Create ChaCha20.java

Author: Keykyrios

src/main/java/com/thealgorithms/ciphers/ChaCha20.java

@@ -0,0 +1,201 @@
+package com.thealgorithms.ciphers;
+
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+import java.util.Arrays;
+
+/**
+ * Implements the ChaCha20 stream cipher algorithm as specified in RFC 8439.
+ * ChaCha20 is a refinement of the Salsa20 algorithm and is known for its
+ * speed and security on modern CPUs.
+ *
+ * <p>Wikipedia: https://en.wikipedia.org/wiki/ChaCha20
+ * <p>RFC 8439: https://tools.ietf.org/html/rfc8439
+ *
+ * @author Mitrajit Ghorui(KeyKyrios)
+ */
+public final class ChaCha20 {
+
+    private ChaCha20() {
+    } // Static class
+
+    private static final int KEY_SIZE_BYTES = 32; // 256 bits
+    private static final int NONCE_SIZE_BYTES = 12; // 96 bits
+    private static final int BLOCK_SIZE_BYTES = 64; // 512 bits
+
+    // ChaCha20 constants "expand 32-byte k"
+    private static final int[] CONSTANTS = {0x61707865, 0x3320646e, 0x79622d32, 0x6b206574};
+
+    /**
+     * Encrypts the given plaintext using ChaCha20.
+     * Since ChaCha20 is a stream cipher, encryption and decryption are the same
+     * operation (XOR with keystream).
+     *
+     * @param key       The 256-bit (32-byte) secret key.
+     * @param nonce     The 96-bit (12-byte) nonce. Must be unique for each
+     * encryption with the same key.
+     * @param plaintext The data to encrypt.
+     * @return The resulting ciphertext.
+     * @throws IllegalArgumentException if the key or nonce has an invalid length,
+     * or if any input is null.
+     */
+    public static byte[] encrypt(final byte[] key, final byte[] nonce, final byte[] plaintext) {
+        validateInputs(key, nonce, plaintext);
+        return process(key, nonce, plaintext, 1); // Start with block counter 1 as per RFC 8439
+    }
+
+    /**
+     * Decrypts the given ciphertext using ChaCha20.
+     * Since ChaCha20 is a stream cipher, encryption and decryption are the same
+     * operation (XOR with keystream).
+     *
+     * @param key        The 256-bit (32-byte) secret key.
+     * @param nonce      The 96-bit (12-byte) nonce used during encryption.
+     * @param ciphertext The data to decrypt.
+     * @return The resulting plaintext.
+     * @throws IllegalArgumentException if the key or nonce has an invalid length,
+     * or if any input is null.
+     */
+    public static byte[] decrypt(final byte[] key, final byte[] nonce, final byte[] ciphertext) {
+        validateInputs(key, nonce, ciphertext);
+        return process(key, nonce, ciphertext, 1); // Start with block counter 1
+    }
+
+    /**
+     * Performs the core ChaCha20 processing (XOR with keystream).
+     *
+     * @param key        The 32-byte key.
+     * @param nonce      The 12-byte nonce.
+     * @param data       Plaintext or Ciphertext.
+     * @param counter    The initial block counter.
+     * @return The result of XORing data with the generated keystream.
+     */
+    private static byte[] process(final byte[] key, final byte[] nonce, final byte[] data, final int counter) {
+        byte[] output = new byte[data.length];
+        ByteBuffer keyStreamBlock = ByteBuffer.allocate(BLOCK_SIZE_BYTES).order(ByteOrder.LITTLE_ENDIAN);
+        int offset = 0;
+        int blockCounter = counter;
+
+        while (offset < data.length) {
+            keyStreamBlock.clear();
+            generateChaCha20Block(key, nonce, blockCounter++, keyStreamBlock.array());
+
+            int length = Math.min(BLOCK_SIZE_BYTES, data.length - offset);
+            for (int i = 0; i < length; i++) {
+                output[offset + i] = (byte) (data[offset + i] ^ keyStreamBlock.get(i));
+            }
+            offset += length;
+        }
+        return output;
+    }
+
+    /**
+     * Generates a 64-byte ChaCha20 keystream block.
+     *
+     * @param key       The 32-byte key.
+     * @param nonce     The 12-byte nonce.
