  The security of messaging applications against person-in-the-middle attacks
relies on the authenticity of the exchanged keys. For users unable to meet in
person, a manual key fingerprint verification is necessary to ascertain key
authenticity. Such fingerprints can be exchanged visually or verbally, and it
is not clear in which condition users perform best. This paper reports the
results of a 62-participant study that investigated differences in performance
and perceived usability of visual and verbal comparisons of word-based key
fingerprints, and the influence of the individual's cognitive learning style.
The results show visual comparisons to be more effective against non-security
critical errors and are perceived to provide increased confidence, yet
participants perceive verbal comparisons to be easier and require less mental
effort. Besides, limited evidence was found on the influence of the
individual's learning style on their performance.
