L8 Stream cipher systems

3.4 STREAM CIPHER:
Recall the discussion in Sections 3.1, 3.2 and 3.3 concerning the One-Time-Pad system, the development of stream cipher systems was greatly influenced by the fact that Shanoon had proved the OTP to be unconditionally secure. Stream cipher systems are developed to emulate the OTP system in the sense that they produced long sequence of displacement which were applied character by character to the plaintext message as illustrated in figure 3-1. However, the fundamental difference is that; the sequences 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 posses the following characteristics:
Figure (3-2)Stream cipher System.
Mixer
Key stream
Plaintext
Ciphertext
Key Generator
Figure (3-3) Stream Cipher encryption and decryption.
Ki Ki
Mi
Ci
Mi
Lec 8 : Data Security Stream Cipher Systems
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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.
In the following sections, fundamental background and techniques of stream cipher systems is discussed. A survey of previous stream ciphers cryptanalysis also discussed.
3.5 KEYSTREAM GENERATOR:
The basic element in stream ciphers is the key stream generator, which will generate the key stream (sequence) to be combined with the plaintext stream and produce the cipher text.
3.5.1 LINEAR SHIFT REGISTERES:
A feed back shift register is an implementation of the key stream generator. It is made up of two parts; a shift register and a feedback function. The shift register is a sequence of bits. Each time a bit is needed, all the bits in the register are shifted 1 bit to the right. The new left-most bit is computed as a function of the other bits in the register. The output of the shift register is one bit. The simplest kind of feedback shift register is a linear feedback shift register (LFSR). The feedback function is simply the XOR function. Figure 3-4 shows a 4-bit LFSR. If it is initialized with the value (1111), it produces the following output sequence