chore(model-server): deduplicate authorization logic

The logic determining the verifier was a duplicate of the logic used in the additional validation.

The logic from the validation (aka `ModelixAuthorizationConfig,nullIfInvalid`) is also used elsewhere.
For example, it is used in the workspaces through the modelix-authorization lib.
So it was reused in a custom verifier.

Calling `ModelixAuthorizationConfig.nullIfInvalid` again in `validate` is redundant.
It is now only used to create a custom principal object.

authorization/src/main/kotlin/org/modelix/authorization/AuthorizationPlugin.kt

@@ -18,6 +18,9 @@ package org.modelix.authorization
 
 import com.auth0.jwt.JWT
 import com.auth0.jwt.algorithms.Algorithm
+import com.auth0.jwt.exceptions.JWTVerificationException
+import com.auth0.jwt.interfaces.DecodedJWT
+import com.auth0.jwt.interfaces.JWTVerifier
 import com.google.common.cache.CacheBuilder
 import io.ktor.http.HttpStatusCode
 import io.ktor.server.application.Application
@@ -80,31 +83,18 @@ object ModelixAuthorization : BaseRouteScopedPlugin<IModelixAuthorizationConfig,
             } else {
                 // "Authorization: Bearer ..." header is provided in the header by OAuth proxy
                 jwt(MODELIX_JWT_AUTH) {
-                    val jwkProvider = config.getJwkProvider()
-                    val jwtAlgorithm = config.getJwtSignatureAlgorithmOrNull()
-                    // If JWK URI and JWT algorithm is configured only use the configured algorithm.
-                    // This is the case if MODELIX_JWK_URI and MODELIX_JWK_KEY_ID are configured.
-                    if (jwtAlgorithm != null) {
-                        verifier(getVerifierForSpecificAlgorithm(jwtAlgorithm))
-                    } else if (jwkProvider != null) {
-                        verifier(jwkProvider)
-                    } else {
-                        error("Either an JWT algorithm or a JWK URI must be configured.")
-                    }
+                    verifier(config.getVerifier())
                     challenge { _, _ ->
                         call.respond(status = HttpStatusCode.Unauthorized, "No or invalid JWT token provided")
                         // login and token generation is done by OAuth proxy. Only validation is required here.
                     }
                     validate {
                         try {
-                            val token = jwtFromHeaders()
-                            if (token != null) {
-                                return@validate config.nullIfInvalid(token)?.let { AccessTokenPrincipal(it) }
-                            }
+                            jwtFromHeaders()?.let(::AccessTokenPrincipal)
                         } catch (e: Exception) {
                             LOG.warn(e) { "Failed to read JWT token" }
+                            null
                         }
-                        null
                     }
                 }
             }
@@ -203,3 +193,21 @@ class ModelixAuthorizationPluginInstance(val config: ModelixAuthorizationConfig)
         }
     }
 }
+
+/**
+ * Returns an [JWTVerifier] that wraps our common authorization logic,
+ * so that it can be configured in the verification with Ktor's JWT authorization.
+ */
+internal fun ModelixAuthorizationConfig.getVerifier() = object : JWTVerifier {
+    override fun verify(token: String?): DecodedJWT {
+        val jwt = JWT.decode(token)
+        return verify(jwt)
+    }
+
+    override fun verify(jwt: DecodedJWT?): DecodedJWT {
+        if (jwt == null) {
+            throw JWTVerificationException("No JWT provided.")
+        }
+        return this@getVerifier.nullIfInvalid(jwt) ?: throw JWTVerificationException("JWT invalid.")
+    }
+}