Add DefaultMergeCallback

pgpainless-core/src/main/java/org/pgpainless/certificate_store/DefaultMergeCallback.java

@@ -0,0 +1,63 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.certificate_store;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.bouncycastle.openpgp.PGPPublicKey;
+import org.bouncycastle.openpgp.PGPPublicKeyRing;
+import org.pgpainless.PGPainless;
+import pgp.certificate_store.Certificate;
+import pgp.certificate_store.MergeCallback;
+
+public class DefaultMergeCallback implements MergeCallback {
+
+    @Override
+    public Certificate merge(Certificate data, Certificate existing) throws IOException {
+        // no existing certificate
+        if (existing == null) {
+            return data;
+        }
+
+        if (!data.getFingerprint().equals(existing.getFingerprint())) {
+            throw new IllegalArgumentException("Certificate mismatch! " +
+                    "Existing certificate: " + existing.getFingerprint() + ", New certificate: " + data.getFingerprint());
+        }
+
+        // existing and new certificates are equal
+        if (existing.getTag().equals(data.getTag())) {
+            return existing;
+        }
+
+        PGPPublicKeyRing existingCertificate = PGPainless.readKeyRing().publicKeyRing(existing.getInputStream());
+        PGPPublicKeyRing updatedCertificate = PGPainless.readKeyRing().publicKeyRing(data.getInputStream());
+
+        List<PGPPublicKey> mergedKeys = new ArrayList<>();
+
+        for (PGPPublicKey key : existingCertificate) {
+            PGPPublicKey kay = updatedCertificate.getPublicKey(key.getKeyID());
+            if (kay != null) {
+                // TODO: get https://github.com/bcgit/bc-java/pull/1102 merged
+                PGPPublicKey mkay = PGPPublicKey.join(key, kay);
+                mergedKeys.add(mkay);
+            } else {
+                mergedKeys.add(key);
+            }
+        }
+
+        for (PGPPublicKey key : updatedCertificate) {
+            PGPPublicKey kay = existingCertificate.getPublicKey(key.getKeyID());
+            if (kay == null) {
+                mergedKeys.add(key);
+            }
+        }
+
+        PGPPublicKeyRing merged = new PGPPublicKeyRing(mergedKeys);
+        return CertificateFactory.certificateFromPublicKeyRing(merged);
+    }
+
+}