gchq/CyberChef · error · OperationError

Invalid IV length: ${iv.length} bytes (expected: 0 or 8)

Error message

Invalid IV length: ${iv.length} bytes (expected: 0 or 8)

What it means

Thrown by the Rabbit cipher when an IV is supplied that is neither empty (0 bytes, meaning no IV) nor exactly 8 bytes (64 bits), the two legal IV sizes for Rabbit. The IV is decoded from the user string via Utils.convertToByteArray, so a wrong encoding or length trips this guard.

Source

Thrown at src/core/operations/Rabbit.mjs:78

    /**
     * @param {string} input
     * @param {Object[]} args
     * @returns {string}
     */
    run(input, args) {
        const key = Utils.convertToByteArray(args[0].string, args[0].option),
            iv = Utils.convertToByteArray(args[1].string, args[1].option),
            endianness = args[2],
            inputType = args[3],
            outputType = args[4];

        const littleEndian = endianness === "Little";

        if (key.length !== 16) {
            throw new OperationError(`Invalid key length: ${key.length} bytes (expected: 16)`);
        }
        if (iv.length !== 0 && iv.length !== 8) {
            throw new OperationError(`Invalid IV length: ${iv.length} bytes (expected: 0 or 8)`);
        }

        // Inner State
        const X = new Uint32Array(8), C = new Uint32Array(8);
        let b = 0;

        // Counter System
        const A = [
            0x4d34d34d, 0xd34d34d3, 0x34d34d34, 0x4d34d34d,
            0xd34d34d3, 0x34d34d34, 0x4d34d34d, 0xd34d34d3
        ];
        const counterUpdate = function() {
            for (let j = 0; j < 8; j++) {
                const temp = C[j] + A[j] + b;
                b = (temp / ((1 << 30) * 4)) >>> 0;
                C[j] = temp;
            }
        };

View on GitHub (pinned to 4290ea7539)

Solutions

  1. Supply an 8-byte IV (8 Latin1 chars, 16 hex chars, or Base64 decoding to 8 bytes), or leave it empty for no IV.
  2. Check the 'IV format' option matches the IV string encoding.
  3. Strip whitespace/newlines from the IV input before running.
  4. Confirm the IV came from the same keying material as the original encryption (length must be 8, not 16).

Example fix

// before
//   iv string: "00112233445566778899" (hex, 10 bytes) -> error
// after
//   iv string: "0011223344556677" (hex, 16 chars -> 8 bytes)
Defensive patterns

Strategy: validation

Validate before calling

const ivBytes = Utils.convertToByteArray(ivStr, ivOption);
if (ivBytes.length !== 0 && ivBytes.length !== 8) {
  throw new Error(`IV must be 0 or 8 bytes, got ${ivBytes.length}`);
}

Type guard

function isValidRabbitIV(ivBytes) { return ivBytes.length === 0 || ivBytes.length === 8; }

Try / catch

try { chef.rabbit(input, { iv: ivStr }); } catch (e) { if (/Invalid IV length/.test(e.message)) fixIV(); else throw e; }

Prevention

When it happens

Trigger: Supplying an IV of any length other than 0 or 8 bytes; selecting the wrong IV format option; passing a partial hex/Base64 IV; leaving stray whitespace that decodes to extra bytes.

Common situations: Using a 16-byte IV (common from AES-GCM contexts) by mistake; mismatching the IV format dropdown; pasting an IV that was truncated or padded.

Related errors

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/8001f53de47ce9ca. Report an issue: GitHub.