Add utility method to write public key

acme4j-utils/src/main/java/org/shredzone/acme4j/util/KeyPairUtils.java

@@ -21,6 +21,7 @@ import java.security.KeyPair;
 import java.security.KeyPairGenerator;
 import java.security.NoSuchAlgorithmException;
 import java.security.NoSuchProviderException;
+import java.security.PublicKey;
 import java.security.SecureRandom;
 
 import org.bouncycastle.jce.ECNamedCurveTable;
@@ -123,4 +124,20 @@ public class KeyPairUtils {
         }
     }
 
+    /**
+     * Writes a {@link PublicKey} as PEM file.
+     *
+     * @param key
+     *            {@link PublicKey}
+     * @param w
+     *            {@link Writer} to write the PEM file to. The {@link Writer} is closed
+     *            after use.
+     * @since 3.0.0
+     */
+    public static void writePublicKey(PublicKey key, Writer w) throws IOException {
+        try (var jw = new JcaPEMWriter(w)) {
+            jw.writeObject(key);
+        }
+    }
+
 }

acme4j-utils/src/test/java/org/shredzone/acme4j/util/KeyPairUtilsTest.java

@@ -140,6 +140,19 @@ public class KeyPairUtilsTest {
         assertThat(pair).isNotSameAs(readPair);
         assertThat(pair.getPublic().getEncoded()).isEqualTo(readPair.getPublic().getEncoded());
         assertThat(pair.getPrivate().getEncoded()).isEqualTo(readPair.getPrivate().getEncoded());
+
+        // Write Public Key
+        String publicPem;
+        try (var out = new StringWriter()) {
+            KeyPairUtils.writePublicKey(pair.getPublic(), out);
+            publicPem = out.toString();
+        }
+
+        // Make sure PEM file is properly formatted
+        assertThat(publicPem).matches(
+                  "-----BEGIN PUBLIC KEY-----[\\r\\n]+"
+                + "([a-zA-Z0-9/+=]+[\\r\\n]+)+"
+                + "-----END PUBLIC KEY-----[\\r\\n]*");
     }
 
     /**