International Association
for Cryptologic Research

Over time, cryptographically deniable systems have come to be associated in computer-science literature with the idea of "denying" evidence in court — specifically, with the ability to convincingly forge evidence in courtroom scenarios, and relatedly, an inability to authenticate evidence in such contexts. Indeed, in some cryptographic models, the ability to falsify mathematically implies the inability to authenticate. Evidentiary processes in courts, however, have been developed over centuries to account for the reality that evidence has always been forgeable, and relies on factors outside of cryptographic models to seek the truth "as well as possible" while acknowledging that all evidence is imperfect. We argue that deniability does not and need not change this paradigm.

Our analysis highlights a gap between technical deniability notions and their application to the real world. There will essentially always be factors outside a cryptographic model that influence perceptions of a message's authenticity, in realistic situations. We propose the broader concept of credibility to capture these factors. The credibility of a system is determined by (1) a threshold of quality that a forgery must pass to be "believable" as an original communication, which varies based on sociotechnical context and threat model, (2) the ease of creating a forgery that passes this threshold, which is also context- and threat-model-dependent, and (3) default system retention policy and retention settings. All three aspects are important for designing secure communication systems for real-world threat models, and some aspects of (2) and (3) may be incorporated directly into technical system design. We hope that our model of credibility will facilitate system design and deployment that addresses threats that are not and cannot be captured by purely technical definitions and existing cryptographic models, and support more nuanced discourse on the strengths and limitations of cryptographic guarantees within specific legal and sociotechnical contexts.