Cryptography Reference
In-Depth Information
cryptanalysis is rather complicated. Astonishingly, there is a free program to
break this code. But let's look at these things in turn.
The
pkzip
tool widely used in the PC world groups several files into a single
one — an archive. The files are compressed using a suitable method, and the entire
archive can be encrypted. We are interested in the latter feature, of course. Despite
it being complicated, this section is important, since you will come to understand
at least roughly how today cryptanalysts can disentangle complex things.
The
pkzip
Cipher
The
pkzip
encryption is a stream cipher. Similarly to RC4, it creates a key-
dependent byte sequence which is XORed with the ciphertext or the plaintext. In
pkzip
, the key-dependent byte sequence depends additionally on the plaintext
previously encrypted. For this reason, and because twelve partially random
bytes, which serve as an initialization vector, among other things, are set prior
to the encryption, an insertion attack won't work.
The method uses three secret 32-bit words,
key0, key1
, and
key2
, and a secret
byte,
K
. Byte
K
is XORed with the plaintext for encryption, or ciphertext
for decryption. Subsequently, the method computes new values for
key0, key1,
key2
, and
K
, depending on the plaintext. This actualization procedure looks
like this:
C=P
⊕
K
key0 = crc32(key0,P)
(a)
key1 = (key1 + (key0 & 0xff)) * 134775813 + 1
(b)
key2 = crc32(key2, key1 >> 24)
(c)
tmp
= key2 | 3
(d)
= (tmp * (tmp
⊕
1)) >> 8
K
(e)
where
P
is a plaintext byte,
C
is a ciphertext byte, and
tmp
is a 16-bit number.
Similarly to the C programming language,
A
n
denotes the right shift of
A
by
n
bytes, i.e.,
key1
24 is the most significant byte of
key1
, and
K
is the
most significant byte of
tmp*
(
tmp
⊕
1). The operation
key0 & 0xff
creates the
least significant byte of
key0
, and the expression
key2
|
3
just means that the
two least significant bits of
key2
are set equal to 1. Finally,
crc32()
is an easily
computable CRC polynomial:
crc32(key,c) = (key >> 8)
⊕
crctab[(key&0xff)
⊕
c]
(5)
