Encryption Type: Secret Chats utilize true end-to-end encryption (E2EE). This is the "gold standard" for privacy, meaning:
Messages are encrypted on the sender's device and can only be decrypted by the recipient's device.
Telegram (the company) has no access to the content of Secret Chats. They are not stored on Telegram's servers (only on the devices of the participants).
Role of Phone Number in Key Exchange: The phone number's role in Secret Chats is indirect and primarily related to the identity of the participants, rather than directly being part of the cryptographic key material itself.
Identity Verification: The phone number, combined telegram number database with the User ID, identifies the two parties who are attempting to establish a Secret Chat. Telegram's servers act as a conduit to initiate the Secret Chat setup between the two user accounts.
Diffie-Hellman Key Exchange: Once a Secret Chat is initiated between two identified users (identified by their User IDs, which are tied to their phone numbers), the cryptographic key exchange process begins. This uses a Diffie-Hellman key exchange protocol.
Each participant's device generates a unique, ephemeral (short-lived) secret key.
They exchange public keys (over Telegram's servers, but securely wrapped within the MTProto session).
Both devices then use their own secret key and the other party's public key to independently compute a shared secret key for that specific Secret Chat. This shared secret key is never transmitted.
Telegram provides a visual representation of this shared key (an "encryption key visualization" or "emoticon sequence"). Users are encouraged to compare this fingerprint with their chat partner out-of-band (e.g., in person, via another secure channel) to verify that no Man-in-the-Middle (MITM) attack has occurred. If the fingerprints match, they can be confident they are talking directly to each other.
Device-Specific: Secret Chats are strictly device-specific. If you start a Secret Chat on your phone, it will not appear on your tablet or desktop Telegram app. This is because the encryption keys are stored only on the devices involved in that specific chat, not on Telegram's servers.