feat(ciphers): add Lamport one-time signature

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

@@ -0,0 +1,145 @@
+package com.thealgorithms.ciphers;
+
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.security.SecureRandom;
+import java.util.Arrays;
+
+/**
+ * Lamport one-time signatures (OTS) are a hash-based post-quantum signature scheme.
+ *
+ * <p>The scheme is based on a private key consisting of 256 pairs of random 32-byte secrets,
+ * one pair per bit position of a SHA-256 digest. The public key contains the SHA-256 hashes of
+ * those secrets.
+ *
+ * <p>To sign a message, the algorithm hashes the message and reveals the secret corresponding to
+ * each bit value in the digest. Verification recomputes the digest and checks that each revealed
+ * secret hashes to the expected public-key value.
+ *
+ * <p>This implementation is educational and intentionally keeps dependencies minimal. Each key pair
+ * is strictly one-time: signing a second message with the same key is rejected.
+ *
+ * <p>Reference: <a href="https://en.wikipedia.org/wiki/Lamport_signature">Wikipedia: Lamport signature</a>
+ */
+public final class LamportSignature {
+
+    private static final int DIGEST_BITS = 256;
+    private static final int SECRET_BYTES = 32;
+    private static final int NUM_VALUES = 2;
+
+    private LamportSignature() {
+    }
+
+    /**
+     * A Lamport key pair containing both the private and public material.
+     */
+    public static final class KeyPair {
+        private final byte[][][] privateKey;
+        private final byte[][][] publicKey;
+        private boolean used;
+
+        private KeyPair(byte[][][] privateKey, byte[][][] publicKey) {
+            this.privateKey = privateKey;
+            this.publicKey = publicKey;
+        }
+
+        /**
+         * Signs a message using this key pair.
+         *
+         * @param message the message to sign
+         * @return the signature bytes; exactly 256 secret values ordered by digest bit positions
+         * @throws IllegalArgumentException if the message is null
+         * @throws IllegalStateException if this key pair has already been used to sign a message
+         */
+        public byte[] sign(byte[] message) {
+            if (message == null) {
+                throw new IllegalArgumentException("message must not be null");
+            }
+            if (used) {
+                throw new IllegalStateException("This Lamport key pair can only sign one message");
+            }
+            used = true;
+
+            byte[] digest = sha256(message);
+            byte[] signature = new byte[DIGEST_BITS * SECRET_BYTES];
+            for (int bit = 0; bit < DIGEST_BITS; bit++) {
+                int value = (digest[bit / 8] >> (7 - (bit % 8))) & 0x01;
+                byte[] secret = privateKey[bit][value];
+                System.arraycopy(secret, 0, signature, bit * SECRET_BYTES, SECRET_BYTES);
+            }
+            return signature;
+        }
+
+        /**
+         * Verifies a message signature against this public key.
+         *
+         * @param message the message to verify
+         * @param signature the signature to verify
+         * @return true if the signature is valid for the provided message and public key
+         * @throws IllegalArgumentException if message or signature is null or malformed
+         */
+        public boolean verify(byte[] message, byte[] signature) {
+            if (message == null) {
+                throw new IllegalArgumentException("message must not be null");
+            }
+            if (signature == null) {
+                throw new IllegalArgumentException("signature must not be null");
+            }
+            if (signature.length != DIGEST_BITS * SECRET_BYTES) {
+                throw new IllegalArgumentException("signature length must be exactly 8192 bytes");
+            }
+
+            byte[] digest = sha256(message);
+            for (int bit = 0; bit < DIGEST_BITS; bit++) {
+                int value = (digest[bit / 8] >> (7 - (bit % 8))) & 0x01;
+                byte[] revealedSecret = Arrays.copyOfRange(signature, bit * SECRET_BYTES, (bit + 1) * SECRET_BYTES);
+                byte[] expectedHash = publicKey[bit][value];
+                byte[] actualHash = sha256(revealedSecret);
+                if (!MessageDigest.isEqual(actualHash, expectedHash)) {
+                    return false;
+                }
+            }
+            return true;
+        }
+
+        public byte[][][] getPrivateKey() {
+            return privateKey;
+        }
+
+        public byte[][][] getPublicKey() {
+            return publicKey;
+        }
+    }
+
+    /**
+     * Generates a new Lamport key pair.
+     *
+     * @return a fresh Lamport key pair
+     */
+    public static KeyPair generateKeyPair() {
+        SecureRandom secureRandom = new SecureRandom();
+        byte[][][] privateKey = new byte[DIGEST_BITS][NUM_VALUES][SECRET_BYTES];
+        byte[][][] publicKey = new byte[DIGEST_BITS][NUM_VALUES][SECRET_BYTES];
+
+        for (int bit = 0; bit < DIGEST_BITS; bit++) {
+            for (int value = 0; value < NUM_VALUES; value++) {
+                secureRandom.nextBytes(privateKey[bit][value]);
+                publicKey[bit][value] = sha256(privateKey[bit][value]);
+            }
+        }
+        return new KeyPair(privateKey, publicKey);
+    }
+
+    static byte[] sha256(byte[] input) {
+        try {
+            MessageDigest digest = MessageDigest.getInstance("SHA-256");
+            return digest.digest(input);
+        } catch (NoSuchAlgorithmException e) {
+            throw new AssertionError("SHA-256 is required by the Java SE specification", e);
+        }
+    }
+
+    static byte[] sha256(byte[] message, int offset, int length) {
+        return sha256(Arrays.copyOfRange(message, offset, offset + length));
+    }
+}

