gchq/CyberChef · warning · 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 GeneratePrime when the requested bit length exceeds 4096. The cap exists for performance reasons — Miller-Rabin probable-prime testing on very large numbers is expensive in the browser/Node environment.

Source

Thrown at src/core/operations/GeneratePrime.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. Set bits to 4096 or less.
2. For larger primes, use a dedicated native crypto tool or OpenSSL outside CyberChef.

Example fix

// before
args = [8192, true, "Hexadecimal"];
// after
args = [4096, true, "Hexadecimal"];

if (bits > 4096) {
  // cap or reject; explain the 4096-bit performance limit
}

function isWithinPrimeBitCap(n) {
  return Number.isInteger(n) && n >= 2 && n <= 4096;
}

When it happens

Trigger: Passing bits > 4096 (e.g. 8192) expecting RSA-key-sized primes.

Common situations: User accustomed to 8192-bit RSA trying to generate equally large primes here.

Related errors

• Bit length must be at least 2
• Length must be less than ${maxLoremWords}
• Length must be less than ${maxLoremCharacters}
• Bit length limited to 4096 bits for performance reasons
• Invalid block cipher mode: ${mode}

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/84c8cb6c8f06d3fd. Report an issue: GitHub.