System and method for protecting data files by periodically refreshing a decryption key

• US 6,937,726 B1
• Filed: 12/21/1999
• Issued: 08/30/2005
• Est. Priority Date: 04/06/1999
• Status: Expired due to Fees

• Current Assignee: ContentGuard Holdings, Inc. (Pendrell Corporation)
• Original Assignee: ContentGuard Holdings, Inc. (Pendrell Corporation)
• Inventors: Wang, Xin
• Primary Examiner(s): Barrón, Gilberto
• Assistant Examiner(s): KIM, JUNG W
• Application Number:
  US09/468,747
• Time in Patent Office: 2,079 Days
• Field of Search: 380/44, 380/277, 380/28, 380/30
• US Class Current: 380/30
• CPC Class Codes: H04L 2209/603 Digital right managament [DRM]
• CPC Class Codes: H04L 2209/76 Proxy, i.e. using intermediary entity to perform cryptographic operations
• CPC Class Codes: H04L 9/0891 Revocation or update of secret information, e.g. encryption key update or rekeying
• CPC Class Codes: H04L 9/0897 involving additional devices, e.g. trusted platform module [TPM], smartcard or USB
• CPC Class Codes: H04L 9/3013 involving the discrete logarithm problem, e.g. ElGamal or Diffie-Hellman systems

Abstract

Methods for transferring among key holders in encoding and cryptographic systems the right to decode and decrypt messages in a way that does not explicitly reveal decoding and decrypting keys used and the original messages. Such methods are more secure and more efficient than typical re-encoding and re-encryption schemes, and are useful in developing such applications as document distribution and long-term file protection.

Claims

1. A method for protecting a data file on a computer system, comprising the steps of:

providing a grantee'"'"'s encryption key, a grantee'"'"'s decryption key, a grantor'"'"'s encryption key, and a grantor'"'"'s decryption key;

using asymmetric encryption, encrypting the data file using the grantor'"'"'s encryption key to create an encrypted data file;

generating a transformation key from the grantor'"'"'s decryption key, the grantee'"'"'s encryption key and other data which is data file independent;

transforming the encrypted data file with the transformation of the encrypted data file does not reveal the data file during the process of transforming;

providing the transformed encrypted data file to the grantee; and

decrypting the transformed encrypted file by the grantee with the grantee'"'"'s decryption key;

wherein the transformation key does not allow the grantee to determine the grantor'"'"'s decryption key.

2. The method of claim 1, further comprising the step of repeating the generating step, and transforming step and the providing step on a periodic basis.

7. The method of claim 1, wherein said generating step comprises generating the transformation key from the grantor'"'"'s decryption key, the grantee'"'"'s encryption key, and the ciphertext of the encrypted data file.

8. The method of claim 1 wherein said generating step comprises generating the transformation key from the grantor'"'"'s decryption key, the grantee'"'"'s encryption key, and a random variable.

9. The method of claim 1 wherein said generating step comprises generating the transformation key from the grantor'"'"'s decryption key, the grantee'"'"'s encryption key, the ciphertext of the encrypted data file, and a random variable.

3. A processor-driven system adapted to protect a data file, the system comprising:

a processor; and

a memory coupled to the processor for storing the data file;

wherein the processor is programmed to perform the steps of;

providing a grantee'"'"'s encryption key, a grantee'"'"'s decryption key, a grantor'"'"'s encryption key, and a grantor'"'"'s decryption key;

using asymmetric encryption, encrypting the data file using the grantor'"'"'s encryption key to create an encrypted data file;

generating a transformation key from the grantor'"'"'s decryption key, the grantee'"'"'s encryption key and other data which is data file independent;

transforming the encrypted data file with the transformation key to generate a transformed encrypted data file wherein the transforming does not reveal the data file during the process of transforming;

providing the transformed encrypted data file to the grantee; and

decrypting the transformed encrypted file by the grantee with the grantee'"'"'s decryption key;

wherein the transformation key does not allow the grantee to determine the grantor'"'"'s decryption key.

4. The processor-driven system of claim 3, further comprising a communication interface.

5. The processor-driven system of claim 3, wherein the processor and the memory are included within a portable device.

6. The processor-driven system of claim 3, wherein the processor and the memory are included within a smart card.

10. The processor driven system of claim 3, wherein said generating step comprises generating the transformation key from the grantor'"'"'s decryption key, the grantee'"'"'s encryption key, and the ciphertext of the encrypted data file.

11. The processor driven system of claim 3, wherein said generating step comprises generating the transformation key from the grantor'"'"'s decryption key, the grantee'"'"'s encryption key, and a random variable.

12. The processor driven system of claim 3, wherein said generating step comprises generating the transformation key from the grantor'"'"'s decryption key, the grantee'"'"'s encryption key, the ciphertext of the encrypted data file, and a random variable.