TheAlgorithms/Java · error · IllegalArgumentException

Message must be non-negative.

Error message

Message must be non-negative.

What it means

Thrown by ElGamalCipher.encrypt when the message BigInteger is negative (compareTo(ZERO) < 0). ElGamal encrypts a message in the range [0, p-1]; a negative message has no valid representation under the modulus, so it is rejected after the null check and before the upper-bound check.

Source

Thrown at src/main/java/com/thealgorithms/ciphers/ElGamalCipher.java:113

        return new KeyPair(p, g, y, x);
    }

    /**
     * Encrypts a message using the public key.
     *
     * @param message The message converted to BigInteger.
     * @param p       The prime modulus.
     * @param g       The generator.
     * @param y       The public key component.
     * @return The CipherText pair (a, b).
     * @throws IllegalArgumentException if inputs are null, negative, or message >= p.
     */
    public static CipherText encrypt(BigInteger message, BigInteger p, BigInteger g, BigInteger y) {
        if (message == null || p == null || g == null || y == null) {
            throw new IllegalArgumentException("Inputs cannot be null.");
        }
        if (message.compareTo(BigInteger.ZERO) < 0) {
            throw new IllegalArgumentException("Message must be non-negative.");
        }
        if (message.compareTo(p) >= 0) {
            throw new IllegalArgumentException("Message must be smaller than the prime modulus p.");
        }

        BigInteger k;
        BigInteger pMinus1 = p.subtract(BigInteger.ONE);

        // Select ephemeral key k such that 1 < k < p-1 and gcd(k, p-1) = 1
        do {
            k = new BigInteger(p.bitLength(), RANDOM);
        } while (k.compareTo(BigInteger.ONE) <= 0 || k.compareTo(pMinus1) >= 0 || !k.gcd(pMinus1).equals(BigInteger.ONE));

        BigInteger a = g.modPow(k, p);
        BigInteger b = y.modPow(k, p).multiply(message).mod(p);

        return new CipherText(a, b);
    }

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Ensure the message BigInteger is non-negative before encrypting.
2. Construct BigIntegers from byte arrays using the 1-arg BigInteger(byte[]) constructor only after confirming the sign, or use BigInteger(1, bytes) to force positive.
3. Reduce or reject negative values upstream.

Example fix

// before
BigInteger msg = new BigInteger(messageBytes);
CipherText ct = ElGamalCipher.encrypt(msg, p, g, y);

// after
BigInteger msg = new BigInteger(1, messageBytes); // force non-negative
if (msg.compareTo(p) >= 0) msg = msg.mod(p);
CipherText ct = ElGamalCipher.encrypt(msg, p, g, y);

if (message == null || message.signum() < 0) {
    throw new IllegalArgumentException("message must be non-negative");
}
CipherText ct = ElGamalCipher.encrypt(message, p, g, y);

static boolean isNonNegative(BigInteger m) { return m != null && m.signum() >= 0; }

try {
    CipherText ct = ElGamalCipher.encrypt(message, p, g, y);
} catch (IllegalArgumentException e) {
    // message negative; rebuild as positive using BigInteger(1, bytes)
}

When it happens

Trigger: Passing a message whose BigInteger value is negative. Common when converting a signed numeric or a hash interpreted as signed.

Common situations: Message derived from arithmetic that can go negative; interpreting a byte array's leading bit as a sign; converting a long with the high bit set via BigInteger.valueOf.

Related errors

• Message must be smaller than the prime modulus p.
• Base, secret, and prime must be non-null and positive values
• Other public value must be non-null and positive.
• Bit length must be at least {} for security.
• Inputs cannot be null.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/9582ab5443c71b7c. Report an issue: GitHub.