... is impossible. Here's why:
According to Telegram, even seizing the data center and physically accessing the servers will not let you see private conversations of users. Each cluster (a unit of storage) is encrypted with a unique key that is stored in another cluster under a different jurisdiction.
Secret chats go even further — they use the end-to-end encryption. The access key used for private chats is stored only on the user device and nowhere else. That's why Telegram declares that it is impossible to decrypt the data transmitted between the Telegram users.
The icing on the cake is the decentralization which Telegram employed long before the blockchain hype. The messenger data is stored on servers located in different parts of the world. Due to the absence of a single server, the user data can move between the server vaults and remain inaccessible to intruders, whoever they'd be.