Here's a fun toy problem: A company wants to design a service with three operations. Signup(username, password), SetData(username, password, new_data) and GetD

Here's a fun toy problem: A company wants to design a service with three operations. Signup(username, password), SetData(username, password, new_data) and GetData(username). The client app is assumed to be trusted, but any server (no more than one though) could be compromised. Design a protocol so that the compromise does neither allow an attacker to update someone elses data, return wrong data, nor cause a denial of service.