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In cryptography, a substitution cipher is a method of encrypting in which units of plaintext are replaced with the ciphertext, in a defined manner, with the help of a key; the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth.
The main classical cipher types are transposition ciphers, which rearrange the order of letters in a message (e.g., 'hello world' becomes 'ehlol owrdl' in a trivially simple rearrangement scheme), and substitution ciphers, which systematically replace letters or groups of letters with other letters or groups of letters (e.g., 'fly at once ...
In cryptography, an S-box (substitution-box) is a basic component of symmetric key algorithms which performs substitution. In block ciphers, they are typically used to obscure the relationship between the key and the ciphertext, thus ensuring Shannon's property of confusion. Mathematically, an S-box is a nonlinear [1] vectorial Boolean function ...
In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code, or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet .
Although ciphers can be confusion-only (substitution cipher, one-time pad) or diffusion-only (transposition cipher), any "reasonable" block cipher uses both confusion and diffusion. [2] These concepts are also important in the design of cryptographic hash functions , and pseudorandom number generators , where decorrelation of the generated ...
This substitution should be one-to-one, to ensure invertibility (hence decryption). In particular, the length of the output should be the same as the length of the input (the picture on the right has S-boxes with 4 input and 4 output bits), which is different from S-boxes in general that could also change the length, as in Data Encryption ...
Mathematically, a cryptosystem or encryption scheme can be defined as a tuple (,,,,) with the following properties.. is a set called the "plaintext space". Its elements are called plaintexts.; is a set called the "ciphertext space". Its elements are called ciphertexts.; is a set called the "key space". Its elements are called keys.; = {:} is a set of functions :. Its elements are called ...
Lucifer uses a combination of transposition and substitution crypting as a starting point in decoding ciphers. [clarification needed] One variant, described by Feistel in 1971, [2] uses a 48-bit key and operates on 48-bit blocks. The cipher is a substitution–permutation network and uses two 4-bit S-boxes. The key selects which S-boxes are used.