Yes, it's possible. To be honest, I find it very sad that we have grown so dependent on ISP and big telecom companies to have a working network.

In theory, you could have an infrastructure in your neighborhood and be able to play Quake with your neighbors without making use of the phone line at all, completely free of monthly fees and with a very efficient and fast connection too! you'd just need cabling connecting the apartments/houses and some decent routers controlling/restricting access on each subnet. It's a pity that's not a standard thing when designing residences.

Though less efficient and more limited in range, you can technically do it with Wifi and mesh networking too... there are projects like B.A.T.M.A.N (https://www.open-mesh.org/), however, it's not very user-friendly to set up. I believe there have been some projects that attempted to launch embedded devices to act as mini routers for this, but the spread has not been wide enough to make it worth it, sadly.

Briar has a mesh mode. And i think there was a matrix app doing this too?

If you don't want to use internet the only ways are to use radio or deploy your own network infrastructure (optic fiber or cell tower), so there's no really any messaging app that can be used without internet. Briar can use Bluetooth but with a limited range, needing an actual dense mesh network.

Meshtastic can be encrypted and is LoRa based. Can easily hit nodes dozens of miles away with a good line of sight. It also relays messages across nodes to reach even further distances.

Check out Reticulum Network Stack using LORA radio. Works really well.

http://reticulum.network/

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.

The first thing that comes to mind is Meshtastic: https://meshtastic.org/

This one works, if you don't mind a little diy and texting only: https://circuitmess.com/products/chatter-lora-communication-device

I mean this is a terrible answer, but DS pictochat fits that

Besides the already mentioned [Briar] (https://briarproject.org/), there's Berty, can't speak to its quality since I never used it, but I always found the project neat in and of itself

Thereis stuff like

https://meshtastic.org/

https://unsigned.io/private-messaging-over-lora/

scratch telegram off that list, but Session messenger there instead.

Telegram isn't private, one guy has the master key to the whole thing

wouldn't a cheap walkie-talkie be more practical in that situation?

i remember using this app some time ago ☞ https://f-droid.org/packages/org.jsl.wfwt/

yes, a lot of people were using those kinds of apps during the free hong kong protests, they go from device-to-device with no internet in between.

No idea what the app is called, but apps like those exist

There used to be one years ago that used WiFi radios or Bluetooth or whatever so you could chat to people near you... I totally forget what it was called though.

Isn't this what airdrop was?