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Telegram doesn't have encryption for groups though.

Actually they do: https://telegram.org/faq#q-so-how-do-you-encrypt-data , but since group chats are using Telegram's servers the encryption is not client-to-client. Here is the actual specification of how does their srever-client encryption work: https://core.telegram.org/mtproto

>> “ since group chats are using Telegram's servers the encryption is not client-to-client”

That is, it is not end-to-end-encrypted (E2EE) — which is the whole point of apps like this.