Require TLS 1.3 on client-facing ports

[link2xt]

I tested with -tls1_2 option
of openssl s_client
that TLS 1.2 connections
are no longer possible
on any ports except port 25.

Port 25 requires at least TLS 1.2
for encrypted connections.

[hpk42]

I think "client-facing" means here Delta <-> Chatmail server (submission and imap ports) and not MTA SMTP (port 25) server-to-server connections.

…

On Mon, Nov 11, 2024 at 17:58 -0800, feld wrote: I'm not up to speed on how widespread TLSv3 is for SMTP right now. Could this break the ability to send messages to other mail servers that don't support TLSv3? -- Reply to this email directly or view it on GitHub: #454 (comment) You are receiving this because you are subscribed to this thread. Message ID: ***@***.***>

[missytake]

I'm a bit hesitant to approve this; some people with outdated desktop clients might have followed our instructions in https://delta.chat/en/2023-12-13-chatmail to setup a nine account. Yes, it's an unlikely scenario, but for them it would break.

And I don't see why it's necessary? Wouldn't any more recent official client use TLSv1.3 anyway? No need to force it server-side.

[link2xt]

I'm a bit hesitant to approve this; some people with outdated desktop clients might have followed our instructions in https://delta.chat/en/2023-12-13-chatmail to setup a nine account. Yes, it's an unlikely scenario, but for them it would break.

We do not support non-delta chat clients and any recent client supports TLS 1.3. It has existed for more than 5 years.

And I don't see why it's necessary? Wouldn't any more recent official client use TLSv1.3 anyway? No need to force it server-side.

It makes development easier because I don't have to check if openssl-s_client or python IMAP library or core has by chance decided to connect using TLS 1.2. Security-wise this avoids downgrade attacks. TLS 1.3 downgrade protection is a hack to make it compatible with TLS 1.2 and by the time it detects downgrade it has already finished TLS 1.2 handshake, exposing server TLS certificate to middleboxes, so it's not prefect. Not having TLS 1.2 on the server makes it impossible to even start connecting with TLS 1.2.

[link2xt]

Let's postpone this until we make Delta Chat releases that use Rustls to connect to chatmail servers. Otherwise Delta Chat on Windows or macOS may not support TLS 1.3. Unlikely because macOS forces users to upgrade all the time and minimum supported version is Windows 10 due to Electron requirements.