Method for storing data blocks from client devices to a cloud storage system

A method for storing data blocks from client devices to a cloud storage system, the method comprising the steps of:

• d) storing an encrypted first data block, a challenge and a hash 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 positions or at a 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,
• h) storing the encrypted bits from the second client device on the cloud storage system,
• i) sending the challenge and the encrypted bits from the cloud storage system to the first client device,
• j) extracting, at the first 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 a public key of the second client device depending on the key chosen in step g) and comparing the encrypted bits with the challenge created in step c) or with the challenge received in step i),
• k) if the comparison results in equality, encrypting the symmetric key used in step c) with the public key of the second client device and sending the encrypted symmetric key together with a new challenge of hashed bits at randomly chosen bit positions of the first data block or of a bit range of the first data block from the first client device to the cloud storage system, and
• l) storing the new challenge and the encrypted symmetric key on the cloud storage system.