Add KeyIdUtil with method to convert from 16 digit hex key-id to long

pgpainless-core/src/main/java/org/pgpainless/key/util/KeyIdUtil.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2021 Paul Schaub.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pgpainless.key.util;
+
+import java.math.BigInteger;
+import java.util.regex.Pattern;
+
+public class KeyIdUtil {
+
+    private static final Pattern LONG_KEY_ID = Pattern.compile("^[0-9A-Fa-f]{16}$");
+
+    /**
+     * Convert a long key-id into a key-id.
+     * A long key-id is a 16 digit hex string.
+     *
+     * @param longKeyId 16-digit hexadecimal string
+     * @return key-id converted to {@link Long}.
+     */
+    public static long fromLongKeyId(String longKeyId) {
+        if (!LONG_KEY_ID.matcher(longKeyId).matches()) {
+            throw new IllegalArgumentException("Provided long key-id does not match expected format. " +
+                    "A long key-id consists of 16 hexadecimal characters.");
+        }
+
+        return new BigInteger(longKeyId, 16).longValue();
+    }
+}