Well it's pretty easy as Telegram doesn't encrypt anything at all and stores all contacts and message history server side. (yes, there is a secret chat feature

Well it's pretty easy as Telegram doesn't encrypt anything at all and stores all contacts and message history server side. (yes, there is a secret chat feature which is encrypted, but unlike Signal it can only connect with a single client and you can't even choose which one so it's very frustrating in practice when chatting with people who have more than 1 client setup)