Surf provides end-to-end encryption in chats for users through their private keys. Messages are not stored in the Free TON Blockchain.

Currently TON Labs uses a database where all messages are encrypted based on the curve25519-xsalsa20-poly1305 asymmetric algorithm.

The database for the time being, is centralized, which is not a critical problem, as only you have the private key needed to decrypt your messages, but in the future messages will be stored in a decentralized Free TON Storage database.