Add tests for A5Cipher.java, improve class & function documentation (#5594)

src/main/java/com/thealgorithms/ciphers/a5/A5Cipher.java

@@ -2,17 +2,43 @@ package com.thealgorithms.ciphers.a5;
 
 import java.util.BitSet;
 
-// https://en.wikipedia.org/wiki/A5/1
+/**
+ * The A5Cipher class implements the A5/1 stream cipher, which is a widely used
+ * encryption algorithm, particularly in mobile communications.
+ *
+ * This implementation uses a key stream generator to produce a stream of bits
+ * that are XORed with the plaintext bits to produce the ciphertext.
+ *
+ * <p>
+ * For more details about the A5/1 algorithm, refer to
+ * <a href="https://en.wikipedia.org/wiki/A5/1">Wikipedia</a>.
+ * </p>
+ */
 public class A5Cipher {
 
     private final A5KeyStreamGenerator keyStreamGenerator;
-    private static final int KEY_STREAM_LENGTH = 228; // 28.5 bytes so we need to pad bytes or something
+    private static final int KEY_STREAM_LENGTH = 228; // Length of the key stream in bits (28.5 bytes)
 
+    /**
+     * Constructs an A5Cipher instance with the specified session key and frame counter.
+     *
+     * @param sessionKey a BitSet representing the session key used for encryption.
+     * @param frameCounter a BitSet representing the frame counter that helps in key stream generation.
+     */
     public A5Cipher(BitSet sessionKey, BitSet frameCounter) {
         keyStreamGenerator = new A5KeyStreamGenerator();
         keyStreamGenerator.initialize(sessionKey, frameCounter);
     }
 
+    /**
+     * Encrypts the given plaintext bits using the A5/1 cipher algorithm.
+     *
+     * This method generates a key stream and XORs it with the provided plaintext
+     * bits to produce the ciphertext.
+     *
+     * @param plainTextBits a BitSet representing the plaintext bits to be encrypted.
+     * @return a BitSet containing the encrypted ciphertext bits.
+     */
     public BitSet encrypt(BitSet plainTextBits) {
         // create a copy
         var result = new BitSet(KEY_STREAM_LENGTH);
@@ -24,6 +50,13 @@ public class A5Cipher {
         return result;
     }
 
+    /**
+     * Resets the internal counter of the key stream generator.
+     *
+     * This method can be called to re-initialize the state of the key stream
+     * generator, allowing for new key streams to be generated for subsequent
+     * encryptions.
+     */
     public void resetCounter() {
         keyStreamGenerator.reInitialize();
     }