The invention relates to a method for storing data blocks from client devices to a cloud storage system, the method includes the steps of: d) storing an encrypted first data block and a challenge of the first data block of a first client device on the cloud storage system, e) determining if a hash of a second data block of a second client device stored on the cloud storage system equals the hash of the first data block, f) if yes, transmitting the challenge of the first data block from the cloud storage system to the second client device, g) extracting, at the second client device, the bits at the positions or at the range contained in the challenge, hashing the extracted bits, encrypting the hashed bits with a public key of the first client device or of the second client device and uploading the encrypted bits from the second client device to the cloud storage system, and h) storing the encrypted bits from the second client device on the cloud storage system.