The present disclosure provides a system for storing encrypted data. The system comprises a server and a plurality of clients. A first client of the plurality of clients is configured to send to the server a first reference value calculated from data to be encrypted and stored. The server is configured to determine a group of second clients from the plurality of clients, the second clients having each sent to the server data with a second reference value equal to the first reference value. The group of second clients is configured to perform a passive key exchange protocol with the first client, and the server is configured to determine, based on a result of the passive key exchange protocol, whether the data is to be stored in full or as deduplicated data.