gchq/CyberChef · error · OperationError

Value a must be defined

Error message

Value a must be defined

What it means

Thrown by the Extended GCD operation when computing the Extended Euclidean Algorithm. The operation accepts values a and b either as arguments or falls back to the input field. This specific error fires when 'Value a' is left blank as a parameter AND the input field is also empty or whitespace, while 'Value b' was provided. The operation cannot compute gcd(a,b) without a value for a.

Source

Thrown at src/core/operations/ExtendedGCD.mjs:73

    run(input, args) {
        const [aStr, bStr] = args;

        // Trim everything so "" and "   " count as empty
        const aParam = aStr?.trim();
        const bParam = bStr?.trim();
        const inputVal = input?.trim();

        let a, b;

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

        const aBI = parseBigInt(a, "Value a");
        const bBI = parseBigInt(b, "Value b");

        const [g, x, y] = egcd(aBI, bBI);
        const gcd = g < 0n ? -g : g;

        // Format output string bearing in mind that crypto-grade numbers
        // may greatly exceed the line length.

View on GitHub (pinned to 4290ea7539)

Solutions

1. Enter a numeric value in the 'Value a' argument field.
2. Alternatively, enter a numeric value in the Input field (it will be used as Value a when the argument is blank).
3. Ensure the value is a valid integer string (decimal or hex) accepted by parseBigInt.

Example fix

// before: args = ['', '48'], input = ''
// Value a blank, input blank -> error

// after: args = ['36', '48'], input = ''
// Both values supplied as arguments
// or: args = ['', '48'], input = '36'
// a taken from input

// Before calling ExtendedGCD, ensure Value a is available
const aParam = (args[0] || '').trim();
const inputVal = (input || '').trim();
if (!aParam && !inputVal) {
  // prompt user or supply default
  throw new Error('Value a must be provided via argument or input');
}

When it happens

Trigger: run(input, args) where args[0] ('Value a') is empty/whitespace, args[1] ('Value b') is non-empty, and the input string (inputVal = input?.trim()) is also empty/whitespace. Specifically the !aParam && bParam branch at line 69 where inputVal is falsy.

Common situations: User provides only Value b in the arguments panel and forgets to type a number into either Value a or the Input field. Also occurs when the input field contains only whitespace characters that get trimmed to empty.

Related errors

• Value b must be defined
• Values a and b must be defined
• Modulus must be defined
• Base must be defined
• Exponent must be defined

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/bd193ff3ca67484c. Report an issue: GitHub.