Introduction to Cryptography with Coding Theory, 3rd Edition

Chapter 5 Stream Ciphers

The one-time pad provides a strong form of secrecy, but since key transmission is difficult, it is desirable to devise substitutes that are easier to use. Stream ciphers are one way of achieving this goal. As in the one-time pad, the plaintext is written as a string of bits. Then a binary keystream is generated and XORed with the plaintext to produce the ciphertext.

Figure 5.1 Stream cipher encryption

$p_n=$ plaintext bit, $x_n=$ key bit, $c_n=$ ciphertext bit.

Figure 5.1 Full Alternative Text

For the system to be secure, the keystream needs to approximate a random sequence, so we need a good source of random-looking bits. In ...