Serious Cryptography

5STREAM CIPHERS

Symmetric ciphers can be either block ciphers or stream ciphers. Recall from Chapter 4 that block ciphers mix chunks of plaintext bits together with key bits to produce chunks of ciphertext of the same size, usually 64 or 128 bits. Stream ciphers, on the other hand, don’t mix plaintext and key bits; instead, they generate pseudorandom bits from the key and encrypt the plaintext by XORing it with the pseudorandom bits, in the same fashion as the one-time pad explained in Chapter 1.

Stream ciphers are sometimes shunned because historically they’ve been more fragile than block ciphers and are more often broken—both the experimental ...