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Cryptography


Cryptographic systems are characterized in three independent dimensions:
  1. The type of operations used for transforming plaintext to ciphertext. All encryption algorithms are based on two general principles: substitution, in which each element in the plaintext (bit, letter, group of bits or letters) is mapped into another element, and transposition, in which elements in the plaintext are realign. The fundamental requirement is that no information be lost (i.e., that all operations are reversible). Maximum systems, referred to as product systems, involve multiple stages of substitutions and transpositions.
  2. The number of keys used. If sender and receiver use the same key, the system is referred to as symmetric, single-key, secret-key, or conventional encryption. If the sender and receiver use different keys, the system is referred to as asymmetric, two-key, or public-key encryption.
  3. The way in which the plaintext is processed. A block cipher processes the input one block of elements at a time, producing an output block for each input block. A stream cipher processes the input elements continuously, producing output one element at a time, as it goes along.
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