PSEUDORANDOM BINARY SEQUENCE GENERATION FOR STREAM CIPHERS

Abstract

Pseudorandom binary sequences find their application in diverse fields but security and cryptography is probably the best known field of their application. One-Time Pad (OTP) is a simple, fast and the most secure encryption algorithm. It provides the perfect secrecy. The encryptiondecryption process of the OTP is based on exclusive-or function computed on the plaintext/ciphertext and the key bits. The requirements for the OTP key are that: it must be a cryptographically strong truly random or pseudorandom binary sequence; must be as long as plaintext size; and must not be reused. The difference between a truly random and a pseudorandom sequence is that the truly random sequence is generated with the help of nondeterministic physical phenomenon but the pseudorandom sequence is generated from some deterministic mechanism and a seed value. In case of pseudorandom binary sequences, given the same seed the pseudorandom number generator will always output the same sequence of numbers or bits. The fundamental difficulty with a truly random sequence is its generation and distribution. Therefore pseudorandom sequences are a popular choice for the practical implementation of the OTP scheme. Many researchers have devoted their time and effort to the family of shift register based pseudorandom sequence generators. But they could not gain a key sequence having very large period equal to the plaintext length. They also tried the complex versions of shift registers but it is yet not very useful and secure