private static void populateExtensions(Wallet wallet, Protos.Wallet.Builder walletBuilder) {
   for (WalletExtension extension : wallet.getExtensions().values()) {
     Protos.Extension.Builder proto = Protos.Extension.newBuilder();
     proto.setId(extension.getWalletExtensionID());
     proto.setMandatory(extension.isWalletExtensionMandatory());
     proto.setData(ByteString.copyFrom(extension.serializeWalletExtension()));
     walletBuilder.addExtension(proto);
   }
 }