Most cryptographic algorithms provide a means for secret and authentic communication. However, under many circumstances, the ability to repudiate messages or deny a conversation is no less important than secrecy and authenticity. For whistleblowers, informants, political dissidents and journalists --- to name a few --- it is most important to have means for deniable conversation, where electronic communication must mimic face-to-face private meetings. Off-the-Record Messaging, proposed in 2004 by Borisov, Goldberg and Brewer, and its subsequent improvements, simulate private two-party meetings. Despite some attempts, the multi-party scenario remains unresolved.

To combat the rapid infection rate of today's Internet worms, signatures for novel worms must be generated soon after an outbreak. This is especially critical in the case of polymorphic worms, whose binary representation changes frequently during the infection process. In this paper, we examine the assumptions underlying two leading network-based signature generation systems for polymorphic worms: polygraph [14] and Hamsa [12]. By identifying an assumption of both systems not met by all vulnerabilities, we discover a class of vulnerabilities (feature omission vulnerabilities) that neither system can accurately characterize. We demonstrate the limitations of polygraph and Hamsa by testing the signatures that they generate for exploits targeting a feature omission vulnerability. We discuss why feature omission vulnerabilities are difficult to characterize and how increased semantic awareness can help the signature generation process.