gchq/CyberChef · error · OperationError

Unsupported JWK key type

Error message

Unsupported JWK key type '${inputJson.kty}'

What it means

Thrown when a key's kty is a string but not one of the two supported types (RSA or EC). NOTE: there is a bug in the message - it interpolates inputJson.kty instead of jwk.kty, so when iterating a JWKS the reported value is the wrong one (often 'undefined'). Supported kty values like 'oct' (symmetric) or 'OKP' (EdDSA) trigger this.

Solutions

1. Use only RSA or EC keys with this operation.
2. For symmetric (oct) keys, extract 'k' and base64url-decode it instead.
3. For OKP/EdDSA keys, use a dedicated Ed25519 conversion path.
4. Filter the key set to kty in {'RSA','EC'} before calling.

Example fix

// before: symmetric key (unsupported)
chef.JWKToPem(JSON.stringify({ kty: 'oct', k: 'GawgguFyGrWKav7AX4VKUg' }));
// after: use an RSA/EC JWK, or handle oct separately
chef.JWKToPem(JSON.stringify({ kty: 'EC', crv: 'P-256', x: '...', y: '...' }));

const SUPPORTED_KTY = new Set(['RSA', 'EC']);
function filterSupportedKty(keys) {
  const arr = Array.isArray(keys) ? keys : [keys];
  const supported = arr.filter(k => k && SUPPORTED_KTY.has(k.kty));
  if (supported.length !== arr.length) {
    throw new Error('Only RSA/EC keys are supported; found: ' +
      arr.map(k => k && k.kty).join(', '));
  }
  return supported;
}

function isSupportedKty(jwk) {
  return jwk !== null && typeof jwk === 'object' &&
    (jwk.kty === 'RSA' || jwk.kty === 'EC');
}

try {
  return chef.JWKToPem(input);
} catch (e) {
  if (/Unsupported JWK key type/.test(e.message))
    throw new Error('Use only RSA/EC keys; note the reported kty may be wrong due to a message bug');
  throw e;
}

When it happens

Trigger: A symmetric key (kty 'oct'), an Octet Key Pair (kty 'OKP'), or any future/proprietary kty value. Feeding a JWKS that mixes supported RSA/EC keys with unsupported oct keys.

Common situations: Trying to convert a symmetric 'oct' key meant for HMAC, or an Ed25519 'OKP' key. CyberChef's JWK-to-PEM only wires RSA and EC through jsrsasign.

Related errors

• Invalid JWK format
• Input is not a JSON Web Key
• Input private key must be in hex; and should be 32 bytes
• Invalid key length: bytes. Salsa20 uses a key of 16 or 32…
• Invalid key length: bytes SM4 uses a key length of 16 bytes…

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/d8afa47fd05947b7. Report an issue: GitHub.

Appendix: source

Thrown at src/core/operations/JWKToPem.mjs:66

            // list of keys => transform all keys
            keys = inputJson;
        } else if (Array.isArray(inputJson.keys)) {
            // JSON Web Key Set => transform all keys
            keys = inputJson.keys;
        } else if (typeof inputJson === "object") {
            // single key
            keys.push(inputJson);
        } else {
            throw new OperationError("Input is not a JSON Web Key");
        }

        let output = "";
        for (let i=0; i<keys.length; i++) {
            const jwk = keys[i];
            if (typeof jwk.kty !== "string") {
                throw new OperationError("Invalid JWK format");
            } else if ("|RSA|EC|".indexOf(jwk.kty) === -1) {
                throw new OperationError(`Unsupported JWK key type '${inputJson.kty}'`);
            }

            const key = r.KEYUTIL.getKey(jwk);
            const pem = key.isPrivate ? r.KEYUTIL.getPEM(key, "PKCS8PRV") : r.KEYUTIL.getPEM(key);

            // PEM ends with '\n', so a new key always starts on a new line
            output += pem;
        }

        return output;
    }
}

export default PEMToJWK;

View on GitHub (pinned to 4290ea7539)