‘cMixx+’ – Enhanced Privacy & Speed in Various Use Cases

On November 17th 2023, a paper was published by the Foundation, authored by David Chaum and Mário Yaksetig, which is a proposed update to the cMixx mix network currently employed by the XX Network, called cMixx+.

The update would give the network the ability to mix batch sizes other than 1,000 messages (potentially much more) and proposes a new format for these new messages which would not require user keys to be passed.

The use case cited by Mário when announcing this was for user authentication. As an example, presently, there may be privacy eroding implications with fixed batch sizes whereby if an application provider is having to split up an authentication database over multiple batches in order for a user to verify they exist within it, it may be possible to use batch number data by the application owner to match a specific part of their authentication database being used and therefore potentially identify who is authenticating. With this new parameter/message format, a large amount of data, such as a user database could be randomised/mixed and sent across the network in one go, so that only the user who authenticates knows they have done so, and potentially allows them to modify their record without revealing they did to anyone else.

In introducing this paper, Mário commented on the XX Telegram:

“The main idea is that we tweaked the protocol slightly and we now have a property where we can essentially have a never ending mix that can contain one (or multiple) databases that are constantly being shuffled (mixed).

Think routing every single iris code reading from Worldcoin without knowing who is being scanned or what the actual iris code is. Companies can now keep an encrypted version of the hashed passwords so that no one can even know which hashes of passwords the users in the system have, while allowing for users to successfully authenticate themselves…”

There are potentially many more use cases for a constantly-mixed dataset which will come into fruition as they are imagined. In conversing with Mário to understand this update better, he implied there already is one which he couldn’t yet talk about. On first look at the paper however, it seems there is a glaring one in the first paragraph and beyond (within the context of a CBDC in conjunction with a private ledger) – whether it was this use case or not he referred to remains to be seen!

The update to cMixx will be trialled by volunteer node runners with a view to becoming live on the XX Network as soon as possible.

You can read / download the paper here:

cmixxplusDownload

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