question: public static PGPPublicKey mergeSignatures ( PGPPublicKey targetKey , PGPPublicKey sourceKey ) throws PGPException { if ( ! Objects . deepEquals ( targetKey . getFingerprint ( ) , sourceKey . getFingerprint ( ) ) ) { throw new IllegalArgumentException ( "Signature merge can be done for different instances of the same public key only" ) ; } return copySignatures ( targetKey , sourceKey ) ; }
target:  Returns a public key containing signatures of two keys.