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Signal and Telegram are both mobile applications that "try" to ensure secure chats.

Signal uses end-to-end encryption, Telegram does this as well (in 'secret chats') but it does not use end-to-end encryption in group chats.

How come? Is it difficult to implement?

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    Not too big a worry as Telegram has already been shown to have garbage encryption.
    – Fiasco Labs
    Mar 18, 2016 at 18:13

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As far as guessing anyones motivation for not doing something goes:

Yes, especially group chat is not easy to implement as secure key sharing in groups is quite a challenge when you consider what happens if anyone leaves the group for example.

Threema does (claim to) securely encrypt group chats, as does Tox. As Tox is open source, you may want to have a look there. Given: their mobile apps are in beta right now (at least for iOS).

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Please specify your question, are you asking wether Telegram does not use end-to-end encryption or does not use multi-party encryption or do you want to know if its difficult to implement?

Encryption is hard and especially group chat encryption is nothing to underestimate. Signal implementes a flavor of OTR, called Multi-party Off-the-Record Messaging (end-to-end encrypted group chat).

Telegram "Secret" Chats are end-to-end encrypted using an own protocol called MTProto, "Normal" Chats and Group-Chats on the other side are not end-to-end encrypted, allowing users to use multiple devices for the same chat and download chatlogs on multiple devices too.

Seemingly, the developers of Telegram value the usability higher than messaging security.

Take a look at the Whisper Systems Blog Post and the MPOTR Whitepaper: https://whispersystems.org/blog/private-groups/ http://www.cypherpunks.ca/~iang/pubs/mpotr.pdf

Also related: Are Telegram secret chats secure assuming MTProto isn't?

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  • Telegram allows you to use different clients with the same account at the same time, e.g. Telegram on your mobile and Telegram Desktop. So at any given time, a new "party" (new client from existing user) can join.
    – Johannes Kuhn
    Mar 18, 2016 at 11:59
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    You are describing the telegram (not-end-to-end) encrypted chats. A new client is able to join and accessing the vanilla chat and chatlogs. Logically this would not be possible if the chats are end-to-end encrypted. Possibly the developers value this feature higher than messaging security.
    – hub
    Mar 18, 2016 at 12:03
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    Exactly. It's always a weighting between usability and security. Telegrams end-to-end encrypted chats ("secret chats") are bound to the devices that were used to start that chat.
    – Johannes Kuhn
    Mar 18, 2016 at 13:46

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