alg value for Ed25519? - jwt

Question

The RFC7518 has a list of algorithms values used in JWT. However there is no value for EdDSA, such as Ed25519. Also Ed25519 is not accepted as a valid value when verifying in Jose. What is the correct alg value for Ed25519?

Answer 1

ED25519 is a EdDSA (Edwards-curve DSA) signature scheme. See also RFC8037 and RFC8032.
According to the jose documentation alg needs to be set to EdDSA:
JWS Algorithm: Edwards-curve DSA
alg: EdDSA
EdDSAis also listed in the section JSON Web Signature and Encryption Algorithms of the IANA Registry (thanks #Florent Morselli for the hint)
Here I show an example how to generate a ed25519 keypair and a signed token in Node.js with jose and crypto and then verify the token with the public key in Python:
Generate key pair and token:
const { SignJWT } = require('jose/jwt/sign')
const { generateKeyPairSync } = require('crypto')
const { publicKey, privateKey } = generateKeyPairSync('ed25519');
console.log(publicKey.export({format:'pem',type:'spki'}))
console.log(privateKey.export({format:'pem',type:'pkcs8'}))
const jwt = await new SignJWT({ 'id': 1 })
.setProtectedHeader({ alg: 'EdDSA' })
.setExpirationTime('2h')
.sign(privateKey)
console.log(jwt)
-----BEGIN PUBLIC KEY-----
MCowBQYDK2VwAyEA7fySb/9h7hVH8j1paD5IoLfXj4prjfNLwOPUYKvsTOc=
-----END PUBLIC KEY-----
-----BEGIN PRIVATE KEY-----
MC4CAQAwBQYDK2VwBCIEIIJtJBnTuKbIy5YjoNiH95ky3DcA3kRB0I2i7DkVM6Cf
-----END PRIVATE KEY-----
eyJhbGciOiJFZERTQSJ9.eyJpZCI6MX0.RAxBAQPFOxrCfgqb56eaAz9u2lByj-WEO-
JWgJH3Cyx1o1Hwjn1pA2M4NgJeob9vb2Oaw4FOeYFr6_33XMTnAQ
the decoded token header:
{
"alg": "EdDSA"
}
Verify the token in Python with PyJWT:
import jwt
public_key = """-----BEGIN PUBLIC KEY-----
MCowBQYDK2VwAyEA7fySb/9h7hVH8j1paD5IoLfXj4prjfNLwOPUYKvsTOc=
-----END PUBLIC KEY-----"""
token = 'eyJhbGciOiJFZERTQSJ9.eyJpZCI6MX0.RAxBAQPFOxrCfgqb56eaAz9u2lByj-WEO-JWgJH3Cyx1o1Hwjn1pA2M4NgJeob9vb2Oaw4FOeYFr6_33XMTnAQ'
decoded = jwt.decode(token, public_key, algorithms='EdDSA', verify=True)
print(decoded)
Note: jwt.io currently doesn't support the EdDSA/ED25519 algorithm, so you can't verify the token on that site. I also don't know about any other JWT website that can verify EdDSA signed tokens.