On the Credibility of Deniable Communication in Court
Jacob Leiken, Sunoo Park
TL;DR
This paper reframes cryptographic deniability within real-world evidentiary practice by introducing a credibility framework that accounts for sociotechnical factors, record retention, forgery effort, and contextual persuasion. It argues that courts rely on traditional fact-finding and corroboration, not purely cryptographic guarantees, and that deniable communications should be evaluated using a broader lens of credibility. The authors outline three gaps (record preservation, forgery effort, and context) and three credibility elements (retention, ease of forgery, threshold of believability), and discuss how these influence evidence authentication, relevance, and expert testimony. The work emphasizes that credibility modeling can guide secure-system design and deployment to better address threats outside purely technical definitions, with practical implications for deepfakes and future legal proceedings.
Abstract
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 an inability to authenticate evidence in such contexts. 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 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.