There's some really interesting stuff we've been looking into on the Matrix side to solve this - e.g. https://github.com/asonnino/arke https://github.com/asonni

There's some really interesting stuff we've been looking into on the Matrix side to solve this - e.g. https://github.com/asonnino/arke https://github.com/asonnino/arke aka https://eprint.iacr.org/2023/1218 https://eprint.iacr.org/2023/1218 or https://martin.kleppmann.com/2024/07/05/pudding-user-discovery-anonymity-networks.html https://martin.kleppmann.com/2024/07/05/pudding-user-discove.... Meanwhile, Matrix for now does support hashed contact lookup, although few clients implement it given the privacy considerations at https://spec.matrix.org/unstable/identity-service-api/#security-considerations https://spec.matrix.org/unstable/identity-service-api/#secur...