There are two issue with hashing I can envisage: 1. Nothing user has is truly hash-able, (email can be replaced, there are people with the same name/dob/place

There are two issue with hashing I can envisage: 1. Nothing user has is truly hash-able, (email can be replaced, there are people with the same name/dob/place of birth, address is not permanent attribute...) 2. Hash key can have duplicates - so those collisions would block different users (probably not for small companies but for FB with 2 billion users something worth considering.)