The Caesar Cipher is one of the simplest and most widely known encryption techniques. It is a form of substitution cipher in which each letter of the original text, known as the "plaintext", is replaced by a letter some fixed number of positions further down the alphabet. This technique is named after Julius Caesar, who used it with a left shift of 3 to protect messages of military significance.
Decryption occurs by performing a shift of the same magnitude but in the opposite direction on each letter of the encrypted text, known as the "ciphertext".
For example, with a right shift of 3: A would be replaced with D, B would be replaced with E, and so on until W is replaced with Z. Then, the beginning of the alphabet must wrap around so that X is replaced with A, Y is replaced with B, and finally, Z is replaced with C.
Encryption using a Caesar cipher can be represented using modular arithmetic by first transforming the letters of the alphabet into numbers with A = 0, B = 1, C = 2, ... , Y = 24, Z = 25.
Encryption of the letter x with a shift of n can be described as:
Decryption of the same letter x with a shift of n can then be described as:
Like all types of monoalphabetic substitution, encryptions made using a Caesar cipher can be easily broken through the frequency analysis of letters and brute force attacks.
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