In cryptography, a Caesar cipher, also known as shift cipher, 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. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence.

The action of a Caesar cipher is to replace each plaintext letter with a different one a fixed number of places down the alphabet.

The cipher illustrated above uses a left shift of three, so that each occurrence of E in the plaintext becomes B in the ciphertext.

The transformation can be represented by aligning two alphabets; the cipher alphabet is the plain alphabet rotated left or right by some number of positions. For instance, here is a Caesar cipher using a left rotation of three places, equivalent to a right shift of 23 (the shift parameter is used as the key):

`Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ`

Cipher: XYZABCDEFGHIJKLMNOPQRSTUVW

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.

`Plaintext: THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG`

Ciphertext: QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD

Deciphering is done in reverse, with a right shift of 3.

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.[2] Encryption of a letter x by a shift n can be described mathematically as,

Decryption is performed similarly,

The Caesar cipher is named after Julius Caesar, who used it, more than 2000 years ago, to protect messages of military significance.

*Vercingetorix throws down his arms at the feet of Julius Caesar.* Painting by Lionel Royer – Musée CROZATIER du Puy-en-Velay – France, Public Domain.

#### Encrypted Message

Use a Python program to decode the following secret message:

`N qtaj uwtlwfrrnsl zxnsl Udymts!`

`Key: right shift by 5 characters`