Groups a user name with both a PublicKey and a PrivateKey. The class is named a ``signer'' because it has the ability to digitally sign Challenges in order to generate Responses. The FileSystemSigner class is used only on the client side of JNFS. Since it contains sensitive information (the user's PrivateKey), it should be stored in a secure place (preferably a smart card that can be inserted into the NC at login time).