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)
this PR adds the type DC_QR_SOCKS5_PROXY to `dc_check_qr()` for
**supporting telegram proxy QR codes**. if returned, the UI should ask
the user if they want to us the proxy and call
`dc_set_config_from_qr();` afterwards (plus maybe `dc_configure()`).
idea is to improve our proxy story, follow ups may be:
- in UI, - move proxy out of "Account & Password", as a **separate
"Proxy Activity"** (it should stay in "Advanced" for now, however, below
"Server", which might be moved up)
- allow **opening the "Proxy Activity" from the welcome screens**
three-dot-menu (that would also solve a long standing issue that
entering the email address bypasses the proxy
- show proxy usage in the "Connectivity View" and/or add an **icon** to
the main chatlist screen (beside three-dot menu) in case some proxy is
in use; tapping this icon will open the "Proxy Activity"
- the the new "Proxy Activity", add a **share / show proxy QR code**
button. that would generate invite links in the form
`https://i.delta.chat/socks#...` - so that tapping then opens the app.
support for these links need to be added to core then.
- handle a list of proxies in core, offer selection in UI. the list
could be one for all profiles and could be filled eg. by normal invite
links or other channels
---------
Co-authored-by: iequidoo <117991069+iequidoo@users.noreply.github.com>
New protocol streams .tar into iroh-net
stream without traversing all the files first.
Reception over old backup protocol
is still supported to allow
transferring backups from old devices
to new ones, but not vice versa.
This adds functionality to send and receive a backup over the network
using a QR code.
The sender or provider prepares the backup, sets up a server that
waits for clients. It provides a ticket in the form of a QR code
which contains connection and authentication information.
The receiver uses the QR code to connect to the provider and fetches
backup, restoring it locally.
This makes the contact ID its own newtype instead of being a plain
u32. The change purposefully does not yet try and reap any benefits
from this yet, instead aiming for a boring change that's easy to
review. Only exception is the ToSql/FromSql as not doing that yet
would also have created churn in the database code and it is easier to
go straight for the right solution here.
