Cryptography is the study of secure communications techniques that allow only the sender and intended recipient of a message to view its contents. The term is derived from the Greek word kryptos, which means hidden.
Cryptographic systems are characterized along three independent dimensions:
1. The type of operations used for transforming plaintext to cipher text. All encryption algorithms are based on two general principles:
(i) Substitution: In which each element in the plaintext (bit, letter, group of bits or letters) is mapped or replaced by another element,
(ii) Transposition: In which elements in the plaintext are rearranged. The fundamental requirement is that no information be lost (that is, that all operations are reversible).
2. The number of keys used. There is two types of method that uses either:
(i) Single Key: If both sender and receiver use the same key, the system is referred to as symmetric, single-key, secret-key, or conventional encryption techniques.
(ii) Two Key: If the sender and receiver use different keys, the system is referred to as asymmetric, two-key, or public-key encryption techniques.
3. The way in which the plaintext is processed. There are two ways through which plaintext is processed as:
(i) Block Cipher: A block cipher processes the input one block of elements at a time, producing an output block for each input block.
(ii) Stream Cipher: A stream cipher processes the input elements continuously, producing output one element at a time, as it goes along.