Strengthening public key authentication against key theft

Martin Kleppmann and Conrad Irwin

9th International Conference on Passwords, Cambridge, UK, December 2015.

• Paper (open access)
• doi:10.1007/978-3-319-29938-9_9
• Slides
• Video

Abstract

Authentication protocols based on an asymmetric keypair provide strong authentication as long as the private key remains secret, but may fail catastrophically if the private key is lost or stolen. Even when encrypted with a password, stolen key material is susceptible to offline brute-force attacks. In this paper we demonstrate a method for rate-limiting password guesses on stolen key material, without requiring special hardware or changes to servers. By slowing down offline attacks and enabling easy key revocation our algorithm reduces the risk of key compromise, even if a low-entropy password is used.