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Improving Signal's Sealed Sender

Ian Martiny (University of Colorado Boulder), Gabriel Kaptchuk
(Boston University), Adam Aviv (The George Washington University),
Dan Roche (U.S. Naval Avademy), Eric Wustrow (University of Colorado
Boulder)

The Signal messaging service recently deployed a emph{sealed sender}
feature that provides sender anonymity by cryptographically hiding a
message's sender from the service provider. We demonstrate, both
theoretically and empirically, that this one-sided anonymity is
broken when two parties send multiple messages back and forth; that
is, the promise of sealed sender does not emph{compose} over a
conversation of messages. Our attack is in the family of Statistical
Disclosure Attacks (SDAs), and is made particularly effective by emph
{delivery receipts} that inform the sender that a message has been
successfully delivered, which are enabled by default on Signal. We
show using theoretical and simulation-based models that Signal could
link sealed sender users in as few as 5 messages.

Our attack goes beyond tracking users via network-level identifiers
by working at the application layer of Signal. This make our attacks
particularly effective against users that employ Tor or VPNs as
anonymity protections, who would otherwise be secure against network
tracing. We present a range of practical mitigation strategies that
could be employed to prevent such attacks, and we prove our protocols
secure using a new simulation-based security definition for one-sided
anonymity over any sequence of messages. The simplest provably-secure
solution uses many of the same mechanisms already employed by the
(flawed) sealed-sender protocol used by Signal, which means it could
be deployed with relatively small overhead costs; we estimate that
the extra cryptographic cost of running our most sophisticated
solution in a system with millions of users would be less than $40
per month.

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