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In this decryption example, the ciphertext that will be decrypted is the ciphertext from the encryption example. The corresponding decryption function is D(y) = 21(y − b) mod 26, where a −1 is calculated to be 21, and b is 8. To begin, write the numeric equivalents to each letter in the ciphertext, as shown in the table below.
The inverse S-box is simply the S-box run in reverse. For example, the inverse S-box of b8 16 is 9a 16. It is calculated by first calculating the inverse affine transformation of the input value, followed by the multiplicative inverse. The inverse affine transformation is as follows:
In cryptography, linear cryptanalysis is a general form of cryptanalysis based on finding affine approximations to the action of a cipher. Attacks have been developed for block ciphers and stream ciphers. Linear cryptanalysis is one of the two most widely used attacks on block ciphers; the other being differential cryptanalysis.
The ciphertext is sent through an insecure channel to the recipient. The recipient decrypts the ciphertext by applying an inverse decryption algorithm, recovering the plaintext. To decrypt the ciphertext, the recipient requires a secret knowledge from the sender, usually a string of letters, numbers, or bits, called a cryptographic key. The ...
To avoid attacks based on simple algebraic properties, the S-box is constructed by combining the inverse function with an invertible affine transformation. The S-box is also chosen to avoid any fixed points (and so is a derangement ), i.e., S ( a i , j ) ≠ a i , j {\displaystyle S(a_{i,j})\neq a_{i,j}} , and also any opposite fixed points, i ...
Some words will, when transformed with ROT13, produce another word. Examples of 7-letter pairs in the English language are abjurer and nowhere, and Chechen and purpura. Other examples of words like these are shown in the table. [12] The pair gnat and tang is an example of words that are both ROT13 reciprocals and reversals.
Mathematically, an S-box is a nonlinear [1] vectorial Boolean function. [2] In general, an S-box takes some number of input bits, m, and transforms them into some number of output bits, n, where n is not necessarily equal to m. [3] An m×n S-box can be implemented as a lookup table with 2 m words of n bits each.
Under the standard affine convention, an alphabet of m letters is mapped to the numbers 0, 1, ... , m − 1. (The Hebrew alphabet has m = 22, and the standard Latin alphabet has m = 26). The Atbash cipher may then be enciphered and deciphered using the encryption function for an affine cipher by setting a = b = (m − 1):