The Internet Engineering Task Force (IETF) is in the process of adopting standards for IP-layer encryption and authentication (IPSEC). We describe how “probable plaintext” can be used to aid in cryptanalytic attacks, and analyze the protocol to show how much probable plaintext is available. We also show how traffic analysis is a powerful aid to the cryptanalyst. We conclude by outlining some likely changes to the underlying protocols that may strengthen them against these attacks.
Steven M. Bellovin