gchq/CyberChef · error · OperationError

Exponent must be defined

Error message

Exponent must be defined

What it means

Thrown by the Modular Exponentiation operation in 'Case 3': the Base argument is supplied, the Exponent argument is blank, and the Input field is also blank — so there is no value to use as the exponent. The operation resolves a missing Exponent from the Input; if both are empty it throws at ModularExponentiation.mjs:90.

Solutions

1. Provide an Exponent value either in the Exponent argument or in the Input field.
2. If using Input as the exponent, ensure the upstream step emits a value.
3. Validate that at least one of Exponent-arg / Input is non-empty when Base is set.

Example fix

// before — exponent missing both as arg and input
modExp.run('', ['2', '15', '']);   // exp blank, input blank

// after — supply exponent via input
modExp.run('6', ['2', '15', '']);  // 2^6 mod 15

// Case 3: Exponent blank, Base set -> Exponent must come from Input.
function resolveExpCase3(expArg, input) {
  const exp = (expArg ?? '').trim() || (input ?? '').trim();
  if (!exp) throw new Error('Exponent must be defined (fill Exponent arg or Input)');
  return exp;
}

function hasExpForCase3(expArg: unknown, input: unknown): boolean {
  const e = (typeof expArg === 'string' ? expArg.trim() : '') || (typeof input === 'string' ? input.trim() : '');
  return e.length > 0;
}

When it happens

Trigger: args: Base=<set>, Exponent='', and input='' (or whitespace). The code path at ModularExponentiation.mjs:85-91 takes exp from inputVal, then rejects an empty exponent. Triggered when the user supplies only a base and forgets the input.

Common situations: User filled the Base field, left Exponent empty intending to use Input, but also left Input empty; upstream recipe step emitted nothing; whitespace-only input.

Related errors

• Base and Exponent must be defined
• Base must be defined
• Modulus must be defined
• A minimum of one reflector must be supplied
• A minimum of three rotors must be supplied

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/dea20db4d7e9ddb7. Report an issue: GitHub.

Appendix: source

Thrown at src/core/operations/ModularExponentiation.mjs:90

        // if their boxes are empty.
        let base, exp;
        if (baseParam && expParam) {
            // Case 1: base and exponent both given as parameters
            base = baseParam;
            exp  = expParam;
        } else if (!baseParam && expParam) {
            // Case 2: base missing - take from input
            base = inputVal;
            exp  = expParam;
            if (!base) {
                throw new OperationError("Base must be defined");
            }
        } else if (baseParam && !expParam) {
            // Case 3: exponent missing - take from input
            base = baseParam;
            exp  = inputVal;
            if (!exp) {
                throw new OperationError("Exponent must be defined");
            }
        } else if (!inputVal) {
            // Case 4: base and exponent both missing
            throw new OperationError("Base and Exponent must be defined");
        } else throw new OperationError("Ambiguous input: specify either Base or Exponent when using Input");

        // Parse numbers
        const baseBI = parseBigInt(base, "Base");
        const expBI  = parseBigInt(exp, "Exponent");
        const modBI  = parseBigInt(mod, "Modulus");

        // Check for invalid modulus (parseBigInt eliminates negatives)
        if (modBI === 0n) {
            throw new OperationError("Modulus must be greater than zero");
        }

        return modPow(baseBI, expBI, modBI).toString();
    }

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