Implement PreferredKeyServer signature subpacket

pg/src/main/java/org/bouncycastle/bcpg/SignatureSubpacketInputStream.java

@@ -16,6 +16,7 @@
 import org.bouncycastle.bcpg.sig.PolicyURI;
 import org.bouncycastle.bcpg.sig.PreferredAEADCiphersuites;
 import org.bouncycastle.bcpg.sig.PreferredAlgorithms;
+import org.bouncycastle.bcpg.sig.PreferredKeyServer;
 import org.bouncycastle.bcpg.sig.PrimaryUserID;
 import org.bouncycastle.bcpg.sig.RegularExpression;
 import org.bouncycastle.bcpg.sig.Revocable;
@@ -152,6 +153,8 @@ else if (flags[StreamUtil.flag_partial])
             return new PreferredAlgorithms(type, isCritical, isLongLength, data);
         case PREFERRED_AEAD_ALGORITHMS:
             return new PreferredAEADCiphersuites(isCritical, isLongLength, data);
+        case PREFERRED_KEY_SERV:
+            return new PreferredKeyServer(isCritical, isLongLength, data);
         case KEY_FLAGS:
             return new KeyFlags(isCritical, isLongLength, data);
         case POLICY_URL:

pg/src/main/java/org/bouncycastle/bcpg/sig/PreferredKeyServer.java

@@ -0,0 +1,46 @@
+package org.bouncycastle.bcpg.sig;
+
+import org.bouncycastle.bcpg.SignatureSubpacket;
+import org.bouncycastle.bcpg.SignatureSubpacketTags;
+import org.bouncycastle.util.Arrays;
+import org.bouncycastle.util.Strings;
+
+/**
+ * Signature Subpacket containing the URI of the users preferred key server.
+ * This is a URI of a key server that the key holder prefers be used for updates.
+ * Note that keys with multiple User IDs can have a preferred key server for each User ID.
+ * Note also that since this is a URI, the key server can actually be a copy of the key
+ * retrieved by ftp, http, finger, etc.
+ *
+ * @see <a href="https://datatracker.ietf.org/doc/html/rfc4880#section-5.2.3.18">
+ *     RFC4880 - Preferred Key Server</a>
+ * @see <a href="https://www.rfc-editor.org/rfc/rfc9580.html#name-preferred-key-server">
+ *     RFC9580 - Preferred Key Server</a>
+ */
+public class PreferredKeyServer
+        extends SignatureSubpacket
+{
+    public PreferredKeyServer(boolean critical, boolean isLongLength, byte[] data)
+    {
+        super(SignatureSubpacketTags.PREFERRED_KEY_SERV, critical, isLongLength, data);
+    }
+
+    public PreferredKeyServer(boolean critical, String uri)
+    {
+        this(critical, false, Strings.toUTF8ByteArray(uri));
+    }
+
+    /**
+     * Return the URI of the users preferred key server.
+     * @return key server uri
+     */
+    public String getURI()
+    {
+        return Strings.fromUTF8ByteArray(data);
+    }
+
+    public byte[] getRawURI()
+    {
+        return Arrays.clone(data);
+    }
+}

pg/src/main/java/org/bouncycastle/openpgp/PGPSignatureSubpacketGenerator.java

@@ -18,6 +18,7 @@
 import org.bouncycastle.bcpg.sig.NotationData;
 import org.bouncycastle.bcpg.sig.PolicyURI;
 import org.bouncycastle.bcpg.sig.PreferredAlgorithms;
+import org.bouncycastle.bcpg.sig.PreferredKeyServer;
 import org.bouncycastle.bcpg.sig.PrimaryUserID;
 import org.bouncycastle.bcpg.sig.RegularExpression;
 import org.bouncycastle.bcpg.sig.Revocable;
@@ -202,6 +203,18 @@ public void setPreferredAEADAlgorithms(boolean isCritical, int[] algorithms)
             algorithms));
     }
 
+    /**
+     * Specify the preferred key server for the signed user-id / key.
+     * Note, that the key server might also be a http/ftp etc. URI pointing to the key itself.
+     *
+     * @param isCritical true if the subpacket should be treated as critical
+     * @param uri key server URI
+     */
+    public void setPreferredKeyServer(boolean isCritical, String uri)
+    {
+        packets.add(new PreferredKeyServer(isCritical, uri));
+    }
+
     public void addPolicyURI(boolean isCritical, String policyUri)
     {
         packets.add(new PolicyURI(isCritical, policyUri));