There are no Delta Chat messenger servers. Each profile is an email account: Delta Chat sends each message via SMTP and receives via IMAP, so any address that has both protocols works, with no phone number and no registration. Chats and regular mail share the account, and Delta Chat stores the chats in a separate DeltaChat folder; a user who prefers a quiet inbox can disable the “Watch Inbox folder” option or use a filter rule to move chats to a sub-address. Chatmail is a network of relays based on Dovecot and Postfix, where Delta Chat generates a random address and password and the user only provides a nickname: the profile is created with one click. Chatmail relays allow only encrypted mail, messages are usually removed after a week or so, and accounts are removed after ninety days of no activity.
Delta Chat uses a subset of OpenPGP for encryption and automatically exchanges keys via Autocrypt. On chatmail relays, unencrypted messages cannot leave at all; on classic email servers they can, and Delta Chat marks them with an email icon. There is no padlock or other indicator on encrypted messages. OpenPGP is based on long-lived keys, so once a key is leaked, all past traffic is compromised — Delta Chat has no forward secrecy yet, and the Autocrypt v2 specification is designed to introduce it in conjunction with post-quantum algorithms. The subject line of all chat messages is always “…”, and servers can still see who wrote to whom and when. When a QR code is scanned or an invite link is opened, the Secure-Join protocol runs and the contact gets a green checkmark. Checkmarked contacts can vouch for others: anyone who adds a checkmarked contact to an all-checkmarked group becomes an “introducer”, and tapping the “Introduced by” line in a profile repeatedly brings you back to the one person you scanned face to face.
A webxdc app is a small web app bundled into a file and added to a chat as an attachment; everyone in the chat opens the app in Delta Chat, and the copies communicate with each other using encrypted messages. The apps have no internet connection of their own, meaning that a shared chess game, calendar, poll or document cannot reach any server. One profile is used on multiple devices simultaneously, and pairing a second device is done via QR scan, with a copy and paste fallback several menus deep. There is no history stored on the Delta Chat servers, and a newly paired device has no chats until the user transfers a backup, which is not encrypted on its own.
The daily use of the email substrate is visible. Messages can take from seconds to minutes between server hops, and an aggressive spam filter can kill one altogether. Delta Chat does not try to attach files larger than the provider’s size limit. Audio and video calls are not available, since the email network cannot support them. On iPhone, instant push is only possible when the chatmail relay pushes a wake-up via Apple’s Push Notification service; other clients talk IMAP IDLE directly.
