OIDC JWT Token signature algorithm?

[olivluca]

I'm trying to configure oidc against lemonldap. I configured lemonldap to sign using ES256 (because another service needs it) and I get this error:

reitti-1  | 2026-09-29T15:56:49.091Z TRACE 1 --- [nio-3001-exec-2] o.s.s.oauth2.jwt.NimbusJwtDecoder        : Failed to process JWT
reitti-1  | 
reitti-1  | com.nimbusds.jose.proc.BadJOSEException: Signed JWT rejected: Another algorithm expected, or no matching key(s) found

Does reitti only accept RSA256 or is it a misconfiguration on my part?
The problem is that I cannot change it because I can only set one global signing key and lemonldap doesn't allow me to set RSA256 with an EC key.

[dgraf-gh]

Hi @olivluca,

I have never heard of LemonLDAP. Maybe there is some configuration we can add here to support it. I will need to set up an instance to test this out. I checked the docs and found this: https://lemonldap-ng.org/documentation/latest/openidconnectservice.html#security

Have you set the key type to EC? If you could share your setup or some form of tutorial, it would really be helpful. I will then put some effort into it to get this working or at least have a definite answer for you.

[dgraf-gh]

I would have expected LemonLDAP to have per-client settings, as both Authelia and PocketID offer this functionality per client. In any case, I am more than willing to provide the necessary settings in Reitti once I have an instance running with your setup. We should be able to get this working, you are simply the first to encounter this issue to my knowledge.

[olivluca]

So, with some clients (immich, homeassistant(*)) I need to specify the signing algorithm, other (proxmox, fressrss) work without specifying it, reitti seems to be hardcoded to RS256.

(*) it's an external component providing oidc authentication

[olivluca]

I would have expected LemonLDAP to have per-client settings, as both Authelia and PocketID offer this functionality per client.

Well, the algorithm is per client but it doesn't accept it if doesn't match the only, global, signing key that you can set. I guess you can only change the bits (ES256, ES385, ES512).

[dgraf-gh]

Ah ok, I see. The web and gemini have quite different opinions how this can be configured. It should be possible in the long run to set this. I will spin up an instance and try to make this work.

[olivluca]

In any case don't worry too much, I'm just testing reitti and I can do that with local authentication.
As it is now it can work with lemonldap provided you use RS256 as the signature algorithm.

The configuration is simple, just add a new entry "OpenID Connect Relying (sic) Parties" then set

Basic -> Client ID (what you put in OIDC_CLIENT_ID)
Basic -> Client Secret (OIDC_CLIENT_SECRET)
Basic -> Allowed redirection addresses for login "https://your.host/login/oauth2/code/oauth https://your.host/login"
Options -> Algorithms -> ID Token signature algorithm RS256
Options -> Algorithms -> Access token signature algorithom RS256

everything else is default. In order to set RS256 you need to have an RSA key in

OpenID Connect Service -> Security -> Keys -> Signature Keys

you can either paste a key generated elsewhere or click on "New certificate".

I don't know how clients that don't need to specify the signing algorithm cope, maybe they try every algorithm returned in the autoconfig under "id_token_signing_alg_values_supported" and/or "userinfo_signing_alg_values_supported" or get the available key and its algorithm from the auth2/jwks endpoint.