How to reduce the size of the access/refresh tokens in Keycloak?

Question

I am setting a keycloack authentication server to allow authorized users to access a protected resource (OAuth2.0).

The access will be done from an embedded device that has certain restrictions. The main restriction is that the access and refresh tokens cannot be saved if they are longer than 256 characters.

While in The OAuth 2.0 Authorization Framework is silent about the token size, all the identity providers are free to decide about the token size. For example, Facebook's token is less than 256 bytes, the same for Google. But for keycloack, I get a token around 850 bytes! I have tried several encryption algorithms available in the admin console by I still get a large jwt token. Decoding that jwt gives the following:

{
  "jti": "d654564qsd-5fqsdf5-4qsdf-8b25qs-b556456",
  "exp": 1556284611,
  "nbf": 0,
  "iat": 1556270211,
  "iss": "http://myadress:myport/auth/realms/myrealm",
  "aud": "myapp",
  "sub": "45464-445645-4b45641e-456456-45645646",
  "typ": "Bearer",
  "azp": "myapp",
  "auth_time": 1556269490,
  "session_state": "cb95519c-0bf8-4b6b-94e4-a10d9000dbd2",
  "acr": "0",
  "allowed-origins": [],
  "realm_access": {
    "roles": [
      "user"
    ]
  },
  "resource_access": {},
  "scope": "readwrite"
}

I am actually not interested at all in the data in the tokens and I am not parsing it. I just need the token to be able to access the resource. Hence, is there a way to reduce the size of the token to less than 256? if no, what is the best result I can get?

Thank you in advance

Answer 1

Also try to change signing algorithm. RSA256 ~354 symbols, ESA256 - 86 symbols, HS256 - 43 symbols. Could be configured using realm -> token -> default token algorithm or on client page