Follow-up for https://github.com/chatmail/core/pull/7042, part of
https://github.com/chatmail/core/issues/6884.
This will make it possible to create invite-QR codes for broadcast
channels, and make them symmetrically end-to-end encrypted.
- [x] Go through all the changes in #7042, and check which ones I still
need, and revert all other changes
- [x] Use the classical Securejoin protocol, rather than the new 2-step
protocol
- [x] Make the Rust tests pass
- [x] Make the Python tests pass
- [x] Fix TODOs in the code
- [x] Test it, and fix any bugs I find
- [x] I found a bug when exporting all profiles at once fails sometimes,
though this bug is unrelated to channels:
https://github.com/chatmail/core/issues/7281
- [x] Do a self-review (i.e. read all changes, and check if I see some
things that should be changed)
- [x] Have this PR reviewed and merged
- [ ] Open an issue for "TODO: There is a known bug in the securejoin
protocol"
- [ ] Create an issue that outlines how we can improve the Securejoin
protocol in the future (I don't have the time to do this right now, but
want to do it sometime in winter)
- [ ] Write a guide for UIs how to adapt to the changes (see
https://github.com/deltachat/deltachat-android/pull/3886)
## Backwards compatibility
This is not very backwards compatible:
- Trying to join a symmetrically-encrypted broadcast channel with an old
device will fail
- If you joined a symmetrically-encrypted broadcast channel with one
device, and use an old core on the other device, then the other device
will show a mostly empty chat (except for two device messages)
- If you created a broadcast channel in the past, then you will get an
error message when trying to send into the channel:
> The up to now "experimental channels feature" is about to become an officially supported one. By that, privacy will be improved, it will become faster, and less traffic will be consumed.
>
> As we do not guarantee feature-stability for such experiments, this means, that you will need to create the channel again.
>
> Here is what to do:
> • Create a new channel
> • Tap on the channel name
> • Tap on "QR Invite Code"
> • Have all recipients scan the QR code, or send them the link
>
> If you have any questions, please send an email to delta@merlinux.eu or ask at https://support.delta.chat/.
## The symmetric encryption
Symmetric encryption uses a shared secret. Currently, we use AES128 for
encryption everywhere in Delta Chat, so, this is what I'm using for
broadcast channels (though it wouldn't be hard to switch to AES256).
The secret shared between all members of a broadcast channel has 258
bits of entropy (see `fn create_broadcast_shared_secret` in the code).
Since the shared secrets have more entropy than the AES session keys,
it's not necessary to have a hard-to-compute string2key algorithm, so,
I'm using the string2key algorithm `salted`. This is fast enough that
Delta Chat can just try out all known shared secrets. [^1] In order to
prevent DOS attacks, Delta Chat will not attempt to decrypt with a
string2key algorithm other than `salted` [^2].
## The "Securejoin" protocol that adds members to the channel after they
scanned a QR code
This PR uses the classical securejoin protocol, the same that is also
used for group and 1:1 invitations.
The messages sent back and forth are called `vg-request`,
`vg-auth-required`, `vg-request-with-auth`, and `vg-member-added`. I
considered using the `vc-` prefix, because from a protocol-POV, the
distinction between `vc-` and `vg-` isn't important (as @link2xt pointed
out in an in-person discussion), but
1. it would be weird if groups used `vg-` while broadcasts and 1:1 chats
used `vc-`,
2. we don't have a `vc-member-added` message yet, so, this would mean
one more different kind of message
3. we anyways want to switch to a new securejoin protocol soon, which
will be a backwards incompatible change with a transition phase. When we
do this change, we can make everything `vc-`.
[^1]: In a symmetrically encrypted message, it's not visible which
secret was used to encrypt without trying out all secrets. If this does
turn out to be too slow in the future, then we can remember which secret
was used more recently, and and try the most recent secret first. If
this is still too slow, then we can assign a short, non-unique (~2
characters) id to every shared secret, and send it in cleartext. The
receiving Delta Chat will then only try out shared secrets with this id.
Of course, this would leak a little bit of metadata in cleartext, so, I
would like to avoid it.
[^2]: A DOS attacker could send a message with a lot of encrypted
session keys, all of which use a very hard-to-compute string2key
algorithm. Delta Chat would then try to decrypt all of the encrypted
session keys with all of the known shared secrets. In order to prevent
this, as I said, Delta Chat will not attempt to decrypt with a
string2key algorithm other than `salted`
BREAKING CHANGE: A new QR type AskJoinBroadcast; cloning a broadcast
channel is no longer possible; manually adding a member to a broadcast
channel is no longer possible (only by having them scan a QR code)
`login_param` module is now for user-visible entered login parameters,
while the `transport` module contains structures for internal
representation of connection candidate list
created during transport configuration.
This is similar to old `dcaccount:` with URL,
but creates a 9-character username on the client
and avoids making an HTTPS request.
The scheme is reused to avoid the apps
needing to register for the new scheme.
`http` support is removed because it was
not working already, there is a check
that the scheme is `https` when the URL
is actually used and the core has
no way to make HTTP requests without TLS.
