Lec 9 Computer and Data Security

3.3 Stream ciphers
Stream cipher systems are developed to emulate the OTP system in the sense that they produced long sequence of displacement which was applied character by character to the plaintext message as illustrated in figure (3.1). However, the fundamental difference is that; the sequence generated by the stream system is not random, and it is completely determined by a key.
Thus, a stream cipher is a system in which the key is fed to an algorithm, which uses the key to generate finite sequence. The algorithm is usually referred to as the sequence generator or key stream generator. Figure (3.2) illustrates stream cipher system. Stream Ciphers lies between simple XOR and One-Time Pad. Encryption and decryption processes are shown in figure (3.3).
K M=C
C K=M K K=M
K 0 =k and K K K =0















Stream ciphers should process the following characteristics:
1. Easy to implement.
2. High speed in generating key stream.
3. Computationally secure.
3.1. It should be protected against known-plaintext attack.
3.2. Having a large key space.
3.3. The period of the key sequence should be greater than the message length.
3.4. Key sequence has random characteristics.