In classical cryptography, a permutation cipher is a transposition cipher in which the key is a permutation.To apply a cipher, a random permutation of size E is generated (the larger the value of E the more secure the cipher).The plaintext is then broken into segments of size E and the letters within that segment are permuted according to this key.In theory, any transposition cipher can be viewed as a permutation cipher where E is equal to the length of the plaintext; this is too cumbersome a generalisation to use in actual practice, however.
m = 26), there are a total of 286 non-trivial affine ciphers, not counting the 26 trivial Caesar ciphers.When encrypting, a person looks up each letter of the message in the 'plain' line and writes down the corresponding letter in the 'cipher' line. The encryption can also be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A = 0, B = 1,..., Z = 25.Encryption of a letter x by a shift n can be described mathematically as Plaintext: Decryption is performed similarly, (There are different definitions for the modulo operation. I.e., if x n or x-n are not in the range 0...25, we have to subtract or add 26.) Read more ...Only those letters which occur in the English alphabet are affected; numbers, symbols, whitespace, and all other characters are left unchanged.Because there are 26 letters in the English alphabet and 26 = 2 * 13, the ROT13 function is its own inverse: ROT13(ROT13(x)) = x for any basic Latin-alphabet text x An example plaintext to ciphertext using ROT13: A Polybius Square is a table that allows someone to translate letters into numbers.
To give a small level of encryption, this table can be randomized and shared with the recipient.