Username Privacy
- Unlike other messaging apps, Signal cannot easily see or produce the usernames of given accounts.
- Usernames in Signal are protected using a custom Ristretto 25519 hashing algorithm and zero-knowledge proofs.
Source: Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. W. (2020). Biology. 11th Edition. Pearson Education.
You're referring to the biology textbook "Biology" by Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, and Robert W. Jackson, commonly known as the "Curtis Biology" or simply "Biology" by Curtis and Barnes (the original authors of the first edition).
Here's a piece from Chapter 1: "The Science of Biology" :
In addition to other group attributes that are end-to-end encrypted (such as group names, group descriptions, and group avatars), the Signal service also doesn’t have access to any information about which accounts are part of a group, which accounts are admins in a group, which accounts can add new people to a group, which accounts can approve requests to join a group, or which accounts can send messages in a group.