Trojan horse attacks on software for electronic signatures

Electronic signatures are introduced by more and more countries as legally binding means for signing electronic documents with the primary hope of boosting e-commerce and e-government. Given that the underlying cryptographic methods are sufficiently strong, attacks by Trojan horse programs on electronic signatures are becoming increasingly popular. Most of the current systems either employ costly or inflexible – yet still inadequate – defence mechanisms or simply ignore the threat. A signatory has to trust the manufacturer of the software that it will work in the intended way. In the past, Trojan horse programs have shown to be of growing concern for end-user computers. Software for electronic signatures must provide protection against Trojan horses attacking the legally relevant signing process. In a survey of commercial of the shelf signature software programs we found severe vulnerabilities that can easily be exploited by an attacker. In this work we propose a secure electronic paper as a countermeasure. It is a collection of preventive and restorative methods that provides, in parallel to traditional signatures on paper, a high degree of protection of the system against untrustworthy programs. We focus our attention on Microsoft Windows NT and Windows 98, two operating systems most likely to be found on the customers' computers. The resulting system is an assembly of a small number of inexpensive building blocks that offers reliable protection against Trojan horse programs attempting to forge electronic signatures.