STREAM CIPHER

A stream cipher also breaks the plaintext into units, this time it is normally a single character. It then encrypts the n^th unit of the plaintext with the n^th unit of the key stream. Stream ciphers can be designed to be exceptionally fast, much faster than any block cipher. While the encryption of any particular plaintext with a block cipher will result in the same ciphertext when the same key is used; with a stream cipher, the transformation of the smaller plaintext units will vary, depending on when they are encountered during the encryption process.

A stream cipher generates what is known as a keystream - a sequence of bits, which is used as a key. The encryption process involves combining the keystream with the plaintext.

The keystream can be generated in two ways:

1. Independent of the plaintext and ciphertext (this yields what is known as a synchronous stream cipher).
2. Depending on the data and its encryption (in which case the stream cipher is said to be self-synchronizing).

The majority of stream cipher designs are for synchronous stream ciphers.

Interest in stream ciphers is currently attributed to the appealing properties of the one-time pad. A one-time pad, which is sometimes called the Vernam cipher, uses a keystream which is the same length as the plaintext message and consists of a series of bits generated completely at random. Theoretically this should produce ciphertext which is the most secure possible, because since the keystream is random even a cryptanalyst with infinite computational resources can still only guess at the underlying plaintext. While the one-time pad has occasionally seen use in wartime for ultra secret transmissions the fact that the key is as long as the message introduces severe practical problems and so, while theoretically perfectly secure, the one-time pad is generally impractical. Stream ciphers were developed as an approximation to the one-time pad.

At this time there is no de facto standard for stream ciphers although the most widely used stream cipher is RC4, a stream cipher designed by Rivest for RSA Data Security Inc. It is a variable key-size stream cipher with an algorithm based on the use of a random permutation.

Strangely, certain modes of operation of a block cipher transform it into a keystream generator and so, in this way, any block cipher can be used as a stream cipher. Stream ciphers with a dedicated design and typically much faster, however.

One method for generating a keystream is a Linear Feedback Shift Register (LFSR). This is a mechanism for generating a sequence of binary bits. LFSRs are easy to implement and fast operating in both hardware and software however a single LFSR is not secure because over the years a mathematical framework has been developed which allows for the analysis of their output. This problem can be solved by using a shift register cascade, a set of LFSRs connected together so that the behaviour of one of them depends on another. The detailed operation of LFSRs and shift register cascades is beyond the scope of this essay.