feat: Add XOR Cipher (TheAlgorithms#961)

* Added XOR Cipher

* Remoced repetition of c ^ key in encrypt

* Added explicit type conversion

* Applied Suggested Changes

* Added bullet points

Author: imdeep2905

ciphers/xor_cipher.cpp

@@ -0,0 +1,99 @@
+/**
+ * @file xor_cipher.cpp
+ * @brief Implementation of [XOR cipher](https://en.wikipedia.org/wiki/XOR_cipher) algorithm.
+ *
+ * @details
+ * In cryptography, the simple XOR cipher is a type of additive cipher, an encryption 
+ * algorithm that operates according to the principles: 
+ *
+ * * \f$A {\oplus} 0 = A\f$
+ * * \f$A {\oplus} A = 0\f$
+ * * \f$ (A {\oplus} B) {\oplus} C = A {\oplus} (B {\oplus} C)\f$
+ * * \f$ (B {\oplus} A) {\oplus} B = B {\oplus} 0 = B \f$
+ * 
+ * 
+ * where \f$\oplus\f$ symbol denotes the exclusive disjunction (XOR) operation.
+ * This operation is sometimes called modulus 2 addition (or subtraction, which is identical).
+ * With this logic, a string of text can be encrypted by applying the bitwise XOR operator to
+ * every character using a given key. To decrypt the output, merely reapplying the XOR function 
+ * with the key will remove the cipher.
+ * 
+ * ### Algorithm
+ * Choose the key for encryption and apply XOR operation to each character of a string. 
+ * Reapplying XOR operation to each character of encrypted string will give original string back.
+ * 
+ * \note This program implements XOR Cipher for string with ASCII characters. 
+ * 
+ * @author [Deep Raval](https://github.com/imdeep2905)
+ */
+#include <iostream>
+#include <string>
+#include <cassert>
+
+/** \namespace ciphers
+ * \brief Algorithms for encryption and decryption
+ */
+namespace ciphers {
+    /** \namespace XOR
+     * \brief Functions for [XOR cipher](https://en.wikipedia.org/wiki/XOR_cipher) algorithm.
+     */
+    namespace XOR {   
+        /**
+         * Encrypt given text using XOR cipher.
+         * @param text text to be encrypted
+         * @param key to be used for encyption
+         * @return new encrypted text
+         */
+        std::string encrypt (const std::string &text, const int &key) {
+            std::string encrypted_text = ""; // Empty string to store encrypted text
+            for (auto &c: text) { // Going through each character
+                char encrypted_char = char(c ^ key); // Applying encyption
+                encrypted_text += encrypted_char; // Appending encrypted character
+            }
+            return encrypted_text; // Returning encrypted text
+        }
+        /**
+         * Decrypt given text using XOR cipher.
+         * @param text text to be encrypted
+         * @param key to be used for decryption
+         * @return new decrypted text
+         */        
+        std::string decrypt (const std::string &text, const int &key) {
+            std::string decrypted_text = ""; // Empty string to store decrypted text
+            for (auto &c : text) { // Going through each character
+                char decrypted_char = char(c ^ key); // Applying decryption
+                decrypted_text += decrypted_char; // Appending decrypted character
+            }
+            return decrypted_text; // Returning decrypted text
+        }
+    } // namespace XOR
+} // namespace ciphers
+
+/**
+ * Function to test above algorithm
+ */
+void test() {
+    // Test 1
+    std::string text1 = "Whipalsh! : Do watch this movie...";
+    std::string encrypted1 = ciphers::XOR::encrypt(text1, 17);
+    std::string decrypted1 = ciphers::XOR::decrypt(encrypted1, 17);
+    assert(text1 == decrypted1);
+    std::cout << "Original text : " << text1;
+    std::cout << " , Encrypted text (with key = 17) : " << encrypted1;
+    std::cout << " , Decrypted text : "<< decrypted1 << std::endl;
+    // Test 2
+    std::string text2 = "->Valar M0rghulis<-";
+    std::string encrypted2 = ciphers::XOR::encrypt(text2, 29);
+    std::string decrypted2 = ciphers::XOR::decrypt(encrypted2, 29);
+    assert(text2 == decrypted2);
+    std::cout << "Original text : " << text2;
+    std::cout << " , Encrypted text (with key = 29) : " << encrypted2;
+    std::cout << " , Decrypted text : "<< decrypted2 << std::endl;
+}
+
+/** Driver Code */
+int main() {
+    // Testing
+    test();
+    return 0;
+}