gchq/CyberChef · error · OperationError

Modulus (m) must be defined

Error message

Modulus (m) must be defined

What it means

Thrown by Modular Inverse in Case 3: the Modulus (m) argument is blank, so the operation expects to read 'm' from Input, but Input is also empty/whitespace. A modulus is required to define the ring in which the inverse is computed, so the operation aborts.

Source

Thrown at src/core/operations/ModularInverse.mjs:77

        const mParam = mStr?.trim();
        const inputVal = input?.trim();

        let a, m;

        if (aParam && mParam) {
            // Case 1: value and modulus both given as parameters
            a = aParam;
            m = mParam;
        } else if (!aParam && mParam) {
            // Case 2: value missing - take from input
            a = inputVal;
            m = mParam;
            if (!a) throw new OperationError("Value (a) must be defined");
        } else if (aParam && !mParam) {
            // Case 3: modulus missing - take from input
            a = aParam;
            m = inputVal;
            if (!m) throw new OperationError("Modulus (m) must be defined");
        } else if (!aParam && !mParam) {
            // Case 4: value and modulus both missing
            throw new OperationError("Value (a) and Modulus (m) must be defined");
        }

        const aBI = parseBigInt(a, "Value (a)");
        const mBI = parseBigInt(m, "Modulus (m)");

        if (mBI <= 0n) {
            throw new OperationError("Modulus must be greater than zero");
        }

        const aNorm = ((aBI % mBI) + mBI) % mBI;
        const [g, x] = egcd(aNorm, mBI);

        if (g !== 1n && g !== -1n) {
            throw new OperationError("Inverse does not exist because gcd(a, m) ≠ 1");
        }

View on GitHub (pinned to 4290ea7539)

Solutions

1. Provide a Modulus (m) argument, or place the modulus in the Input field when Modulus is blank.
2. Make sure the Input string contains an actual number, not whitespace.
3. When scripting, fill exactly one of {Value, Modulus} from Input and the other as an argument.

Example fix

// before: Value set, Modulus empty, Input empty
chef.bake("Modular Inverse", ["3", ""], "");
// after: modulus from Input
chef.bake("Modular Inverse", ["3", ""], "7");

const mParam = (args[1] ?? "").trim();
const inputVal = (input ?? "").trim();
if (!mParam && !inputVal) {
  // supply a modulus before calling Modular Inverse
}

const hasModulusSource = (mStr, input) =>
  Boolean((mStr ?? "").trim()) || Boolean((input ?? "").trim());

try { chef.bake("Modular Inverse", [a, ""], input); }
catch (e) { if (e.message.startsWith("Modulus (m) must be defined")) supplyM(); else throw e; }

When it happens

Trigger: run(input, args) with args[0] (Value) populated, args[1] (Modulus) empty/whitespace, and input empty/whitespace-only.

Common situations: User supplies the value as a parameter but forgets that the modulus must then come from Input; recipe where the modulus stream was renamed or disconnected; whitespace-only input.

Related errors

• Modulus must be greater than zero
• Value (a) must be defined
• Value (a) and Modulus (m) must be defined
• Modulus must be greater than zero
• Invalid block cipher mode: ${mode}

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/0dbabc580d8b0cd7. Report an issue: GitHub.