Re-encryption-based digital rights management

1. A method for encrypting a document, the method comprising:

generating and delivering a first nonce to a license manager;

generating and delivering a second nonce to a rights manager;

combining the first nonce received at the license manager and the second nonce received at the rights manager to generate a secret key that is deterministically defined by the first and second nonces;

delivering the generated secret key;

receiving and encrypting the document using the secret key to produce an encrypted document and a document identifier; and

delivering the document identifier to the rights manager.

2. The method of claim 1 , wherein the combination of the first and second nonces is performed as an exclusive-OR (XOR) function.

3. The method of claim 1 , wherein encrypting the document further comprises

generating a pre-license by encrypting the first nonce for the license manager with a public key of the license manager.

4. A method for decrypting an encrypted document, the method comprising:

receiving the encrypted document at a user device;

sending a request to a rights manager to access the encrypted document;

receiving an encrypted second nonce from the rights manager when the rights manager determines to grant the user device access to the encrypted document,

wherein the encrypted second nonce is calculated by encrypting a second nonce with a public key of the user device;

receiving, at the user device, a token from the rights manager signed with a private key of the rights manager,

wherein the rights manager sends the token to a license manager when the rights manager determines to grant the user device access to the encrypted document;

receiving, at the user device, a first nonce from a license manager,

wherein the license manager sends the first nonce to the user device when the token is received at the license manager from the rights manager;

generating a decryption key to decrypt the encrypted document by combining the first nonce with the encrypted second nonce; and

decrypting the encrypted document using the decryption key.

5. The method of claim 4 , further comprising

determining by the license manager whether the token received from the rights manager is valid as defined by a standard.

6. The method of claim 4 , wherein the license manager also receives a pre-license from the rights manager,

wherein the pre-license is generated by encrypting the first nonce with a public key of the license manager.

7. The method of claim 6 , further comprising

receiving a license from the license manager when the pre-license and the token are determined to refer to a same document,

wherein the license is generated at the license manager by decrypting the pre-license using a private key of the license manager and re-encrypting the decrypted pre-license using the public key of the user device.

8. The method of claim 7 , wherein generating the decryption key by combining the first nonce with the encrypted second nonce comprises:

decrypting the license with a private key of the user device;

decrypting the encrypted second nonce with the private key of the user device; and

performing an exclusive OR function of the decrypted license with the decrypted second nonce.

9. A system for encrypting a document, the system comprising:

a first random number generator to generate and deliver a first nonce to a license manager;

a second random generator to generate and deliver a second nonce to a rights manager;

a key generator to receive and combine the first and second nonces into a secret key that is deterministically defined by the first and second nonces; and

an encryptor to receive and encrypt the document using the secret key to produce an encrypted document, and to generate a document identifier,

wherein the document identifier is delivered to and stored in the rights manager.

10. The system of claim 9 , wherein the key generator combines the first and second nonces by performing an exclusive-OR (XOR) function on the first and second nonces.

11. The system of claim 9 , wherein the encryptor generates a pre-license by encrypting the first nonce for the license manager with a public key of the license manager.

12. The system of claim 11 , wherein the encryptor sends the pre-license and the document identifier to the rights manager.

13. A system for decrypting an encrypted document, the system comprising:

a user device including a decryptor and a key generator, the user device to receive the encrypted document and a document identifier, and to send a request to a rights manager to access the encrypted document,

wherein the rights manager sends to a license manager an authorization to generate a license for the encrypted document;

the key generator to receive an encrypted second nonce from the rights manager, when the rights manager determines to grant the user device access to the encrypted document, wherein the encrypted second nonce is calculated by encrypting a second nonce with a public key of the user device,

the key generator to receive a first nonce from the license manager, when the rights manager determines to grant the user device access to the encrypted document,

the key generator to generate a decryption key by combining the first and second nonces, and

the decryptor to decrypt the encrypted document using the decryption key to produce a decrypted document.

14. The system of claim 13 , wherein the request sent to the rights manager includes a certificate of the user device to the rights manager.

15. The system of claim 14 , wherein the user device receives a token from the rights manager signed with a private key of the rights manager,

wherein the token includes certificate of the user device and the document identifier.

16. The system of claim 15 , wherein the rights manager generates a pre-license by encrypting the first nonce with a public key of the license manager,

wherein the user device receives the token, the pre-license, and the encrypted second nonce from the rights manager.

17. The system of claim 16 , wherein the license manager also receives the pre-license from the rights manager.

18. The system of claim 16 , wherein the user device receives a license from the license manager when the pre-license and the token are determined to refer to a same document,

wherein the license is generated at the license manager by decrypting the pre-license using a private key of the license manager and re-encrypting the decrypted pre-license using the public key of the user device.

19. The system of claim 13 , wherein the key generator generates the decryption key as a combination of: (a) decrypting the license with a private key of the user device; and (b) decrypting the encrypted second nonce with the private key of the user device.

20. The system of claim 19 , wherein the combination includes an exclusive OR function of the decrypted license with the decrypted second nonce.