Javadoc

pg/src/main/java/org/bouncycastle/openpgp/api/SignatureSubpacketsFunction.java

@@ -1,19 +1,28 @@
 package org.bouncycastle.openpgp.api;
 
 import org.bouncycastle.openpgp.PGPSignatureSubpacketGenerator;
+import org.bouncycastle.openpgp.PGPSignatureSubpacketVector;
 
 /**
  * Callback to modify the contents of a {@link PGPSignatureSubpacketGenerator}.
+ * The {@link OpenPGPV6KeyGenerator} already prepopulates the hashed subpacket areas of signatures during
+ * key generation. This callback is useful to apply custom changes to the hashed subpacket area during the
+ * generation process.
  */
 @FunctionalInterface
 public interface SignatureSubpacketsFunction
 {
     /**
      * Apply some changes to the given {@link PGPSignatureSubpacketGenerator} and return the result.
      * It is also possible to replace the whole {@link PGPSignatureSubpacketGenerator} by returning another instance.
+     * Tipp: In order to replace a subpacket, make sure to prevent duplicates by first removing subpackets
+     * of the same type using {@link PGPSignatureSubpacketGenerator#removePacketsOfType(int)}.
+     * To inspect the current contents of the generator, it is best to call
+     * {@link PGPSignatureSubpacketGenerator#generate()} and in turn inspect its contents using
+     * {@link PGPSignatureSubpacketVector#toArray()}.
      *
      * @param subpackets original subpackets
-     * @return modified subpackets
+     * @return non-null modified subpackets
      */
     PGPSignatureSubpacketGenerator apply(PGPSignatureSubpacketGenerator subpackets);
 }