Add ConvertKeys example

pgpainless-core/src/test/java/org/pgpainless/example/ConvertKeys.java

@@ -0,0 +1,55 @@
+/*
+ * 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.example;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.security.InvalidAlgorithmParameterException;
+import java.security.NoSuchAlgorithmException;
+
+import org.bouncycastle.openpgp.PGPException;
+import org.bouncycastle.openpgp.PGPPublicKeyRing;
+import org.bouncycastle.openpgp.PGPSecretKeyRing;
+import org.junit.jupiter.api.Test;
+import org.pgpainless.PGPainless;
+import org.pgpainless.key.info.KeyRingInfo;
+import org.pgpainless.key.util.KeyRingUtils;
+
+public class ConvertKeys {
+
+    /**
+     * This example demonstrates how to extract a public key certificate from a secret key.
+     *
+     * @throws PGPException
+     * @throws InvalidAlgorithmParameterException
+     * @throws NoSuchAlgorithmException
+     */
+    @Test
+    public void secretKeyToCertificate() throws PGPException, InvalidAlgorithmParameterException, NoSuchAlgorithmException {
+        String userId = "alice@wonderland.lit";
+        PGPSecretKeyRing secretKey = PGPainless.generateKeyRing()
+                .modernKeyRing(userId, null);
+        // Extract certificate (public key) from secret key
+        PGPPublicKeyRing certificate = KeyRingUtils.publicKeyRingFrom(secretKey);
+
+
+        KeyRingInfo secretKeyInfo = PGPainless.inspectKeyRing(secretKey);
+        assertTrue(secretKeyInfo.isSecretKey());
+        KeyRingInfo certificateInfo = PGPainless.inspectKeyRing(certificate);
+        assertFalse(certificateInfo.isSecretKey());
+    }
+}