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Far from perfect in absolute security sense. That sense is not common afaik. Personally, I'm fine with that no one can read my texts, not that no one can actively make focused changes to my conversations or send fake messages from my device to decipher all previous conversations while I sleep. Non-terrorist people just want their messages mass-unreadable by automated inspection systems. Full protection against active high-tech investigations (i.e. special forces watching special you) is not on high social demand.

Why isn't Signal on the prism break page?

https://github.com/nylira/prism-break/issues/1667

User accessibility, userbase size.