I strongly recommend that you read the first article about the OpenID Federation protocol.
This new article dives deeper into the OpenID Federation support in pac4j and Connect2id.
You should download and use the latest versions of both software (at least version 6.5.1 for pac4j).
1) Let's log in (again)
a) Calling the login page
As we have previously seen, the first login generates several logs on both sides (client = RP = pac4j + server = OP = connect2id).
Several HTTP calls are required to check the JWKS and the entity statements and establish the trust chains.
This could be a performance issue if these HTTP calls were made for each login attempt, though on the second try, the logs are much less verbose before displaying the login page:
The pac4j logs:
DEBUG o.p.o.r.OidcRedirectionActionBuilder : Request Object claim names: [iss, aud, iat, exp, jti, scope, response_type,
redirect_uri, state, code_challenge_method, client_id, code_challenge, response_mode]
DEBUG o.p.o.r.OidcRedirectionActionBuilder : Authz parameter names: [response_type, request, client_id, scope]
DEBUG o.p.o.r.OidcRedirectionActionBuilder : Authentication request URL: http://127.0.0.1:8080/c2id-login
?response_type=code&request=eyJr...hULhwg&client_id=http%3A%2F%2Flocalhost%3A8081&scope=openid%20profile%20email
The Connect2id logs:
INFO AUTHZ-SESSION - [OP2101] Created new auth session: sid=FKEttMylh3MVBAtu59F7CXx5m4BTGXZ2_DKowRh_8eg
client_id=http://localhost:8081 scope=[openid, profile, email]
Hopefully, trust chains have been cached depending on the expiration time and the whole plumbing has not been triggered a second time.
b) After typing in the login and password
After a successful login, we get the following logs:
On the Connect2id side:
INFO SESSION-STORE - [SS0201] Added new session: sub=alice ctx=web sid_key=MhNnqmimFgizG5ALmnp-tg
INFO AUTHZ-SESSION - [OP2103] Created new consent session: sid=FKEttMylh3MVBAtu59F7CXx5m4BTGXZ2_DKowRh_8eg subject=alice
client_id=http://localhost:8081
INFO AUTHZ-SESSION - [OP2108] Created authZ response: subject=alice client_id=http://localhost:8081 response_type=[code]
INFO TOKEN - HTTP POST request: ip=127.0.0.1 path=/c2id/token
INFO TOKEN - [OP6204] Authenticated: client_id=http://localhost:8081 method=private_key_jwt client_auth_id=NgAiEAADRhMrDiZp
INFO AUTHZ-STORE - [AS0280] Issued access token: sub=alice act=null client_id=http://localhost:8081 scope=[openid]
INFO TOKEN - [OP6225] Success response: client_id=http://localhost:8081 grant=code tokens=[access,id]
INFO USERINFO - HTTP GET request: ip=127.0.0.1 path=/c2id/userinfo
INFO AUTHZ-STORE - [AS0213] Inspected valid SELF_CONTAINED Bearer access token: sub=alice act=null client_id
=http://localhost:8081 iat=1775213922: eyJraWQiOiJQUlJ6Iiwid...
INFO USERINFO - [OP7307] Received valid UserInfo request: sub=alice claims=null ia_id=aac191d2-dcc5-4837-8545-692e204bcc07
This is fairly obvious:
- A POST call is performed on the
/c2id/tokenendpoint using theprivate_key_jwtclient authentication method (this is what we have configured on the pac4j side) - A GET call is performed on the
/c2id/userinfoendpoint using theaccess_tokenas bearer (= HTTP header).
This is the regular OIDC login process even if the flow has started in a federation way.
On the pac4j side:
DEBUG o.p.o.c.e.OidcCredentialsExtractor : Authentication response successful
DEBUG o.p.o.c.e.OidcCredentialsExtractor : Request state: d508c0f0ad/response state: d508c0f0ad
DEBUG org.pac4j.oidc.client.OidcClient : clean authentication attempt from session
DEBUG o.p.o.c.authenticator.OidcAuthenticator : Token response: status=200, content={"access_token":"eyJ...bng","token_type":"Bearer","expires_in":600}
DEBUG o.p.o.c.authenticator.OidcAuthenticator : Token response successful
DEBUG org.pac4j.oidc.client.OidcClient : clean authentication attempt from session (second call)
DEBUG org.pac4j.oidc.client.OidcClient : Credentials validation took: 32 ms
DEBUG org.pac4j.oidc.client.OidcClient : credentials : OidcCredentials(code=sjtox4...NQw, accessToken=...