It's used in `fetch_existing_msgs()`, but we can remove it and tell users that they need to
move/copy messages from Sentbox to Inbox so that Delta Chat adds all contacts from them. This way
users will be also informed that Delta Chat needs users to CC/BCC/To themselves to see messages sent
from other MUAs.
The motivation is to reduce code complexity, get rid of the extra IMAP connection and cases when
messages are added to chats by Inbox and Sentbox loops in parallel which leads to various message
sorting bugs, particularly to outgoing messages breaking sorting of incoming ones which are fetched
later, but may have a smaller "Date".
We already have both rand 0.8 and rand 0.9
in our dependency tree.
We still need rand 0.8 because
public APIs of rPGP 0.17.0 and Iroh 0.35.0
use rand 0.8 types in public APIs,
so it is imported as rand_old.
Flakiness was introduced in e7348a4fd8.
This change removes a call to joining_chat_id() which created a chat,
now we check for existing group chat
without creating it if it does not exist.
Context: PR #7116 is backwards-incompatible with versions older than
v2.21, and since the release hasn't reached all users yet, we currently
can't release from main; for details see #7326.
Issue #7326 explains how we can make this less breaking, but this only
works if many contacts are verified. So, this PR here proposes to
postpone the stricter rules for who is verified a bit:
- Set verification timeout for invite codes to 1 week (this is still
stricter than no timeout at all, which we had in the past)
- Don't reset indirect verifications yet
In a few months (when everyone has v2.22.0), we can revert the PR here,
then.
---------
Co-authored-by: l <link2xt@testrun.org>
We do not try to delete resent messages
anymore. Previously resent messages
were distinguised by having duplicate Message-ID,
but future Date, but now we need to download
the message before we even see the Date.
We now move the message to the destination folder
but do not fetch it.
It may not be a good idea to delete
the duplicate in multi-device setups anyway,
because the device which has a message
may delete the duplicate of a message
the other device missed.
To avoid triggering IMAP busy move loop
described in the comments
we now only move the messages
from INBOX and Spam folders.
Make it return the correct value for non-Group chats.
This also should improve performance, thanks to the fact that
we now don't have to query all the chat's contacts.
Instead we only need confirm that self-contact
is among the group members, and only when the chat type is `Group`.
tokio-tar is unmaintained and has unpatched CVE-2025-62518.
More details on CVE are in <https://edera.dev/stories/tarmageddon>.
tokio-tar is only used for transferring backups
and worst case is that by manually inspecting
a carefully crafted backup user will not see
the same files as get unpacked when importing a backup.
This way, the statistics / self-reporting bot will be made into an
opt-in regular sending of statistics, where you enable the setting once
and then they will be sent automatically. The statistics will be sent to
a bot, so that the user can see exactly which data is being sent, and
how often. The chat will be archived and muted by default, so that it
doesn't disturb the user.
The collected statistics will focus on the public-key-verification that
is performed while scanning a QR code. Later on, we can add more
statistics to collect.
**Context:**
_This is just to give a rough idea; I would need to write a lot more
than a few paragraphs in order to fully explain all the context here_.
End-to-end encrypted messengers are generally susceptible to MitM
attacks. In order to mitigate against this, messengers offer some way of
verifying the chat partner's public key. However, numerous studies found
that most popular messengers implement this public-key-verification in a
way that is not understood by users, and therefore ineffective - [a 2021
"State of Knowledge" paper
concludes:](https://dl.acm.org/doi/pdf/10.1145/3558482.3581773)
> Based on our evaluation, we have determined that all current E2EE
apps, particularly when operating in opportunistic E2EE mode, are
incapable of repelling active man-in-the-middle (MitM) attacks. In
addition, we find that none of the current E2EE apps provide better and
more usable [public key verification] ceremonies, resulting in insecure
E2EE communications against active MitM attacks.
This is why Delta Chat tries to go a different route: When the user
scans a QR code (regardless of whether the QR code creates a 1:1 chat,
invites to a group, or subscribes to a broadcast channel), a
public-key-verification is performed in the background, without the user
even having to know about this.
The statistics collected here are supposed to tell us whether Delta Chat
succeeds to nudge the users into using QR codes in a way that is secure
against MitM attacks.
**Plan for statistics-sending:**
- [x] Get this PR reviewed and merged (but don't make it available in
the UI yet; if Android wants to make a release in the meantime, I will
create a PR that removes the option there)
- [x] Set the interval to 1 week again (right now, it's 1 minute for
testing)
- [ ] Write something for people who are interested in what exactly we
count, and link to it (see `TODO[blog post]` in the code)
- [ ] Prepare a short survey for participants
- [ ] Fine-tune the texts at
https://github.com/deltachat/deltachat-android/pull/3794, and get it
reviewed and merged
- [ ] After the next release, ask people to enable the
statistics-sending
If we use modules (which are actually namespaces), we can use shorter names. Another approach is to
only use modules for internal code incapsulation and use full names like deltachat-ffi does.