+     * @param counter   The block counter.
+     * @param output    The 64-byte array to store the generated block.
+     */
+    private static void generateChaCha20Block(final byte[] key, final byte[] nonce, final int counter, final byte[] output) {
+        int[] state = initializeState(key, nonce, counter);
+        int[] workingState = Arrays.copyOf(state, state.length);
+
+        // 20 rounds (10 double rounds)
+        for (int i = 0; i < 10; i++) {
+            // Column rounds
+            quarterRound(workingState, 0, 4, 8, 12);
+            quarterRound(workingState, 1, 5, 9, 13);
+            quarterRound(workingState, 2, 6, 10, 14);
+            quarterRound(workingState, 3, 7, 11, 15);
+            // Diagonal rounds
+            quarterRound(workingState, 0, 5, 10, 15);
+            quarterRound(workingState, 1, 6, 11, 12);
+            quarterRound(workingState, 2, 7, 8, 13);
+            quarterRound(workingState, 3, 4, 9, 14);
+        }
+
+        // Add initial state to the final state
+        for (int i = 0; i < state.length; i++) {
+            workingState[i] += state[i];
+        }
+
+        // Serialize state to output bytes (Little Endian)
+        ByteBuffer buffer = ByteBuffer.wrap(output).order(ByteOrder.LITTLE_ENDIAN);
+        for (int val : workingState) {
+            buffer.putInt(val);
+        }
+    }
+
+    /**
+     * Initializes the 16-word (512-bit) ChaCha20 state.
+     */
+    private static int[] initializeState(final byte[] key, final byte[] nonce, final int counter) {
+        int[] state = new int[16];
+        ByteBuffer keyBuffer = ByteBuffer.wrap(key).order(ByteOrder.LITTLE_ENDIAN);
+        ByteBuffer nonceBuffer = ByteBuffer.wrap(nonce).order(ByteOrder.LITTLE_ENDIAN);
+
+        // Constants
+        state[0] = CONSTANTS[0];
+        state[1] = CONSTANTS[1];
+        state[2] = CONSTANTS[2];
+        state[3] = CONSTANTS[3];
+
+        // Key (8 words)
+        for (int i = 0; i < 8; i++) {
+            state[4 + i] = keyBuffer.getInt(i * 4);
+        }
+
+        // Counter (1 word)
+        state[12] = counter;
+
+        // Nonce (3 words)
+        for (int i = 0; i < 3; i++) {
+            state[13 + i] = nonceBuffer.getInt(i * 4);
+        }
+
+        return state;
+    }
+
+    /**
+     * The ChaCha20 quarter round function. Modifies the state array in place.
+     */
+    private static void quarterRound(final int[] state, final int a, final int b, final int c, final int d) {
+        state[a] += state[b];
+        state[d] = rotl(state[d] ^ state[a], 16);
+        state[c] += state[d];
+        state[b] = rotl(state[b] ^ state[c], 12);
+        state[a] += state[b];
+        state[d] = rotl(state[d] ^ state[a], 8);
+        state[c] += state[d];
+        state[b] = rotl(state[b] ^ state[c], 7);
+    }
+
+    /**
+     * Rotates the bits of an integer to the left.
+     */
+    private static int rotl(final int value, final int shift) {
+        return (value << shift) | (value >>> (32 - shift));
+    }
+
+    /**
+     * Validates key, nonce, and data inputs.
+     */
+    private static void validateInputs(final byte[] key, final byte[] nonce, final byte[] data) {
+        if (key == null) {
+            throw new IllegalArgumentException("Key cannot be null.");
+        }
+        if (key.length != KEY_SIZE_BYTES) {
+            throw new IllegalArgumentException("Invalid key size. Key must be " + KEY_SIZE_BYTES + " bytes (256 bits).");
+        }
+        if (nonce == null) {
+            throw new IllegalArgumentException("Nonce cannot be null.");
+        }
+        if (nonce.length != NONCE_SIZE_BYTES) {
+            throw new IllegalArgumentException("Invalid nonce size. Nonce must be " + NONCE_SIZE_BYTES + " bytes (96 bits).");
+        }
+        if (data == null) {
+            throw new IllegalArgumentException("Plaintext/Ciphertext cannot be null.");
+        }
+    }
+}