This is a longstanding issue with Telegram, and deceptive advertising that a lot of people have an issue with. They use client-server encryption sure, but unlik

This is a longstanding issue with Telegram, and deceptive advertising that a lot of people have an issue with. They use client-server encryption sure, but unlike Signal or WhatsApp, they do not employ end-to-end encryption by default; you must enable it for two person chats, and it is not available for group chats. Unless something has changed, you get full chat history when you log in from a new device or are invited to a pre-existing group, meaning that the chat history is stored on their servers, and if not in plain text it can be unencrypted for new group members or devices.