gchq/CyberChef · error · OperationError

Modulus must be defined

Error message

Modulus must be defined

What it means

Thrown by the Modular Exponentiation operation when the 'Modulus' argument (args[1]) is empty or whitespace-only. run() trims modStr and rejects a falsy result at ModularExponentiation.mjs:68 before base/exponent resolution. The default modulus is '1', so this only fires when the field is explicitly cleared or set to blanks.

Source

Thrown at src/core/operations/ModularExponentiation.mjs:68

    }

    /**
      * @param {string} input
      * @param {Object[]} args
      * @returns {string}
      */
    run(input, args) {
        const [baseStr, modStr, expStr] = args;

        // Trim everything so "" and "   " count as empty
        const baseParam = baseStr?.trim();
        const expParam  = expStr?.trim();
        const modParam  = modStr?.trim();
        const inputVal  = input?.trim();

        const mod = modParam;
        if (!mod) {
            throw new OperationError("Modulus must be defined");
        }

        // Base *or* Exponent (but not both) are taken from the Input
        // 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

View on GitHub (pinned to 4290ea7539)

Solutions

1. Provide a non-empty, positive integer Modulus (e.g. the default '1', or your RSA/DH modulus).
2. When building recipes programmatically, default modulus to '1' if blank.
3. Validate modStr.trim() is non-empty before running.

Example fix

// before
modExp.run('', ['', '', '']);   // modulus blank -> throws

// after
modExp.run('2', ['', '15', '']);  // modulus 15 -> computes 2^input mod 15

function definedModulus(m) {
  const v = String(m ?? '').trim();
  if (!v) throw new Error('Modulus must be defined');
  return v;
}
const mod = definedModulus(args[1]);

function isDefinedModulus(v: unknown): v is string {
  return typeof v === 'string' && v.trim().length > 0;
}

When it happens

Trigger: Invoking Modular Exponentiation with args[1] ('Modulus') set to '', ' ', or omitted. Raised at ModularExponentiation.mjs:67-69. A later check (line 103) separately rejects a modulus that parses to 0.

Common situations: User cleared the Modulus field expecting it to be optional; a recipe JSON with a missing/blank modulus; copy/paste that left only whitespace.

Related errors

• Base must be defined
• Exponent must be defined
• Base and Exponent must be defined
• Letter ${letter} is not included in LS47
• Unknown padding type: ${padding}

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/1d34f3ba6c3cdaca. Report an issue: GitHub.