Briar (Android Only) - Uses Bluetooth, Wifi, or Internet via Tor to communicate. You can theretically create a large mesh group with enough users. Think of protests where the government shuts down the internet. Downside is, bluetooth range is 10 meters 😓.

Also: You can send encrypted text over SMS using Secure Space Encryptor (SSE) (known as Paranoia Text Encryption on iOS). It's an Open Source app that can encrypt text.

1. Type text
2. Copy the Ciphertext
3. Send and tell the recipient to use SSE fo decrypt.

You both have to share a password/passphrase over a secure channel, then use that to encrypt and decrypt.

Or PGP (there are mobile apps), but they aren't quantum resistant. If someone intercept and stores them, it could be decrypted later. So I recomment Symmetric Encryption like AES 256 (so use SSE for better security, since they use AES 256)

You can also encrypt a radio:

Rattlegram is an app on iOS/Android that alllows converting text to audio and play it over your phone’s speaker.

As mentioned before, SSE.

1. Use SSE to encrypt text
2. Copy-Paste the Ciphertext to Rattlegram
3. Sent it over the radio
4. On the other end, use Raddlegram to turn the audio back to the ciphertext
5. Use SSE to decrypt.

Voila! Off-Grid Encrypted communications.

Warning: Encryption over Ham Radio bands is illegal in many countries 😉 (but fuck the law lol, who cares)

There's also Meshtastic, but it has much shorter range, but, in the USA at least, they aren't "Ham Radio" so they (supposedly) can be encrypted legally.