Delta Chat

Summary

Delta Chat is a messenger built on email transport: IMAP receives messages and SMTP sends them. Its current default is a profile on a chatmail relay, with a randomly generated address, rather than an existing personal mailbox. Mobile and desktop clients are available. The transport standards are documented in the chatmail core repository; the current help explains profiles and relay choice.

How it works

Autocrypt distributes encryption information, OpenPGP protects message contents, and Secure-Join establishes contacts through QR codes or invite links. These are separate jobs: opportunistic key exchange alone does not authenticate a contact against a malicious mail provider.

Delta Chat's guaranteed encryption announcement explains the extension of its earlier verified-group model to direct chats: a verified connection must not silently fall back to plaintext. Groups formed from verified contacts inherit that protection. The subsequent security analysis and fixes cover attacks on that setup and group membership information. These older announcements explain the guarantee; their interface markers are not instructions for today's app.

The current rule is clearer: default chatmail relays reject messages without end-to-end encryption. A classic email server can still carry unencrypted messages, which Delta Chat marks with an email icon. Do not extend chatmail's guarantee to arbitrary email correspondence. Chatmail's relay documentation also distinguishes encrypted message forwarding from a conventional mailbox.

Trust model / threat model

The devices and the contact invitation are the trust boundary for content. Mail relays still see sender and recipient addresses and message sizes; the relay you connect to also sees your IP address. Group details and attachments are inside the encrypted message. This reduces exposed metadata without making the transport anonymous; compare metadata resistance and SimpleX.

Readiness

Usable. Documented mobile, desktop and multi-device workflows make this a practical messenger. Start with a chatmail profile and establish contacts using the documented invitation flow.

Limits

The current FAQ explicitly says perfect forward secrecy is not yet supported. A leaked private decryption key can expose previously recorded ciphertext. Planned Autocrypt changes are not a present guarantee. Endpoint compromise and recipients retaining messages also remain outside transport encryption's protection.

For the broader choice, see Choosing a private messenger.

Primary sources

• Delta Chat help: encryption, metadata, profiles and forward secrecy.
• Transport and encryption standards used by chatmail.
• Guaranteed encryption and verified groups.
• Secure-Join security analysis and fixes.
• Chatmail relay guarantees and interoperability.