Cryptography Reference
In-Depth Information
Keystream
generator
Key
Keystream
K
1
,
K
2
,…,
K
n
P
1
⊕
K
1
,
P
2
⊕
K
2
,…,
P
n
⊕
K
n
⊕
Ciphertext bits
Plaintext bits
P
1
,
P
2
,…,
P
n
Figure 4.2.
Model of a simple stream cipher
this simple stream cipher and the Vernam Cipher is that, rather than using a
sequence of randomly generated key bits that is as long as the plaintext, a stream
cipher:
1. Uses a key that is normally much shorter than the length of the plaintext
(a typical key length would be 128 bits).
2. Converts this (short) key into a continuous stream of bits known as the
keystream
bymeans of a
keystreamgenerator
. The keystreamgenerator, which
is the main 'engine' of the stream cipher, converts the short key into continuous
keystream by means of a carefully designed mathematical process.
In order to decrypt the resulting ciphertext, the receiver needs to be in
possession of:
• the same short key; and
• the same keystream generator.
The keystream generator is part of the stream cipher specification. If two parties
agree on which stream cipher to use then they are, by default, agreeing on which
keystream generator to use. In fact, technically, the encryption of the plaintext
in this simple stream cipher is just the XOR of the plaintext with the keystream,
and decryption is just the XOR of the ciphertext with the keystream. Thus the
real work in designing a good stream cipher goes into designing the keystream
generator. For this reason, when we refer to a particular
stream cipher
we tend to
really be referring to the design of a particular keystream generator. Note that:
• Keystream generators are examples of
deterministic generators
(see
Section 8.1.4). Keystream generators produce output that appears to be
randomly generated, but is actually not randomly generated. We often term
this
pseudorandom
(literally, 'fake random').
• The Vernam Cipher can be thought of as a stream cipher whose keystream is
truly randomly generated (see Section 8.1.3).
