Secure messaging apps like Signal let you deny having sent certain messages, but do these denials only really work for cryptographers?
Cryptographers have long been concerned with secure messaging protocols threatening deniability. Many messaging protocols—including, surprisingly, modern email—contain digital signatures which definitively tie the author to their message. If stolen or leaked, these signatures make it impossible to deny authorship. As illustrated by events surrounding leaks from Hilary Clinton’s 2016 U.S. presidential campaign, this concern has proven well founded. Deniable protocols are meant to avoid this very outcome, letting politicians and dissidents alike safely disavow authorship. Despite being deployed on billions of devices in Signal and WhatsApp, the effectiveness of such protocols in convincing people remains unstudied. While the absence of cryptographic evidence is clearly necessary for an effective denial, is it sufficient?
We conduct a survey study (n = 1, 200) to understand how people perceive evidence of deniability related to encrypted messaging protocols. Surprisingly, in a world of "fake news" and Photoshop, we find that simple denials of message authorship, when presented in a courtroom setting without supporting evidence, are not effective. In contrast, participants who were given access to a screenshot forgery tool or even told one exists were much more likely to believe a denial. Similarly, but to a lesser degree, we find an expert cryptographer’s assertion that there is no evidence is also effective.