Surely the specific use case could be achieved in some privacy respecting way?
For example, you could send hashes of the user's contacts, they could be compared to each other, but not reverse engineered?
For example, you could send hashes of the user's contacts, they could be compared to each other, but not reverse engineered?
Hashes with salts are also useless for preserving privacy, but at slightly higher than zero computational cost.