ABSTRACT
The concept of perfect secrecy was proposed by Claude Shannon in 1949. A cryptosystem is said to be perfectly secure if the attacker cannot get any information about the plaintext by observing only the ciphertext. A probability model consists of three components: sample space, event, and a probability function or experiment. Birthday paradox is one of the good examples to visualize the application of probability theory in cryptography. It is also useful to check security of a cryptosystem under collision attack also called birthday attack. The probability distribution of ciphertext space depends on the probability distribution of plaintext and key space. A pseudo-random number generator is an algorithm that produces a long sequence of bits from a given short sequence of random bits. The probability theory is applicable in cryptography usually to estimate “how probable it is that an insecure event may occur under certain conditions”.
