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Improving Signal’s Sealed Sender - NDSS Symposium https://www.ndss-symposium.org/ndss-paper/improving-signals-sealed-sender/

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