src/test/java/com/thealgorithms/ciphers/LamportSignatureTest.java

@@ -0,0 +1,101 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.nio.charset.StandardCharsets;
+import java.util.Arrays;
+import org.junit.jupiter.api.Test;
+
+class LamportSignatureTest {
+
+    @Test
+    void testValidSignatureVerifies() {
+        LamportSignature.KeyPair keyPair = LamportSignature.generateKeyPair();
+        byte[] message = "hello lamport".getBytes(StandardCharsets.UTF_8);
+
+        byte[] signature = keyPair.sign(message);
+
+        assertTrue(keyPair.verify(message, signature));
+    }
+
+    @Test
+    void testTamperedMessageFailsVerification() {
+        LamportSignature.KeyPair keyPair = LamportSignature.generateKeyPair();
+        byte[] message = "hello lamport".getBytes(StandardCharsets.UTF_8);
+        byte[] signature = keyPair.sign(message);
+
+        assertFalse(keyPair.verify("hello lamport!".getBytes(StandardCharsets.UTF_8), signature));
+    }
+
+    @Test
+    void testTamperedSignatureFailsVerification() {
+        LamportSignature.KeyPair keyPair = LamportSignature.generateKeyPair();
+        byte[] message = "hello lamport".getBytes(StandardCharsets.UTF_8);
+        byte[] signature = keyPair.sign(message);
+
+        signature[0] = (byte) (signature[0] ^ 0xFF);
+
+        assertFalse(keyPair.verify(message, signature));
+    }
+
+    @Test
+    void testDifferentKeyPairDoesNotVerify() {
+        LamportSignature.KeyPair keyPair1 = LamportSignature.generateKeyPair();
+        LamportSignature.KeyPair keyPair2 = LamportSignature.generateKeyPair();
+        byte[] message = "post-quantum".getBytes(StandardCharsets.UTF_8);
+
+        byte[] signature = keyPair1.sign(message);
+
+        assertFalse(keyPair2.verify(message, signature));
+    }
+
+    @Test
+    void testSecondSignatureThrowsException() {
+        LamportSignature.KeyPair keyPair = LamportSignature.generateKeyPair();
+        byte[] first = "first message".getBytes(StandardCharsets.UTF_8);
+        byte[] second = "second message".getBytes(StandardCharsets.UTF_8);
+
+        keyPair.sign(first);
+
+        assertThrows(IllegalStateException.class, () -> keyPair.sign(second));
+    }
+
+    @Test
+    void testNullAndMalformedInput() {
+        LamportSignature.KeyPair keyPair = LamportSignature.generateKeyPair();
+        byte[] validMessage = "message".getBytes(StandardCharsets.UTF_8);
+        byte[] validSignature = keyPair.sign(validMessage);
+
+        assertThrows(IllegalArgumentException.class, () -> keyPair.sign(null));
+        assertThrows(IllegalArgumentException.class, () -> keyPair.verify(null, validSignature));
+        assertThrows(IllegalArgumentException.class, () -> keyPair.verify(validMessage, null));
+        assertThrows(IllegalArgumentException.class, () -> keyPair.verify(validMessage, new byte[1]));
+    }
+
+    @Test
+    void testPublicKeyMatchesPrivateKeyHash() {
+        LamportSignature.KeyPair keyPair = LamportSignature.generateKeyPair();
+        byte[][][] publicKey = keyPair.getPublicKey();
+        byte[][][] privateKey = keyPair.getPrivateKey();
+
+        for (int bit = 0; bit < 256; bit++) {
+            for (int value = 0; value < 2; value++) {
+                byte[] hash = LamportSignature.sha256(privateKey[bit][value]);
+                assertArrayEquals(hash, publicKey[bit][value]);
+            }
+        }
+    }
+
+    @Test
+    void testSha256SliceHelperMatchesRangeHash() {
+        byte[] message = "hello lamport".getBytes(StandardCharsets.UTF_8);
+
+        byte[] expected = LamportSignature.sha256(Arrays.copyOfRange(message, 2, 12));
+        byte[] actual = LamportSignature.sha256(message, 2, 10);
+
+        assertArrayEquals(expected, actual);
+    }
+}