If you don’t trust a communication channel, you could always do a Diffie-Hellman key exchange in the clients which lets you create a shared encrypted channel between two parties by sharing public keys. This works even if they are trying to monitor you.
It can work with simplex channels if you have the public key of the receiver. Both parties just need to know each other's public keys to create encrypted communication. After exchanging the public keys, it can be one-way communication.
I guess it wouldn't work for a one-to-many channel though, just individual one-to-one channels.
