Check out Briar if you need a secure way of communicating, it's fully end to end encrypted and can send messages via bluetooth to allow you to continue communicating with nearby people even when there is no internet, for example, if a cell phone jammer is being used.
opsec/protest PSA; request
If anyone has the platform, be sure to let people on twitter know about this as well
@pomworm has a thread of safe messaging apps there and has not been made aware of Briar's existence as far as i can tell
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