DEBUG org.pac4j.oidc.profile.OidcProfile : adding => key: access_token / value: eyJh...MDB9 / class java.lang.String
DEBUG org.pac4j.oidc.profile.OidcProfile : adding => key: expiration / value: 1775214522541 / class java.lang.Long
DEBUG org.pac4j.oidc.profile.OidcProfile : adding => key: id_token / value: eyJ...bng / class java.lang.String
DEBUG o.p.oidc.profile.creator.TokenValidator : Trying IDToken validator, issuer: http://127.0.0.1:8080/c2id, type: null, JWS:
DEBUG o.p.oidc.profile.creator.TokenValidator : Validated: {"iss":"http:\/\/127.0.0.1:8080\/c2id","sub":"alice",
"aud":"http:\/\/localhost:8081","exp":1775214522,"iat":1775213922,"amr":["pwd"]}
DEBUG o.p.o.p.creator.OidcProfileCreator : User info response: status=200, content={"sub":"alice","groups":["admin","audit"]}
DEBUG o.p.oidc.profile.OidcProfileDefinition : converted to => key: sub / value: alice / class java.lang.String
DEBUG org.pac4j.oidc.profile.OidcProfile : adding => key: sub / value: alice / class java.lang.String
...
DEBUG org.pac4j.oidc.profile.OidcProfile : adding => key: token_expiration_advance / value: 0 / class java.lang.Integer
DEBUG org.pac4j.oidc.client.OidcClient : profile: Optional[OidcProfile(super=AbstractJwtProfile(super=CommonProfile(
super=BasicUserProfile(logger=Logger[org.pac4j.oidc.profile.OidcProfile], id=alice, attributes={access_token=eyJ...MDB9,
token_expiration_advance=0, sub=alice, aud=[http://localhost:8081], amr=[pwd], id_token=eyJr...Hsbng,
iss=http://127.0.0.1:8080/c2id, groups=[admin, audit], expiration=1775214522541, exp=Fri Apr 03 13:08:42 CEST 2026,
The logs are straightforward on the pac4j side as well: we see the successful authentication, the token and the userprofile calls.
c) After the login process
Even though we’re not doing anything, new logs keep appearing for the Connect2id server:
INFO AUTHZ-STORE - [AS0228] Revoking multiple authzs: client_id=http://localhost:8081
INFO CLIENT-REG - [OP5184] Deleted client: client_id=http://localhost:8081 num_revoked_authz=1
INFO FED-REG - [OP8041] Reaped 1 expired federation clients
These logs are related to the fact that we have performed an automatic registration. The logs indicate that the temporarily created client is deleted.
Indeed, as Connect2id does not know the pac4j client, it has temporarily registered this client and after some time, the registered client is cleaned.
While this is a very convenient mechanism, it can impact server performance.
Therefore, it could be useful to consider explicitly and permanently registering our OIDC pac4j client.
2) Let's log in with explicit registration
a) The client identifier
And this is a feature supported by the OpenID Federation protocol:
the explicit registration of the OIDC client.
This must be of course supported by the OIDC server and this is the case of the Connect2id server.
pac4j supports both modes depending on the OIDC server, so the configuration must only be updated on the Connect2id server.
Stop the server (tomcat/bin/shutdown.sh), edit the tomcat/webapps/c2id/WEB-INF/oidcProvider.properties file:
op.federation.clientRegistrationTypes=explicit
and restart the server (tomcat/bin/startup.sh).
On the pac4j side:
DEBUG o.p.o.m.r.FederationClientRegister : Registration endpoint exists and only explicit registration by OP (and RP)
-> performing explicit registration
INFO .f.e.DefaultEntityConfigurationGenerator : Generating entity configuration for: http://localhost:8081
DEBUG o.p.o.m.r.FederationClientRegister : Received response registration: eyJraW...mkaMxZQ
WARN o.p.o.m.r.FederationClientRegister : /!\ ================================================
WARN o.p.o.m.r.FederationClientRegister : /!\ Explicit registration of the client 'http://localhost:8081' returns
id: [t4j746kwjax6s]. This information won't be repeated. You MUST add this value to your configuration before the next
application startup!
