Comment by sarahjamielewis

SimpleX relies on out-of-band key material transfer between clients, in addition to the honesty of routing server to protect privacy and metadata.

Cwtch uses the existing infrastructure of Tor and v3 onion services to establish p2p chat sessions, thus relying on the underlying security of the Tor network. There is some nuances regarding how different kinds of groups work, we have a security handbook that goes into it a deeper: https://docs.cwtch.im/security/intro