ID Collisions and Notifications
Typically, user IDs are unique identifiers and have the property of being matched to one user. This allows the users to be strongly identified, based upon an ID alone, making it easy to tell which user is which.
When sending messages to users, the xx Network intentionally has users share their IDs to receive messages–the sender ID details are removed when the message is sent through the xx Network.
The goal here is to hide the identification details of which user receives a message and prevent a special type of tagging attack possible within the x protocol. You can read about the tagging attack in cMixx Tagging Attack.
All users have a unique reception ID where they receive messages. When you add a user as a contact, one can see their ID and use it at any time to calculate their Ephemeral ID for a specific moment. Ephemeral IDs rotate once per day, at a random time unique to the recipient.
Ephemeral IDs are calculated within a small bit space that is designed to allocate at any given time roughly half as many IDs as the number of users in the network. This ensures that users will be receiving messages meant for other users to guarantee further privacy. However, due to end-to-end encryption, users cannot read these messages or access any data about the sender or recipient.
At launch, and at any point where usage drops below a few hundred thousand users, the number of users will be insufficient for this system to function, requiring alternate protections.
Notifications
The notifications system relies on shared ephemeral IDs to keep notifications private. When a user registers for notifications, a server run by the xx Network team tracks when their current ephemeral ID receives a message and sends a notification. Then their phone processes the notification to detect if the message is actually for the user and will notify if it is. This bot, as well as the notifications provider (currently available for Apple and Google operating systems), do not know when a message is for you due to ID collisions, ensuring message content is private.
Cryptographic Primitives Summary
| Algorithm | Length | Description |
|---|---|---|
| Ephemeral ID | 64 bits (max) | Weak identification of a network user. Designed such that collisions are probable. |
| User ID | 264 bits | Strong identification of a network user. Designed such that one’s user ID is unique under the Birthday Paradox. |
| BLAKE2B | 256 bits | Used for ID generation (ephemeral and user IDs). |