gchq/CyberChef · error · OperationError

Invalid key length: ${key.length} bytes. Ascon-AEAD128 requ

Error message

Invalid key length: ${key.length} bytes.

Ascon-AEAD128 requires a key of exactly 16 bytes (128 bits).

What it means

Thrown by AsconDecrypt.run when the supplied key is not exactly 16 bytes (128 bits). Ascon-AEAD128 (the NIST-LWC standardized variant implemented here) fixes the key size at 128 bits, so the operation hard-validates key.length === 16 before invoking JsAscon.decrypt. The key is converted via Utils.convertToByteArray using the chosen input option (Hex, Base64, UTF8, etc.), so the length is measured in decoded bytes, not input characters.

Source

Thrown at src/core/operations/AsconDecrypt.mjs:76

            }
        ];
    }

    /**
     * @param {string} input
     * @param {Object[]} args
     * @returns {string}
     * @throws {OperationError} if invalid key or nonce length, or authentication fails
     */
    run(input, args) {
        const key = Utils.convertToByteArray(args[0].string, args[0].option),
            nonce = Utils.convertToByteArray(args[1].string, args[1].option),
            ad = Utils.convertToByteArray(args[2].string, args[2].option),
            inputType = args[3],
            outputType = args[4];

        if (key.length !== 16) {
            throw new OperationError(`Invalid key length: ${key.length} bytes.

Ascon-AEAD128 requires a key of exactly 16 bytes (128 bits).`);
        }

        if (nonce.length !== 16) {
            throw new OperationError(`Invalid nonce length: ${nonce.length} bytes.

Ascon-AEAD128 requires a nonce of exactly 16 bytes (128 bits).`);
        }

        // Convert input to byte array
        const inputData = Utils.convertToByteArray(input, inputType);

        const keyUint8 = new Uint8Array(key);
        const nonceUint8 = new Uint8Array(nonce);
        const adUint8 = new Uint8Array(ad);
        const ciphertextUint8 = new Uint8Array(inputData);

View on GitHub (pinned to 4290ea7539)

Solutions

Provide exactly 16 bytes of key material (32 hex chars, or ~22 base64 chars).
If you only have a passphrase, hash it to 16 bytes first (e.g. SHA-256 then truncate, or use a KDF) before Ascon.
Confirm the input option matches your key encoding (Hex vs UTF8) so the byte count is what you expect.

Example fix

// before - 32 hex chars = 16 bytes? No: 'abcd...' of wrong length
chef.asconDecrypt(ct, { key: "short", keyOption: "UTF8" });

// after - 32 hex chars = exactly 16 bytes
chef.asconDecrypt(ct, { key: "00112233445566778899aabbccddeeff", keyOption: "Hex" });

Defensive patterns

Strategy: validation

Validate before calling

import Utils from "src/core/Utils.mjs";
function assertAsconKey(keyStr, keyOption) {
  const bytes = Utils.convertToByteArray(keyStr, keyOption);
  if (bytes.length !== 16) {
    throw new Error(`Ascon key must be 16 bytes, got ${bytes.length}`);
  }
  return bytes;
}
assertAsconKey(key, keyOption);

Type guard

function isExactlyNBytes(s, option, n) {
  // caller must decode; structural guard for hex:
  if (option === "Hex") return /^[0-9a-f]{2*n}$/i.test(s);
  return false; // decode and measure for other options
}

Prevention

Use exactly 16 bytes (32 hex chars) for the Ascon key.
Derive keys from passphrases with a KDF before Ascon.
Verify the input option matches the key encoding.

When it happens

Trigger: Supplying a 32-byte key (common when reusing an AES-256 key), a short passphrase as UTF-8, a hex string of wrong length, or a base64 value that decodes to != 16 bytes.

Common situations: Reusing an AES-256 key or SHA-256 digest as the Ascon key; pasting a human passphrase expecting it to be padded; hex/base64 miscount causing off-by-one byte length.

Related errors

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/72fe059d452789a9. Report an issue: GitHub.