WARN o.p.o.m.r.FederationClientRegister : /!\ ================================================
On the Connect2id side:
INFO FED-REG - [OP8014] Registered entity http://localhost:8081 as explicit client with client_id=xkqolxvshcjv6 exp=1783005293
INFO FED-REG - [OP8019] Explicit registration response statement for entity http://localhost:8081: {sub=http://localhost:8081,
aud=[http://localhost:8081], metadata={openid_relying_party={client_registration_types=[explicit, automatic],
token_endpoint_auth_signing_alg=RS256, grant_types=[authorization_code], jwks={keys=[{kty=RSA, e=AQAB, use=sig, kid=...
The explicit registration is duly taken into account by Connect2id which generates a specific client_id for the OIDC client, returns it to pac4j to be displayed in its logs.
Let's follow the instruction given in the logs and add this client_id in the pac4j configuration:
@Bean
public Config config() {
final var config = new OidcConfiguration();
// the new clientId!
config.setClientId("xkqolxvshcjv6");
final var rpJwks = config.getRpJwks();
rpJwks.setJwksPath("file:./metadata/rpjwks.jwks");
rpJwks.setKid("defaultjwks0426");
config.setClientAuthenticationMethod(ClientAuthenticationMethod.PRIVATE_KEY_JWT);
final var privateKeyJwtConfig = new PrivateKeyJwtClientAuthnMethodConfig(rpJwks);
config.setPrivateKeyJWTClientAuthnMethodConfig(privateKeyJwtConfig);
config.setRequestObjectSigningAlgorithm(JWSAlgorithm.RS256);
final var federation = config.getFederation();
federation.setTargetOp("http://127.0.0.1:8080/c2id");
final var trust = new OidcTrustAnchorProperties();
trust.setIssuer("http://localhost:8081/trustanchor");
trust.setJwksPath("classpath:trustanchor.jwks");
federation.getTrustAnchors().add(trust);
federation.getJwks().setJwksPath("file:./metadata/oidcfede.jwks");
federation.getJwks().setKid("mykeyoidcfede26");
federation.setContactName("New RP test");
federation.setContactEmails(List.of("jerome@casinthecloud.com"));
federation.setEntityId("http://localhost:8081");
return new Config(baseUri + "/callback", new OidcClient(config));
}
Here is the complete configuration for reference (and not only the added client_id).
We restart the Spring Boot application and try a new login process.
This time, no registration happens and Connect2id directly recognizes the provided client_id:
INFO AUTHZ-SESSION - [OP2101] Created new auth session: sid=reJ...58w client_id=xkqolxvshcjv6 scope=[openid, profile, email]
b) The client secret
At this point in the article, you may wonder why we only have a client_id and no client_secret.
In fact, we don't need a secret as we use the private_key_jwt client authentication method: the credential is the private key, not the secret.
As this pac4j configuration is revealed in its entity statement, the Connect2id server is aware of that setting and, accordingly, decides to only return a client_id for this OIDC client.
Let's go further and replace this configuration in pac4j:
config.setClientAuthenticationMethod(ClientAuthenticationMethod.PRIVATE_KEY_JWT);
final var privateKeyJwtConfig = new PrivateKeyJwtClientAuthnMethodConfig(rpJwks);
config.setPrivateKeyJWTClientAuthnMethodConfig(privateKeyJwtConfig);
by:
config.setClientAuthenticationMethod(ClientAuthenticationMethod.CLIENT_SECRET_BASIC);
to use the client_secret_basic authentication (and not the private_key_jwt).
We also remove the previous client_id:
config.setClientId("xkqolxvshcjv6");
and completely change the contact name:
federation.setContactName("New RP test");
Restart the Spring Boot application and try to log in.
This time, we call the Connect2id server with explicit registration and no configured client id/secret and a client_secret_basic authentication method.
And we get a new error from pac4j:
org.pac4j.oidc.exceptions.OidcException: Client secret export file is required
This seems definitely a weird one, but it's not! Let me explain: the received client_id is output in the logs, though it would not be safe to output the client_secret in the logs as well.
