gchq/CyberChef · error · OperationError

Bit length limited to 4096 bits for performance reasons

Error message

Bit length limited to 4096 bits for performance reasons

What it means

Thrown by RandomPrime when the requested bit length exceeds 4096. This is an artificial cap to prevent performance problems: Miller-Rabin testing and big-integer arithmetic on >4096-bit numbers are slow in the browser/worker.

Source

Thrown at src/core/operations/RandomPrime.mjs:127

                value: ["Decimal", "Hexadecimal"]
            }
        ];
    }

    /**
     * @param {string} input
     * @param {Object[]} args
     * @returns {string}
     */
    run(input, args) {
        const [bits, cryptoGrade, outputFormat] = args;

        if (bits < 2) {
            throw new OperationError("Bit length must be at least 2");
        }

        if (bits > 4096) {
            throw new OperationError("Bit length limited to 4096 bits for performance reasons");
        }

        const rounds = cryptoGrade ? 40 : 7;
        let attempts = 0;
        const maxAttempts = 10000;

        let n = randBigInt(bits);

        while (!isProbablePrime(n, rounds)) {
            n = randBigInt(bits);
            attempts++;

            if (attempts > maxAttempts) {
                throw new OperationError(`Failed to generate prime after ${maxAttempts} attempts. Try a different bit length.`);
            }
        }

        // Return only the prime for pipeability

View on GitHub (pinned to 4290ea7539)

Solutions

1. Limit the request to <= 4096 bits.
2. If you truly need a larger prime, generate it externally with a native crypto library.
3. Reconsider whether such a large prime is necessary for your use case.

Example fix

// before
//   bits: 8192
// after
//   bits: 4096

if (bits > 4096) throw new Error('Bit length capped at 4096');

const withinCap = b => Number.isInteger(b) && b <= 4096;

try { randomPrime(bits); } catch (e) { if (/4096/.test(e.message)) bits = 4096; else throw e; }

When it happens

Trigger: Requesting 8192-bit or larger primes; copy-pasting an RSA-8192 key size into the bit field.

Common situations: Assuming modern RSA sizes (8192) are supported; wanting extra security margin; scripted requests with large bit counts.

Related errors

• Bit length limited to 4096 bits for performance reasons
• Bit length must be at least 2
• Letter ${letter} is not included in LS47
• Unknown padding type: ${padding}
• Invalid key length: ${key.length} bytes The following algor

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/edce1101a235d057. Report an issue: GitHub.