1Crypt::Eksblowfish::SubUkseeyredC(o3n)tributed Perl DocuCmreynptta:t:iEoknsblowfish::Subkeyed(3)
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6 Crypt::Eksblowfish::Subkeyed - Blowfish/Eksblowfish with access to
7 subkeys
8
10 use Crypt::Eksblowfish::Subkeyed;
11 $block_size = Crypt::Eksblowfish::Subkeyed->blocksize;
13 $cipher = Crypt::Eksblowfish::Subkeyed
15 ->new_from_subkeys(\@p_array, \@s_boxes);
16 $cipher = Crypt::Eksblowfish::Subkeyed->new_initial;
17 $block_size = $cipher->blocksize;
19 $ciphertext = $cipher->encrypt($plaintext);
20 $plaintext = $cipher->decrypt($ciphertext);
21 $p_array = $cipher->p_array;
23 $s_boxes = $cipher->s_boxes;
24 if($cipher->is_weak) { ...
25
27 An object of this class encapsulates a keyed instance of the Blowfish
28 or Eksblowfish block cipher, ready to encrypt and decrypt. Normally
29 this class will not be used directly, but through subclasses such as
30 Crypt::Eksblowfish.
31 Eksblowfish is a variant of the Blowfish cipher with a modified key
33 setup algorithm. This class doesn't implement either form of key
34 setup, but only provides the actual encryption and decryption parts of
35 the ciphers. This part is shared between Blowfish and Eksblowfish, and
36 also any other cipher that uses the core of Blowfish but supplies its
37 own key setup. This class has "Eksblowfish" in its name rather than
38 "Blowfish" merely due to the historical accident that it is derived
39 from the encryption engine that was used to implement Eksblowfish.
40 The key setup phase of a block cipher, also known as the "key
42 schedule", produces a set of "subkeys", which are somewhat like
43 ordinary cryptographic keys (which are the input to the key setup
44 algorithm) but are much larger. In some block ciphers the subkeys also
45 have special interrelationships. In Blowfish the subkeys consist of a
46 "P-array" of 18 32-bit entries (one per encryption round plus two more)
47 and four "S-boxes" ("S" is for "substitution") each of which consists
48 of 256 32-bit entries. There is no special relationship between the
49 values of the subkeys.
50 Methods in this class allow a cipher object to be constructed from a
52 full set of subkeys, and for the subkeys to be extracted from a cipher
53 object. Normal users don't need to do either of these things. It's
54 mainly useful when devising a new key schedule to stick onto the
55 Blowfish core, or when performing cryptanalysis of the cipher
56 algorithm.
57 Generating subkeys directly by a strong random process, rather than by
59 expansion of a smaller random key, is an expensive and slightly bizarre
60 way to get greater cryptographic strength from a cipher algorithm. It
61 eliminates attacks on the key schedule, and yields the full strength of
62 the core algorithm. However, this is always a lot less strength than
63 the amount of subkey material, whereas a normal key schedule is
64 designed to yield strength equal to the length of the (much shorter)
65 key. Also, any non-randomness in the source of the subkey material is
66 likely to lead to a cryptographic weakness, whereas a key schedule
67 conceals any non-randomness in the choice of the key.
68
70 Crypt::Eksblowfish::Subkeyed->blocksize
71 Returns 8, indicating the Eksblowfish block size of 8 octets. This
72 method may be called on either the class or an instance.
73
75 Crypt::Eksblowfish::Subkeyed->new_from_subkeys(ROUND_KEYS, SBOXES)
76 Creates a new Blowfish cipher object encapsulating the supplied
77 subkeys. ROUND_KEYS must be a reference to an array of 18 32-bit
78 integers. SBOXES must be a reference to an array of four
79 references to 256-element arrays of 32-bit integers. These subkeys
80 are used in the standard order for Blowfish.
81 Crypt::Eksblowfish::Subkeyed->new_initial
83 The standard Blowfish key schedule is an iterative process, which
84 uses the cipher algorithm to progressively replace subkeys, thus
85 mutating the cipher for subsequent iterations of keying. The
86 Eksblowfish key schedule works similarly, but with a lot more
87 iterations. In both cases, the key setup algorithm begins with a
88 standard set of subkeys, consisting of the initial bits of the
89 fractional part of pi. This constructor creates and returns a
90 Blowfish block cipher object with that standard initial set of
91 subkeys. This is probably useful only to designers of novel key
92 schedules.
93
95 $cipher->blocksize
96 Returns 8, indicating the Eksblowfish block size of 8 octets. This
97 method may be called on either the class or an instance.
98 $cipher->encrypt(PLAINTEXT)
100 PLAINTEXT must be exactly eight octets. The block is encrypted,
101 and the ciphertext is returned.
102 $cipher->decrypt(CIPHERTEXT)
104 CIPHERTEXT must be exactly eight octets. The block is decrypted,
105 and the plaintext is returned.
106 $cipher->p_array
108 Returns a reference to an 18-element array containing the 32-bit
109 round keys used in this cipher object.
110 $cipher->s_boxes
112 Returns a reference to a 4-element array containing the S-boxes
113 used in this cipher object. Each S-box is a 256-element array of
114 32-bit entries.
115 $cipher->is_weak
117 Returns a truth value indicating whether this is a weak key. A key
118 is considered weak if any S-box contains a pair of identical
119 entries (in any positions). When Blowfish is used with such an
120 S-box, certain cryptographic attacks are possible that are not
121 possible against most keys. The current (as of 2007) cryptanalytic
122 results on Blowfish do not include an actual break of the algorithm
123 when weak keys are used, but if a break is ever developed then it
124 is likely to be achieved for weak keys before it is achieved for
125 the general case.
126 About one key in every 2^15 is weak (if the keys are randomly
128 selected). Because of the complicated key schedule in standard
129 Blowfish it is not possible to predict which keys will be weak
130 without first performing the full key setup, which is why this is a
131 method on the keyed cipher object. In some uses of Blowfish it may
132 be desired to avoid weak keys; if so, check using this method and
133 generate a new random key when a weak key is detected. Bruce
134 Schneier, the designer of Blowfish, says it is probably not worth
135 avoiding weak keys.
136
138 Crypt::Eksblowfish, Crypt::Eksblowfish::Blowfish,
139 <http://www.schneier.com/paper-blowfish-fse.html>
140
142 Eksblowfish guts originally by Solar Designer (solar at openwall.com).
143 Modifications and Perl interface by Andrew Main (Zefram)
145 <zefram@fysh.org>.
146
148 Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Andrew Main (Zefram)
149 <zefram@fysh.org>
150 The original Eksblowfish code (in the form of crypt()) from which this
152 module is derived is in the public domain. It may be found at
153 <http://www.openwall.com/crypt/>.
154
156 This module is free software; you can redistribute it and/or modify it
157 under the same terms as Perl itself.
158perl v5.34.0 2022-01-21 Crypt::Eksblowfish::Subkeyed(3)