diff --git a/Ciphers/SubstitutionCipher.js b/Ciphers/SubstitutionCipher.js
new file mode 100644
index 0000000000..2383a99796
--- /dev/null
+++ b/Ciphers/SubstitutionCipher.js
@@ -0,0 +1,59 @@
+/**
+ * Substitution Cipher
+ *
+ * A monoalphabetic substitution cipher replaces each letter of the plaintext
+ * with another letter based on a fixed permutation (key) of the alphabet.
+ * https://en.wikipedia.org/wiki/Substitution_cipher
+ */
+
+const alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+const defaultKey = 'QWERTYUIOPASDFGHJKLZXCVBNM'
+
+/**
+ * Encrypts a string using a monoalphabetic substitution cipher
+ * @param {string} text - The text to encrypt
+ * @param {string} key - The substitution key (must be 26 uppercase letters)
+ * @returns {string}
+ */
+export function substitutionCipherEncryption(text, key = defaultKey) {
+ if (key.length !== 26 || !/^[A-Z]+$/.test(key)) {
+ throw new RangeError('Key must be 26 uppercase English letters.')
+ }
+
+ let result = ''
+ const textUpper = text.toUpperCase()
+ for (let i = 0; i < textUpper.length; i++) {
+ const char = textUpper[i]
+ const index = alphabet.indexOf(char)
+ if (index !== -1) {
+ result += key[index]
+ } else {
+ result += char
+ }
+ }
+ return result
+}
+/**
+ * Decrypts a string encrypted with the substitution cipher
+ * @param {string} text - The encrypted text
+ * @param {string} key - The substitution key used during encryption
+ * @returns {string}
+ */
+export function substitutionCipherDecryption(text, key = defaultKey) {
+ if (key.length !== 26 || !/^[A-Z]+$/.test(key)) {
+ throw new RangeError('Key must be 26 uppercase English letters.')
+ }
+
+ let result = ''
+ const textUpper = text.toUpperCase()
+ for (let i = 0; i < textUpper.length; i++) {
+ const char = textUpper[i]
+ const index = key.indexOf(char)
+ if (index !== -1) {
+ result += alphabet[index]
+ } else {
+ result += char
+ }
+ }
+ return result
+}
diff --git a/Ciphers/test/SubstitutionCipher.test.js b/Ciphers/test/SubstitutionCipher.test.js
new file mode 100644
index 0000000000..820d0303d6
--- /dev/null
+++ b/Ciphers/test/SubstitutionCipher.test.js
@@ -0,0 +1,44 @@
+import { describe, it, expect } from 'vitest'
+import {
+ substitutionCipherEncryption,
+ substitutionCipherDecryption
+} from '../SubstitutionCipher.js'
+
+describe('Substitution Cipher', () => {
+ const key = 'QWERTYUIOPASDFGHJKLZXCVBNM'
+
+ it('correctly encrypts a message', () => {
+ const encrypted = substitutionCipherEncryption('HELLO WORLD', key)
+ expect(encrypted).toBe('ITSSG VGKSR')
+ })
+
+ it('correctly decrypts a message', () => {
+ const decrypted = substitutionCipherDecryption('ITSSG VGKSR', key)
+ expect(decrypted).toBe('HELLO WORLD')
+ })
+
+ it('handles non-alphabetic characters', () => {
+ const encrypted = substitutionCipherEncryption('Test! 123', key)
+ expect(encrypted).toBe('ZTLZ! 123')
+ })
+
+ it('throws error for invalid key', () => {
+ expect(() => substitutionCipherEncryption('HELLO', 'BADKEY')).toThrow(
+ RangeError
+ )
+ })
+ it('encrypts using default key if none provided', () => {
+ const encrypted = substitutionCipherEncryption('HELLO WORLD')
+ expect(encrypted).toBe('ITSSG VGKSR')
+ })
+
+ it('decrypts using default key if none provided', () => {
+ const decrypted = substitutionCipherDecryption('ITSSG VGKSR')
+ expect(decrypted).toBe('HELLO WORLD')
+ })
+
+ it('throws error for invalid key in decryption', () => {
+ expect(() => substitutionCipherDecryption('HELLO', 'BADKEY')).toThrow(RangeError)
+ })
+
+})
diff --git a/Maths/MobiusFunction.js b/Maths/MobiusFunction.js
index bd268b8bbd..4239d6ab31 100644
--- a/Maths/MobiusFunction.js
+++ b/Maths/MobiusFunction.js
@@ -28,6 +28,6 @@ export const mobiusFunction = (number) => {
return primeFactorsArray.length !== new Set(primeFactorsArray).size
? 0
: primeFactorsArray.length % 2 === 0
- ? 1
- : -1
+ ? 1
+ : -1
}
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