Caesar Cipher Algorithm

In cryptography, a 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. 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.

Caesar Cipher Algorithm

Example

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):

  1. Plain:    ABCDEFGHIJKLMNOPQRSTUVWXYZ
  2. 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.

  1. Plaintext:  THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
  2. Ciphertext: QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD

Complexity

  • Time: O(|n|)
  • Space: O(|n|)

References