Crypto.sign() function to sign a message with given private key

I need to sign a message with crypto.sign() function in NodeJS to get a valid JWT. I have a private key (base 64) like this:

Dm2xriMD6riJagld4WCA6zWqtuWh40UzT/ZKO0pZgtHATOt0pGw90jG8BQHCE3EOjiCkFR2/gaW6JWi+3nZp8A==

And I tried to get a signature:

const getJWT = () => {
  const privateKey =
    "Dm2xriMD6riJagld4WCA6zWqtuWh40UzT/ZKO0pZgtHATOt0pGw90jG8BQHCE3EOjiCkFR2/gaW6JWi+3nZp8A==";
  const payload = {
    iss: "test",
    aud: "test.com",
    iat: 1650101178,
    exp: 1650101278,
    sub: "12345678-1234-1234-1234-123456789123"
  };
  const token = encode(payload, privateKey);
  return token
};

const encode = (payload, key) => {
  const header = {
    typ: "JWT",
    alg: "EdDSA"
  };
  const headerBase64URL = base64url(JSON.stringify(header));
  const payloadBase64URL = base64url(JSON.stringify(payload));
  const msg = Buffer.from(`${headerBase64URL}.${payloadBase64URL}`);
  const keyDecoded = Buffer.from(key, "base64");
  const signature = crypto.sign("Ed25519", msg, keyDecoded); //Here is the problem
  const signatureBase64url = base64url(Buffer.from(signature));
  return `${msg}.${signatureBase64url}`;
};

I received this error:

internal/crypto/sig.js:142
  return _signOneShot(keyData, keyFormat, keyType, keyPassphrase, data,
         ^

Error: error:0909006C:PEM routines:get_name:no start line

  library: 'PEM routines',
  function: 'get_name',
  reason: 'no start line',
  code: 'ERR_OSSL_PEM_NO_START_LINE'

How can I adapt my private key to a valid format?

Solution

The crypto.sign() method requires for Ed25519 a private key in PKCS#8 format. Your key is a raw key consisting of the concatenation of the raw private 32 bytes key and the raw public 32 bytes, base64 encoded. A DER encoded PKCS#8 key can be derived and imported as follows:

Base64 decode your key. Use the first 32 bytes of your raw 64 bytes key (i.e. the raw private key).
Concat the following prefix for a private Ed25519 key (hex): 302e020100300506032b657004220420
Import that DER encoded PKCS#8 key.

Accordingly, the key import in getJWT() must be changed as follows:

const privateKey = toPkcs8der('Dm2xriMD6riJagld4WCA6zWqtuWh40UzT/ZKO0pZgtHATOt0pGw90jG8BQHCE3EOjiCkFR2/gaW6JWi+3nZp8A==');

with

const toPkcs8der = (rawB64) => {
    var rawPrivate = Buffer.from(rawB64, 'base64').subarray(0, 32);
    var prefixPrivateEd25519 = Buffer.from('302e020100300506032b657004220420','hex');
    var der = Buffer.concat([prefixPrivateEd25519, rawPrivate]);
    return crypto.createPrivateKey({key: der, format: "der", type: "pkcs8"})
}

Furthermore, in the encode() function:

Remove the line const keyDecoded = Buffer.from(key, "base64")
Create the signature with
```
const signature = crypto.sign(null, msg, key)
```
Note that for Ed25519, a null must be passed as first parameter in the sign() call. The algorithm comes from the key.

With these changes, the NodeJS code returns the following JWT:

eyJ0eXAiOiJKV1QiLCJhbGciOiJFZERTQSJ9.eyJpc3MiOiJ0ZXN0IiwiYXVkIjoidGVzdC5jb20iLCJpYXQiOjE2NTAxMDExNzgsImV4cCI6MTY1MDEwMTI3OCwic3ViIjoiMTIzNDU2NzgtMTIzNC0xMjM0LTEyMzQtMTIzNDU2Nzg5MTIzIn0.f7WG_02UKljrMeVVOTNNBAGxtLXJUT_8QAnujNhomV18Pn5cU-0lHRgVlmRttOlqI7Iol_fHut3C4AOXxDGnAQ

that matches the expected JWT.