How does one "authenticate" a bitcoin user?
There are different KYC (Know Your Customer) laws in different jurisdictions, but generally it takes information that is difficult or very time consuming to falsify. License/ID scans, utility bills, and SSNs are examples

Suggesting "hashing" as a KYC solution suggests you do not know what hashing is (unless you were referring to the way a server should store such confidential information). I suggest you use the search function or explore bitcoin wikipedia to learn more.