Messenger group chats can be easily manipulated – is Delta Chat affected too? [transcript consistency]

[SWEETGOOD]

I read this (German only) topic at heise.de and I wonder if Delta Chats group chats are also affected by this security flaw?

Is Delta Chat sending a new message to every user or does it behave differently? Does somebody know anything about it?

https://www.heise.de/news/Verschluesselt-aber-falsch-Gruppenchats-anfaellig-fuer-manipulierte-Inhalte-11384095.html

[link2xt]

The paper described is at https://raw.githubusercontent.com/sbaresearch/transcript-consistency/main/paper.pdf (referenced in the end of your article). I have not read it yet, but short answer is yes, you can send different messages to different users in a group chat, or selectively not send the copy to some users, and send different messages with the same Message-ID to different users.

“Transcript consistency” is not a property Delta Chat offers, and it was already known before that it is not offered by WhatsApp (2018) and Signal (2014). While centralized messengers can offer transcript consistency, and e.g. MLS protocol tries to, this comes at the cost of availability, and most messengers don’t even try to, it is “not part of the threat model”.

Imagine netsplit situation, where half of the group members ended up on some “intranet”, and the rest on the larger part of the internet. In Delta Chat users can still send messages to the group and see the messages from the users that are in the same part of the network, assuming they have some “local” relays in their relay list. But obviously the users will see the chat completely differently during the netsplit. If you want to ensure transcript consistency, i.e. that users always see the chat the same way, you will have to decide which part gets the “real” group chat and somehow block the other part of the users from using the chat and tell them they have to wait until they can join the chat again.

In general, Delta Chat assumes that you are in a group with your friends/peers and not in a group with adversaries that tries to trick you.

See also my comment at another topic (and the whole topic, it also comes from the assumption that someone in the same chat is trying to “attack” others by crafting a message that will decrypt differently for different users):

Security concern re: group message wrapping (invisible salamanders attack)

We have already responded this in that thread: While it may be interesting to raise the issue of the lack of key commitment in OpenPGP circles and it may be useful for some applications, in case of Delta Chat usage for messaging there is no need to do this attack because there is no transcript consistency in the first place - you can simply create two different messages with the same Message-ID and send different message to different group members. Providing transcript consistency would requir…

[link2xt]

Also topic about another paper by the same authors:

Careless Whisper on DeltaChat

There is security research paper “Careless Whisper: Exploiting Silent Delivery Receipts to Monitor Users on Mobile Instant Messengers” https://arxiv.org/pdf/2411.11194 Video highlighting key points: https://youtu.be/B9Syj555RQc This research was targeted on other messengers, but the same method probably may be used for gathering information on DC clients. I’m curious, what can be achieved with ‘hidden’ system messages on DC?