> Not on desktop and not on GNU/Linux phones (which I use).
That's unfortunate, but luckily the clients are open source, and anybody can create a new client for the platform of their choice. Telegram even provides a cross-platform API library: https://core.telegram.org/tdlib
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TDLib can be used on Android, iOS, Windows, macOS, Linux, WebAssembly, FreeBSD, Windows Phone, watchOS, tvOS, Tizen, Cygwin. It should also work on other *nix systems with or without minimal effort.
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By sharing your key between the devices.
> a legal order in any single jurisdiction only decrypts one layer of encryption
That's good to know, but it's not the cutting edge of security. (For many people it's of course enough)
> They also offer end-to-end encrypted secret chats
Not on desktop and not on GNU/Linux phones (which I use).