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<H1><A NAME="SECTION00040000000000000000">
Crypto.Cipher: Encryption Algorithms</A>
</H1>
<P>
Encryption algorithms transform their input data, or plaintext,
in some way that is dependent on a variable key, producing
ciphertext. This transformation can easily be reversed, if (and,
hopefully, only if) one knows the key. The key can be varied by the
user or application and chosen from some very large space of possible
keys.
<P>
For a secure encryption algorithm, it should be very difficult to
determine the original plaintext without knowing the key; usually, no
clever attacks on the algorithm are known, so the only way of breaking
the algorithm is to try all possible keys. Since the number of possible
keys is usually of the order of 2 to the power of 56 or 128, this is not
a serious threat, although 2 to the power of 56 is now considered
insecure in the face of custom-built parallel computers and distributed
key guessing efforts.
<P>
Block ciphers take multibyte inputs of a fixed size
(frequently 8 or 16 bytes long) and encrypt them. Block ciphers can
be operated in various modes. The simplest is Electronic Code Book
(or ECB) mode. In this mode, each block of plaintext is simply
encrypted to produce the ciphertext. This mode can be dangerous,
because many files will contain patterns greater than the block size;
for example, the comments in a C program may contain long strings of
asterisks intended to form a box. All these identical blocks will
encrypt to identical ciphertext; an adversary may be able to use this
structure to obtain some information about the text.
<P>
To eliminate this weakness, there are various feedback modes in which
the plaintext is combined with the previous ciphertext before
encrypting; this eliminates any repetitive structure in the
ciphertext.
<P>
One mode is Cipher Block Chaining (CBC mode); another is Cipher
FeedBack (CFB mode). CBC mode still encrypts in blocks, and thus is
only slightly slower than ECB mode. CFB mode encrypts on a
byte-by-byte basis, and is much slower than either of the other two
modes. The chaining feedback modes require an initialization value to
start off the encryption; this is a string of the same length as the
ciphering algorithm's block size, and is passed to the new()
function. There is also a special PGP mode, which is an oddball
variant of CFB used by the PGP program. While you can use it in
non-PGP programs, it's quite non-standard.
<P>
The currently available block ciphers are listed in the following table,
and are in the Crypto.Cipher package:
<P>
<BR>
8#8
<BR>
<P>
In a strict formal sense, stream ciphers encrypt data bit-by-bit;
practically, stream ciphers work on a character-by-character basis.
Stream ciphers use exactly the
same interface as block ciphers, with a block length that will always
be 1; this is how block and stream ciphers can be distinguished.
The only feedback mode available for stream ciphers is ECB mode.
<P>
The currently available stream ciphers are listed in the following table:
<P>
<BR>
9#9
<BR>
<P>
ARC4 is short for `Alleged RC4'. In September of 1994, someone posted
C code to both the Cypherpunks mailing list and to the Usenet
newsgroup sci.crypt, claiming that it implemented the RC4
algorithm. This claim turned out to be correct. Note that there's a
damaging class of weak RC4 keys; this module won't warn you about such keys.
<P>
A similar anonymous posting was made for Alleged RC2 in January, 1996.
<P>
An example usage of the DES module:
<PRE>
>>> from Crypto.Cipher import DES
>>> obj=DES.new('abcdefgh', DES.MODE_ECB)
>>> plain="Guido van Rossum is a space alien."
>>> len(plain)
34
>>> obj.encrypt(plain)
Traceback (innermost last):
File "<stdin>", line 1, in ?
ValueError: Strings for DES must be a multiple of 8 in length
>>> ciph=obj.encrypt(plain+'XXXXXX')
>>> ciph
'\021,\343Nq\214DY\337T\342pA\372\255\311s\210\363,\300j\330\250\312\347\342I\3215w\03561\303dgb/\006'
>>> obj.decrypt(ciph)
'Guido van Rossum is a space alien.XXXXXX'
</PRE>
<P>
All cipher algorithms share a common interface. After importing a
given module, there is exactly one function and two variables
available.
<P>
<BR>
10#10
<BR>
<P>
<BR>
11#11
<BR>
<P>
<BR>
12#12
<BR>
<P>
All cipher objects have at least three attributes:
<P>
<BR>
13#13
<BR>
<P>
<BR>
14#14
<BR>
<P>
<BR>
15#15
<BR>
<P>
All ciphering objects have the following methods:
<P>
<BR>
16#16
<BR>
<P>
<BR>
17#17
<BR>
<P>
<BR><HR>
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<LI><A NAME="tex2html154"
HREF="node9.html">Algorithm-specific Notes for Encryption Algorithms</A>
<LI><A NAME="tex2html155"
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<LI><A NAME="tex2html156"
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