So the received client_secret is planned to be saved on the disk, on a file defined by the secretExportFile property.
Let's define it in the configuration:
federation.setSecretExportFile("./mysecret.tmp");
and try again. It works!
The generated client_id and client_secret have been added on-the-fly to the OIDC configuration and have been used to perform the client_secret_basic authentication method.
The client_id is in the logs: Explicit registration of the client 'http://localhost:8081' returns id: [wzcyln5hdtmck].
The client_secret is in the defined file: The received secret has been saved into the file: ./mysecret.tmp. Its value is: U9UaF99rUopAXnWqXqkWBj_RsOZXfM1Efs67N4KweHo.
These seem to be the right settings as the login process has worked, but we'd like to check that on the Connect2id side.
Let's seek in the Connect2id configuration file oidcProvider.properties for the property: op.reg.apiAccessTokenSHA256. I find:
# Evaluation note: Use token value ztucZS1ZyFKgh0tUEruUtiSTXhnexmd6
op.reg.apiAccessTokenSHA256=cca68b8b82bcf0b96cb826199429e50cd95a042f8e8891d1ac56ab135d096633
Let's try to use the Connect2id API to list the existing clients with this key:
curl -X GET http://127.0.0.1:8080/c2id/clients -H "Authorization: Bearer ztucZS1ZyFKgh0tUEruUtiSTXhnexmd6"
We get two clients:
[
{
"client_registration_types":[
"explicit",
"automatic"
],
"grant_types":[
"authorization_code"
],
"jwks":{
"keys":[
{
"kty":"RSA",
"e":"AQAB",
"use":"sig",
"kid":"defaultjwks0426",
"n":"2moV...aq7Q"
}
]
},
"subject_type":"public",
"application_type":"web",
"registration_client_uri":"http:\/\/127.0.0.1:8080\/c2id\/clients\/wzcyln5hdtmck",
"redirect_uris":[
"http:\/\/localhost:8081\/callback?client_name=OidcClient"
],
"registration_access_token":"0QKFCVfBe5PqVFwgjEaRousXHKEa9My0-IMMMCTPB20.YSxpLHI",
"token_endpoint_auth_method":"client_secret_basic",
"client_id":"wzcyln5hdtmck",
"client_secret_expires_at":0,
"request_object_signing_alg":"RS256",
"client_id_issued_at":1775836027,
"client_secret":"U9UaF99rUopAXnWqXqkWBj_RsOZXfM1Efs67N4KweHo",
"client_name":"New RP test",
"contacts":[
"jerome@casinthecloud.com"
],
"response_types":[
"code"
],
"id_token_signed_response_alg":"RS256"
},
{
"token_endpoint_auth_signing_alg":"RS256",
"grant_types":[
"authorization_code"
],
"jwks":{
"keys":[
{
"kty":"RSA",
"e":"AQAB",
"use":"sig",
"kid":"defaultjwks0426",
"n":"2moV...aq7Q"
}
]
},
"subject_type":"public",
"application_type":"web",
"registration_client_uri":"http:\/\/127.0.0.1:8080\/c2id\/clients\/xkqolxvshcjv6",
"redirect_uris":[
"http:\/\/localhost:8081\/callback?client_name=OidcClient"
],
"registration_access_token":"7ilSWZGsH4wOyehETCNJQz0My8NO-6efLiV1ED2HcN0.YSxpLHI",
"token_endpoint_auth_method":"private_key_jwt",
"client_id":"xkqolxvshcjv6",
"request_object_signing_alg":"RS256",
"client_id_issued_at":1775747499,
"client_name":"C2ID Test RP (Localhost)",
"contacts":[
"jerome@casinthecloud.com"
],
"response_types":[
"code"
],
"id_token_signed_response_alg":"RS256"
}
]
The second one has only the right client_id, no client_secret and is defined with private_key_jwt.
The first one has the right client_id and client_secret and is defined with client_secret_basic.
Notice the appropriate client_name property as well.
The Connect2id configuration perfectly matches what was received by pac4j (not that I had any doubts 😉)
The magic of the federation continues:
The pac4j RP and the Connect2id OP only know and rely on the trust anchor, they don't know each other.
But nonetheless the RP has been able to definitely register itself on the OP!