Added Atbash Cipher! (#136)

Ciphers/AtbashCipher.php

@@ -0,0 +1,39 @@
+<?php
+
+/**
+ * Encrypt a message using the Atbash Cipher.
+ * The Atbash Cipher is a simple substitution cipher where each letter in the plaintext is
+ * replaced with its corresponding letter from the end of the alphabet (reverse alphabet).
+ * Non-alphabet characters are not modified.
+ *
+ * @param string $plainText The plaintext to encrypt.
+ * @return string The encrypted message.
+ */
+function atbash_encrypt($plainText)
+{
+    $result = '';
+    $plainText = strtoupper($plainText);
+    for ($i = 0; $i < strlen($plainText); $i++) {
+        $char = $plainText[$i];
+        if (ctype_alpha($char)) {
+            $offset = ord('Z') - ord($char);
+            $encryptedChar = chr(ord('A') + $offset);
+        } else {
+            $encryptedChar = $char; // Non-alphabet characters remain unchanged
+        }
+        $result .= $encryptedChar;
+    }
+    return $result;
+}
+
+/**
+ * Decrypt a message encrypted using the Atbash Cipher.
+ * Since the Atbash Cipher is its own inverse, decryption is the same as encryption.
+ *
+ * @param string $cipherText The ciphertext to decrypt.
+ * @return string The decrypted message.
+ */
+function atbash_decrypt($cipherText)
+{
+    return atbash_encrypt($cipherText); // Decryption is the same as encryption
+}

DIRECTORY.md

@@ -1,6 +1,7 @@
 # List of all files
 
 ## Ciphers
+  * [Atbashcipher](./Ciphers/AtbashCipher.php)
   * [Caesarcipher](./Ciphers/CaesarCipher.php)
   * [Monoalphabeticcipher](./Ciphers/MonoAlphabeticCipher.php)
   * [Morsecode](./Ciphers/MorseCode.php)
@@ -88,6 +89,7 @@
 
 ## Tests
   * Ciphers
+    * [Atbashciphertest](./tests/Ciphers/AtbashCipherTest.php)
     * [Cipherstest](./tests/Ciphers/CiphersTest.php)
     * [Monoalphabeticciphertest](./tests/Ciphers/MonoAlphabeticCipherTest.php)
     * [Morsecodetest](./tests/Ciphers/MorseCodeTest.php)

tests/Ciphers/AtbashCipherTest.php

@@ -0,0 +1,25 @@
+<?php
+
+use PHPUnit\Framework\TestCase;
+
+require_once __DIR__ . '/../../vendor/autoload.php';
+require_once __DIR__ . '/../../Ciphers/AtbashCipher.php';
+
+class AtbashCipherTest extends TestCase
+{
+    public function testEncryptionAndDecryption()
+    {
+        $plainText = "HELLO";
+        $encryptedText = atbash_encrypt($plainText);
+        $decryptedText = atbash_decrypt($encryptedText);
+        $this->assertEquals($plainText, $decryptedText);
+    }
+
+    public function testWithNonAlphabetCharacters()
+    {
+        $plainText = "HELLO, WORLD!";
+        $encryptedText = atbash_encrypt($plainText);
+        $decryptedText = atbash_decrypt($encryptedText);
+        $this->assertEquals($plainText, $decryptedText);
+    }
+}