There are two scenarios for this mode. First, In case of internet shutdown, blackouts, in places and people with no internet connection. Devices with telegram can still send and receive messages using other wireless technologies (e.g. Bluetooth). Second, just for messaging off the common grids (internet from mobile and wifi networks) to have an extra private mode of communication; for goverment institutions, campuses, companies, labs, public events, etc.
How can it work? In this mode a device with telegram is a node of the off-grid network. Nodes casts its own messages along with received messages from other nodes. A message then jumps from node to node until it reaches its recipient, when this happens, the successful receiver node no longer cast this message but a short acknowledging answer that will indicate for the sender node a successful reception and for the others nodes indicates deleting that message from their cast batch.
Security: Every node has a particular and unique key associated with its telegram account and also has a key list from their contacts list. A message is encrypted using the sender and receiver node keys. At reception of messages the node will try to read all messages using its key and each key from its key list. At unsuccessful opening the messages will be cast.
Key list: users can add its own personalized keys using a short text and emojis and set various keys both for sending and receiving messages.
Public cast: to send messages without encryption. Nodes can set on/off the reading and casting of this messages. Acknowledging reception messages are automatic public cast whose reception cannot be disable. A public message is cast after reading.
Public focused cast: to send message without sender encryption key but with receiver encryption key. Nodes can set on/off reading and casting this messages. Message is read if the receiver encryption key is in its key list.
Telegram app share: allow to send the app to other devices.
Telegram off-grid account: allows to open a new local account with no conection with telegram servers and only relaying on the off-grid network. It can be set to synchronize with telegram servers once internet connection is restablished.
Online syncronization and support of off-grid messages: (can be set on/off) It is necessary a pool in the server for unsuccessful off-grid messages batches. if a node gets online (common internet) it can push their previous off-grid messages batches to the pool. Successful off-grid messages will syncronize in users server history and won't be in the pool. Unsuccessful off-grid messages will be stored in the server pool. Nodes that get online will ask the server pool for messages addressed to them, will read it and delete it from the server pool. All the other messages that haven't being read in the pool will be cast by this online nodes in the off-grid network. This function allow people without internet to communicate with the support of online users. Libraries, hospitals, parks, etc can have a permanent online off-grid-node.
**It sounds like a whole new app inside another app but an standalone app whit this characteristics won't work for most of the scenarios (who is going to download an app to communicate in case of blackout? What is the point if only you have it?) Telegram, among the communication apps, is possibly the only candidate for a functionality like this.