feat: replace random access with round robin

vertx-auth-common/src/main/java/io/vertx/ext/auth/impl/jose/JWT.java

@@ -44,8 +44,46 @@ public final class JWT {
 
   private static final Logger LOG = LoggerFactory.getLogger(JWT.class);
 
-  // simple random as its value is just to create entropy
-  private static final Random RND = new Random();
+  /**
+   * Internal holder of keys and usage tracking
+   */
+  private static class KeyRing implements Iterable<JWS> {
+    private final List<JWS> signers = new ArrayList<>();
+    private int cnt = 0;
+
+    int size() {
+      return signers.size();
+    }
+
+    JWS get(int pos) {
+      return signers.get(pos);
+    }
+
+    JWS get() {
+      int size = signers.size();
+      switch (size) {
+        case 0:
+          return null;
+        case 1:
+          return signers.get(0);
+        default:
+          return signers.get(cnt++ % size);
+      }
+    }
+
+    JWS set(int pos, JWS jws) {
+      return signers.set(pos, jws);
+    }
+
+    boolean add(JWS jws) {
+      return signers.add(jws);
+    }
+
+    @Override
+    public Iterator<JWS> iterator() {
+      return signers.iterator();
+    }
+  }
 
   private static final Charset UTF8 = StandardCharsets.UTF_8;
 
@@ -54,8 +92,8 @@ public final class JWT {
   private MessageDigest nonceDigest;
 
   // keep 2 maps (1 for sing, 1 for verify) this simplifies the lookups
-  private final Map<String, List<JWS>> SIGN = new ConcurrentHashMap<>();
-  private final Map<String, List<JWS>> VERIFY = new ConcurrentHashMap<>();
+  private final Map<String, KeyRing> SIGN = new ConcurrentHashMap<>();
+  private final Map<String, KeyRing> VERIFY = new ConcurrentHashMap<>();
 
   /**
    * Adds a JSON Web Key (rfc7517) to the signature maps.
@@ -66,14 +104,14 @@ public final class JWT {
   public JWT addJWK(JWK jwk) {
 
     if (jwk.use() == null || "sig".equals(jwk.use())) {
-      List<JWS> current;
+      KeyRing current;
       synchronized (this) {
         if (jwk.mac() != null || jwk.publicKey() != null) {
-          current = VERIFY.computeIfAbsent(jwk.getAlgorithm(), k -> new ArrayList<>());
+          current = VERIFY.computeIfAbsent(jwk.getAlgorithm(), k -> new KeyRing());
           addJWK(current, jwk);
         }
         if (jwk.mac() != null || jwk.privateKey() != null) {
-          current = SIGN.computeIfAbsent(jwk.getAlgorithm(), k -> new ArrayList<>());
+          current = SIGN.computeIfAbsent(jwk.getAlgorithm(), k -> new KeyRing());
           addJWK(current, jwk);
         }
       }
@@ -129,21 +167,21 @@ public JWT nonceAlgorithm(String alg) {
     return this;
   }
 
-  private void addJWK(List<JWS> current, JWK jwk) {
+  private void addJWK(KeyRing keyring, JWK jwk) {
     boolean replaced = false;
-    for (int i = 0; i < current.size(); i++) {
-      if (current.get(i).jwk().label().equals(jwk.label())) {
+    for (int i = 0; i < keyring.size(); i++) {
+      if (keyring.get(i).jwk().label().equals(jwk.label())) {
         // replace
         LOG.info("replacing JWK with label " + jwk.label());
-        current.set(i, new JWS(jwk));
+        keyring.set(i, new JWS(jwk));
         replaced = true;
         break;
       }
     }
 
     if (!replaced) {
       // non existent, add it!
-      current.add(new JWS(jwk));
+      keyring.add(new JWS(jwk));
     }
   }
 
@@ -270,7 +308,7 @@ public JsonObject decode(final String token, boolean full, List<X509CRL> crls) t
 
     // verify signature. `sign` will return base64 string.
     if (!unsecure) {
-      List<JWS> signatures = VERIFY.get(alg);
+      KeyRing signatures = VERIFY.get(alg);
 
       if (signatures == null || signatures.size() == 0) {
         throw new NoSuchKeyIdException(alg);
@@ -331,14 +369,14 @@ public String sign(JsonObject payload, JWTOptions options) {
     final String kid;
 
     if (!unsecure) {
-      List<JWS> signatures = SIGN.get(algorithm);
+      KeyRing signatures = SIGN.get(algorithm);
 
       if (signatures == null || signatures.size() == 0) {
         throw new RuntimeException("Algorithm not supported/allowed: " + algorithm);
       }
 
       // lock the crypto implementation
-      jws = signatures.get(signatures.size() == 1 ? 0 : RND.nextInt(signatures.size()));
+      jws = signatures.get();
       kid = jws.jwk().getId();
     } else {
       jws = null;