Design a privacy-preserving crowd sensing system.

Design a privacy-preserving crowd sensing system. This system works in three steps:
Step 1: a remote data collector publishes sensing tasks to all mobile users (e.g., through smartphones carried by users) in the system. A task contains a task identifier and a task description.
Step 2: the mobile users who are interested in a task request the collector to assign the task to them and the collector approves some of requests.
Step 3: those mobile users who are assigned the task submit sensing data to the collector.For privacy reasons, each user does not want the collector to know which tasks she/he has requested and submitted data for.
For trust reasons, the collector wants to make sure only authorized users (i.e., those in the system) can request task and each user can only request each task once. Also only the users assigned a task can submit data for this task. Design a protocol to meet these goals, and clearly describe how it works in each step. Your protocol can have some initialization step before the above Step 1. Here,collusion between authorized users and outside users is not considered. Also, you can assume that no user will be compromised and controlled by the attacker.