expansion of a smaller random key, is an expensive and slightly bizarre
way to get greater cryptographic strength from a cipher algorithm. It
eliminates attacks on the key schedule, and yields the full strength of
the core algorithm. However, this is always a lot less strength than
the amount of subkey material, whereas a normal key schedule is
designed to yield strength equal to the length of the (much shorter)
key. Also, any non-randomness in the source of the subkey material is
likely to lead to a cryptographic weakness, whereas a key schedule
conceals any non-randomness in the choice of the key.
CLASS METHODS
Crypt::Eksblowfish::Subkeyed->blocksize
Returns 8, indicating the Eksblowfish block size of 8 octets. This
method may be called on either the class or an instance.
CONSTRUCTOR
Crypt::Eksblowfish::Subkeyed->new_from_subkeys(ROUND_KEYS, SBOXES)
Creates a new Blowfish cipher object encapsulating the supplied
subkeys. ROUND_KEYS must be a reference to an array of 18 32-bit
integers. SBOXES must be a reference to an array of four
references to 256-element arrays of 32-bit integers. These subkeys
are used in the standard order for Blowfish.
Crypt::Eksblowfish::Subkeyed->new_initial
The standard Blowfish key schedule is an iterative process, which
uses the cipher algorithm to progressively replace subkeys, thus
mutating the cipher for subsequent iterations of keying. The
Eksblowfish key schedule works similarly, but with a lot more
iterations. In both cases, the key setup algorithm begins with a
standard set of subkeys, consisting of the initial bits of the
fractional part of pi. This constructor creates and returns a
Blowfish block cipher object with that standard initial set of
subkeys. This is probably useful only to designers of novel key
schedules.
METHODS
$cipher->blocksize
Returns 8, indicating the Eksblowfish block size of 8 octets. This
method may be called on either the class or an instance.
$cipher->encrypt(PLAINTEXT)
PLAINTEXT must be exactly eight octets. The block is encrypted,
and the ciphertext is returned.
$cipher->decrypt(CIPHERTEXT)
CIPHERTEXT must be exactly eight octets. The block is decrypted,
and the plaintext is returned.
$cipher->p_array
Returns a reference to an 18-element array containing the 32-bit
round keys used in this cipher object.
$cipher->s_boxes
Returns a reference to a 4-element array containing the S-boxes
used in this cipher object. Each S-box is a 256-element array of
32-bit entries.
$cipher->is_weak
Returns a truth value indicating whether this is a weak key. A key
is considered weak if any S-box contains a pair of identical
entries (in any positions). When Blowfish is used with such an
S-box, certain cryptographic attacks are possible that are not
possible against most keys. The current (as of 2007) cryptanalytic
results on Blowfish do not include an actual break of the algorithm
when weak keys are used, but if a break is ever developed then it
is likely to be achieved for weak keys before it is achieved for
the general case.
About one key in every 2^15 is weak (if the keys are randomly
selected). Because of the complicated key schedule in standard
Blowfish it is not possible to predict which keys will be weak
without first performing the full key setup, which is why this is a
method on the keyed cipher object. In some uses of Blowfish it may
be desired to avoid weak keys; if so, check using this method and
generate a new random key when a weak key is detected. Bruce
Schneier, the designer of Blowfish, says it is probably not worth
avoiding weak keys.
SEE ALSO
Crypt::Eksblowfish, Crypt::Eksblowfish::Blowfish,
http://www.schneier.com/paper-blowfish-fse.html
<http://www.schneier.com/paper-blowfish-fse.html>
AUTHOR
Eksblowfish guts originally by Solar Designer (solar at openwall.com).
Modifications and Perl interface by Andrew Main (Zefram)
<zefram@fysh.org>.
COPYRIGHT
Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Andrew Main (Zefram)
<zefram@fysh.org>
The original Eksblowfish code (in the form of crypt()) from which this
module is derived is in the public domain. It may be found at
<http://www.openwall.com/crypt/>.
LICENSE
This module is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.14.1 2011-06-21 Crypt::Eksblowfish::Subkeyed(3)
