Stream Ciphers and Block Ciphers

A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. Examples of classical stream ciphers are the autokeyed Vigenère cipher and the Vernam cipher. In the ideal case, a one-time pad version of the Vernam cipher would be used in which the keystream (k) is as long as the plaintext bit stream (p.). If the cryptographic keystream is random, then this cipher is unbreakable by any means other than acquiring the keystream. However, the keystream must be provided to both users in advance via some free and secure channel. This introduces insurmountable logistical problems if the intended data traffic is very large.

Accordingly, for practical reasons, the bit-stream generator must be applied as an algorithmic procedure, so that the cryptographic bit stream can be produced by both users. In this approach (Figure la), the bit-stream gen- erator is a key-controlled algorithm and must produce a bit stream that is cryp- tographically strong. That is, it must be computationally impractical to predict future portions of the bit stream based on previous portions of the bit stream. The two users need only share the generating key, and each can produce the keystream.

A block cipher is one in which a block of plaintext is treated as a whole and len used to produce a ciphertext block of equal length. Typically, 64 or 128 bits is used for a block size. As with a stream cipher, the two users share a symmetric encryp- tion key (Figure 1b). Far more effort has gone into analyzing block ciphers. In general, they seem applicable to a broader range of applications than stream ciphers. The wide majority of network-based symmetric cryptographic applications make use of block ciphers. Accordingly, the concern in this chapter, will primarily focus on block ciphers.