What causes "Key length must match input length" in TheAlgorithms/Java?

Generate the key with exactly the plaintext length: generateKey(plaintext.length).

TheAlgorithms/Java · error · IllegalArgumentException

Key length must match input length

Error message

Key length must match input length

What it means

Thrown by OneTimePadCipher.validateInputs when input.length != key.length. A one-time pad XORs each plaintext/ciphertext byte with a corresponding key byte, so the key must be exactly the same length as the data. Applies to both encrypt and decrypt, which both route through validateInputs.

Solutions

Generate the key with exactly the plaintext length: generateKey(plaintext.length).
Pre-check key.length == data.length before encrypt/decrypt.
Do not reuse or truncate one-time-pad keys; regenerate per message.

Example fix

// before
byte[] key = OneTimePadCipher.generateKey(16);
byte[] ct = OneTimePadCipher.encrypt(plaintext, key); // fails if plaintext.length != 16

// after
byte[] key = OneTimePadCipher.generateKey(plaintext.length);
byte[] ct = OneTimePadCipher.encrypt(plaintext, key);

Defensive patterns

Strategy: validation

Validate before calling

if (key.length != data.length) {
    throw new IllegalArgumentException("key length (" + key.length + ") must equal data length (" + data.length + ")");
}
byte[] out = OneTimePadCipher.encrypt(data, key);

Type guard

static boolean lengthsMatch(byte[] data, byte[] key) {
    return data != null && key != null && data.length == key.length;
}

Try / catch

try {
    byte[] ct = OneTimePadCipher.encrypt(data, key);
} catch (IllegalArgumentException e) {
    // length mismatch; regenerate key at the right length
    byte[] k = OneTimePadCipher.generateKey(data.length);
    ct = OneTimePadCipher.encrypt(data, k);
}

Prevention

Generate the key at plaintext length: generateKey(plaintext.length).
Never reuse or truncate one-time-pad keys.
Pre-check length equality before encrypt/decrypt.

When it happens

Trigger: Calling encrypt or decrypt with a key whose byte length differs from the data byte length. Also note null inputs throw NullPointerException (via Objects.requireNonNull) before this check.

Common situations: Reusing or truncating a key; deriving key length from a different source than the message; generating a key for the wrong message size; multi-block messages with a single-block key.

Related errors

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/321d686ab8132573. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/com/thealgorithms/ciphers/OneTimePadCipher.java:78

     * <p>For a One-Time Pad, decryption is identical to encryption:
     * {@code plaintext = ciphertext XOR key}.
     *
     * @param ciphertext the ciphertext bytes, must not be {@code null}
     * @param key the one-time pad key bytes, must not be {@code null}
     * @return the decrypted plaintext bytes
     * @throws IllegalArgumentException if the key length does not match ciphertext length
     * @throws NullPointerException if ciphertext or key is {@code null}
     */
    public static byte[] decrypt(byte[] ciphertext, byte[] key) {
        validateInputs(ciphertext, key);
        return xor(ciphertext, key);
    }

    private static void validateInputs(byte[] input, byte[] key) {
        Objects.requireNonNull(input, "input must not be null");
        Objects.requireNonNull(key, "key must not be null");
        if (input.length != key.length) {
            throw new IllegalArgumentException("Key length must match input length");
        }
    }

    private static byte[] xor(byte[] data, byte[] key) {
        byte[] result = new byte[data.length];
        for (int i = 0; i < data.length; i++) {
            result[i] = (byte) (data[i] ^ key[i]);
        }
        return result;
    }
}

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