Cryptography folklore has it that simple substitution ciphers are trivial to break by looking at the letter frequencies of the encrypted text. - Verifying the Substitution Cipher Folklore

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Caesar cipher

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.

ROT13

ROT13 is a simple letter substitution cipher that replaces a letter with the 13th letter after it in the Latin alphabet.

Because there are 26 letters in the Latin alphabet and 26 = 2 × 13, the ROT13 function is its own inverse: