Methods and systems for secure sharing of data between user devices using a proxy re-encryption key (PRE)

A cloud storage system comprising of a plurality of user devices, at least one cloud device and a key server, wherein the key server is to:

• share at least one encrypted data object of a plurality of encrypted data objects of at least one first user device of the plurality of user devices stored in the at least one cloud device with at least one second user device of the plurality of user devices using a Proxy Re-Encryption (PRE);
• and
• wherein the at least one first user device is to:
• generate a master secret key (MSK) at a time of registration with the key server;
• use the generated MSK to derive a private key;
• use a key generation function of the PRE to derive a public key;
• derive a master secret key encryption key (MSKEK) to encrypt the MSK using at least one of a password and a passphrase selected by a user of the at least one first user device;
• share the encrypted MSK and the public key with the key server for storage;
• obtain and decrypt the encrypted MSK from the key server with at least one of the password and the passphrase selected by the user of the at least one first user device;
• derive the private key corresponding to the at least one first user device using the decrypted MSK;
• retrieve a public key corresponding to the at least one second user device from the key server;
• derive the re-key using the private key corresponding to the at least one first user device, the public key corresponding to the at least one second user device and a re-key generation function of the PRE;
• and
• share the re-key with the key server marking as a key from the at least one first user device to the at least one second user device for sharing the at least one encrypted data object stored in the at least one cloud device with the at least one first user device.

A user device of a cloud storage system comprising of a plurality of user devices, at least one cloud device and a key server, wherein the user device is to:

• upload a plurality of encrypted data objects and an associated encrypted file encryption key (FEK) to the at least one cloud device;
• share at least one encrypted data objects of the plurality of encrypted data objects of the user device stored in the at least one cloud device with at least one other user device of the plurality of user devices through the key server using a Proxy-Re-Encryption (PRE);
• generate a master secret key (MSK) at a time of registration with the key server;
• use the generated MSK to derive a private key;
• use a key generation function of the PRE to derive a public key;
• derive a master secret key encryption key (MSKEK) to encrypt the MSK using at least one of a password and a passphrase selected by a user of the user device;
• and
• share the encrypted MSK and the public key with the key server for storage;
• obtain and decrypt the encrypted MSK from the key server with at least one of the password and the passphrase selected by the user of the user device;
• derive the private key corresponding to the user device using the decrypted MSK;
• retrieve a public key corresponding to the at least one other user device from the key server;
• derive the re-key using the private key corresponding to the user device, the public key corresponding to the at least one other user device and a re-key generation function of the PRE;
• share the re-key with the key server marking as a key from the user device to the at least one other user device for sharing the at least one encrypted data object stored in the at least one cloud device with the user device;
• and
• share at least one re-encrypted data of the user with the at least one other user device through the key server using the PRE.