Secret Chats
Official telegram.org Source
When a secret chat is created, the participating devices exchange encryption keys using the so-calledDiffie-Hellman key exchange. After the secure end-to-end connection has been established, we generate a picture that visualizes the encryption key for your chat. You can then compare this image with the one your friend has — if the two images are the same, you can be sure that the secret chat is secure, and noman-in-the-middle attackcan succeed.
Newer versions of Telegram apps will show a larger picture along with a textual representation of the key (this is not the key itself, of course!) when both participants are using an updated app.
Always compare visualizations using a channel that is known to be secure — it's safest if you do this in person, in an offline meeting with the conversation partner.
Official Telegram Reference: https://telegram.org/faq
Part of the TGWAY.com Official Telegram Knowledge Base (98 Verified Q&As).