A signing interface is disclosed for facilitating digital signing of electronic messages by a buyer or other subscribing party in an e-commerce or other context. The signing interface provides a mechanism by which a seller's Web applications may invoke a buyer's smartcard subsystem or other signing module to execute a digital signature. The signing interface preferably comprises a signing interface library and a signing plug-in resident on the buyer's computer. The signing interface library is preferably implemented as a Java package and associated libraries that are installed locally on the buyer's computer and made accessible to signed Java applets provided by the seller's Web server. The signing plug-in is preferably implemented as a plug-in, ActiveX control, or similar code that is installed locally on the buyer's computer and made accessible through the buyer's Web browser using an embedded browser object on a seller's Web page. In a preferred embodiment, the signing interface may be adapted to facilitate access to system services such as a warranty service provided via a four-corner trust model.