Checking possession of a file at the local system of a user
Sometimes a user possess a file which already exists at the remote cloud. The user wants the cloud to store the file for him. Therefore the file can be marked as belonging to the user, provided that the user proves its possession. This can be done effectively without communication overhead required for sending a whole file copy from the user to the server. Idea: reversed regular proof of possession.

The protocol
User:

• Computes a descriptor \(d\) of the file \(f\).
• Sends d to the user.

• Locates the file \(f\) via \(d\).
• Generates \(a\) random challenge \(c\) for the file \(f\).
• Sends \(c\) to the user.

• Computes the proof: \(p = H(f \parallel c)\)
• Sends p to the owner.

• Accepts if \(p == H(f \parallel c)\).