Caesar Cipher Algorithm
Algorithm · Stretch
Complexity
Time Complexity
O(|n|)
Space Complexity
O(|n|)
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.
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):
Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher: XYZABCDEFGHIJKLMNOPQRSTUVWWhen 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 ALDCode
// Create alphabet array: ['a', 'b', 'c', ..., 'z'].
const englishAlphabet = 'abcdefghijklmnopqrstuvwxyz'.split('');
/**
* Generates a cipher map out of the alphabet.
* Example with a shift 3: {'a': 'd', 'b': 'e', 'c': 'f', ...}
*
* @param {string[]} alphabet - i.e. ['a', 'b', 'c', ... , 'z']
* @param {number} shift - i.e. 3
* @return {Object} - i.e. {'a': 'd', 'b': 'e', 'c': 'f', ..., 'z': 'c'}
*/
const getCipherMap = (alphabet, shift) => {
return alphabet
.reduce((charsMap, currentChar, charIndex) => {
const charsMapClone = { ...charsMap };
// Making the shift to be cyclic (i.e. with a shift of 1 - 'z' would be mapped to 'a').
let encryptedCharIndex = (charIndex + shift) % alphabet.length;
// Support negative shifts for creating a map for decryption
// (i.e. with shift -1 - 'a' would be mapped to 'z').
if (encryptedCharIndex < 0) {
encryptedCharIndex += alphabet.length;
}
charsMapClone[currentChar] = alphabet[encryptedCharIndex];
return charsMapClone;
}, {});
};
/**
* @param {string} str
* @param {number} shift
* @param {string[]} alphabet
* @return {string}
*/
export const caesarCipherEncrypt = (str, shift, alphabet = englishAlphabet) => {
// Create a cipher map:
const cipherMap = getCipherMap(alphabet, shift);
return str
.toLowerCase()
.split('')
.map((char) => cipherMap[char] || char)
.join('');
};
/**
* @param {string} str
* @param {number} shift
* @param {string[]} alphabet
* @return {string}
*/
export const caesarCipherDecrypt = (str, shift, alphabet = englishAlphabet) => {
// Create a cipher map:
const cipherMap = getCipherMap(alphabet, -shift);
return str
.toLowerCase()
.split('')
.map((char) => cipherMap[char] || char)
.join('');
};References
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