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Are there any known exploits in Signal's encryption protocol that could lead to messages being decrypted, or any other compromising information being released?

5

No, there are no known public flaw in Signal at the time of writing of this answer.

Source: there was only one CVE for Signal (for Android), in 2018. The only one listed (fixed in version 4.25.0 the 14 Aug 2018) could leak photos attached to disappearing messages.

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It's not so much a flaw but more an expected and documented but nonetheless surprising behavior: since Signal uses the phone number as an ID, it is possible to identify whether a Signal account tied to a specific phone number is a member of a group that you are yourself a member of.

This has apparently been used by Chinese government agencies to identify dissidents:

  1. Get yourself added to the "subversive" group chat.
  2. Check a list of suspected dissident's phone numbers against the members of the group.

[I can't find the article nor remember the details, so I'm leaving this as a community wiki. If someone knows the details, please feel free to edit.]

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    Is this a weakness, though? If you are a member of a group, it's not a weakness that other members know who you are. Sure, it could be architected differently, but I wouldn't call this a weakness of the protocol.
    – schroeder
    Jan 19 at 9:20
  • @schroeder "or any other compromising information being released"
    – psitae
    Jan 20 at 18:47
  • @psitae then you might need to define what you mean, because the identity of those you are communicating with is not "compromising information". Neither is it a "flaw". Your question is now sounding like "tell me all the potentially bad things about Signal".
    – schroeder
    Jan 20 at 18:56

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