provide single-key accessor instead of whole Map

Author: Valentin Brückel

core/src/main/java/com/predic8/membrane/core/interceptor/jwt/Jwks.java

@@ -114,8 +114,8 @@ public Jwks setJwksUris(String jwksUris) {
         return this;
     }
 
-    public HashMap<String, RsaJsonWebKey> getKeysByKid() {
-        return keysByKid;
+    public Optional<RsaJsonWebKey> getKeyByKid(String kid) {
+        return Optional.ofNullable(keysByKid.get(kid));
     }
 
     private HashMap<String, RsaJsonWebKey> buildKeyMap(List<Jwk> jwks) {

core/src/main/java/com/predic8/membrane/core/interceptor/jwt/JwtAuthInterceptor.java

@@ -13,7 +13,6 @@
 package com.predic8.membrane.core.interceptor.jwt;
 
 import com.fasterxml.jackson.core.*;
-import com.fasterxml.jackson.databind.*;
 import com.predic8.membrane.annot.*;
 import com.predic8.membrane.core.exceptions.ProblemDetails;
 import com.predic8.membrane.core.exchange.*;
@@ -129,8 +128,7 @@ public Outcome handleJwt(Exchange exc, String jwt) throws JWTException, JsonProc
 
         // we could make it possible that every key is checked instead of having the "kid" field mandatory
         // this would then need up to n checks per incoming JWT - could be a performance problem
-        RsaJsonWebKey key = Optional.ofNullable(jwks.getKeysByKid().get(kid))
-                .orElseThrow(() -> new JWTException(ERROR_UNKNOWN_KEY, ERROR_UNKNOWN_KEY_ID));
+        RsaJsonWebKey key = jwks.getKeyByKid(kid).orElseThrow(() -> new JWTException(ERROR_UNKNOWN_KEY, ERROR_UNKNOWN_KEY_ID));
 
         Map<String, Object> jwtClaims = createValidator(key).processToClaims(jwt).getClaimsMap();