Can Unicorns Help Users Compare Crypto Key Fingerprints? Joshua Tan, Lujo Bauer, Joseph Bonneau†, Lorrie Faith Cranor, Jeremy Thomas, Blase Ur∗ Carnegie Mellon University, {jstan, lbauer, lorrie, thomasjm}@cmu.edu † Stanford University,
[email protected] * University of Chicago,
[email protected] ABSTRACT key. To learn Bob’s key, Alice would typically look it up Many authentication schemes ask users to manually compare on a web site (e.g., a public key server) that publishes such compact representations of cryptographic keys, known as fin- information. Unfortunately, an attacker seeking to intercept gerprints. If the fingerprints do not match, that may signal a Alice’s communications to Bob might try to add his own key man-in-the-middle attack. An adversary performing an attack to the key server under Bob’s name. When trying to find may use a fingerprint that is similar to the target fingerprint, but Bob’s public key, Alice would then unwittingly download the not an exact match, to try to fool inattentive users. Fingerprint attacker’s key. Any messages she composed for Bob would representations should thus be both usable and secure. then be readable by the attacker, and not by Bob. We tested the usability and security of eight fingerprint repre- A more reliable method would be for Bob to deliver his public sentations under different configurations. In a 661-participant key to Alice in person. Because public keys are long strings between-subjects experiment, participants compared finger- of arbitrary bits, this approach is unfortunately unwieldy and prints under realistic conditions and were subjected to a sim- impractical.