SMAPs: Short Message Authentication Protocols

There is a long history of authentication protocols designed for ease of human use, which rely on users copying a short string of digits. Historical examples include telex test keys and early nuclear firing codes; familiar modern examples include prepayment meter codes and the 3-digit card verification values used in online shopping. In this paper, we show how security protocols that are designed for human readability and interaction can fail to provide adequate protection against simple attacks. To illustrate the problem, we discuss an offline payment protocol and explain various problems. We work through multiple iterations, or ‘evolutions’, of the protocol in order to get better tradeoffs between security and usability. We discuss the limitation of verifying such protocols using BAN logic. Our aim is to develop usable human-friendly protocols that can be used in constrained offline environments. We conclude that protocol designers need to be good curators of security state, and also pay attention to the interaction between online and offline functions. In fact, we suggest that delay-tolerant networking might be a future direction of evolution for protocol research.