[messaging] Value of deniability

Daniel Kahn Gillmor dkg at fifthhorseman.net
Wed Dec 10 14:12:35 PST 2014

On 12/10/2014 04:49 PM, Eleanor Saitta wrote:
> On 2014.12.10 16.31, moderncrypto at mkern.fastmail.fm wrote:
>> The practical value of deniability at the protocol level would be
>> much higher if it was deeply integrated into the user interface of
>> (commonly used) client software.
> Under which specific scenario would this improve security outcomes for
> users?

Under the scenario where a judge or jury is confronted with the
situation that a transcript introduced as evidence might be forged.

Making transcript forgery tools not only easy to use but immediately
visible to anyone who has ever used the tool would most likely make it
easier to make a convincing case that a purported transcript is not
"ground truth".

This isn't a slam-dunk legal argument, and it's certainly not guaranteed
to get anyone out of trouble, but getting any part of the legal system
to cast a more skeptical eye at digital evidence could certainly
"improve security outcomes for users" faced with legal charges.

Concretely: imagine plea deal negotiations in a case where a digital
transcript has been introduced as evidence.  Do you think you might get
a better deal if you can convincingly say in two minutes "look, here's
how you would forge this transcript" or if you have to show the jury
some fancy complicated hacker tools to forge the transcript?  what if
there was some "non-repudiable" cryptographic proof of origin for the

note that cryptographic deniability isn't really the biggest issue here
-- people are convicted all the time based at least in part on
cleartext, non-signed transcripts, possibly because lawyers aren't good
at making arguments about the forgability of digital data yet, in part
because we (the tech community) haven't given them the tools or the
public mindshare to make this argument easy.

cryptographic deniability is still not a huge win.  but if we get it for
free, i see no reason to dismiss it.


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