System and method for transferring the right to decode messages

• US 7,286,665 B1
• Filed: 12/21/1999
• Issued: 10/23/2007
• 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): Vu; Kim
• Assistant Examiner(s): TRUVAN, LEYNNA THANH
• Application Number:
  US09/468,703
• Time in Patent Office: 2,863 Days
• Field of Search: 713/165, 713170-171, 713200-201, 713/189, 713/193, 209/225, 209/228, 380/228, 380277-278, 380281-285, 380 28- 30, 380/259, 380/268, 380/44, 726 2- 5, 726 26- 30
• US Class Current: 380/30
• CPC Class Codes: H04L 2209/76 Proxy, i.e. using intermediary entity to perform cryptographic operations network architectures or network communication protocols using hop-by-hop encryption H04L63/0464
• CPC Class Codes: H04L 9/002 Countermeasures against attacks on cryptographic mechanisms network architectures or network communication protocols for protection against malicious traffic H04L63/1441
• 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 encrypting an original message to be passed to a recipient by way of a grantor, the method comprising the steps of:

obtaining an encrypted message representative of the original message, the encrypted message having been encrypted with a public key corresponding to the grantor according to a public key encryption scheme;

generating a public proxy key based on a private key corresponding to the recipient and on the private key corresponding to said grantor, wherein said grantor'"'"'s private key and said recipient'"'"'s private key are combined, and the combination of the private keys is based on said public key encryption scheme and provides that it is computationally difficult to recover the recipient'"'"'s private key from the public proxy key even with the knowledge of the grantor'"'"'s private key; and

applying the public proxy key to the encrypted message to transform the encrypted message into a transformed message, wherein the transformed message is decryptable by the recipient using information selected from the private key corresponding to the recipient and the available public key information, and wherein the encrypted message remains in an encrypted state while being transformed into the transformed message and is not decrypted to the original message and re-encrypted at any point during the transformation.

2. The method of claim 1, wherein the encrypted message has been encrypted with an ElGamal encryption scheme.

3. The method of claim 1, wherein the encrypted message has been encrypted with a modified ElGamal encryption scheme.

4. The method of claim 1, wherein the receiving, generating, and applying steps are performed by the grantor.

5. The method of claim 1, further comprising the step of providing the transformed message to the recipient.

6. The method of claim 5, further comprising the step of decrypting the transformed message using information selected from the private key corresponding to the recipient and any available public information.

7. The method of claim 5, further comprising the step of decrypting the transformed message using the private key corresponding to the recipient.

8. The method of claim 2, wherein the encrypted message comprises a first portion and a second portion, the first portion encoding a generator and a random key, and the second portion encoding the original message, the public key corresponding to the grantor, and the random key.

9. The method of claim 8, wherein the applying step operates on the second portion of the encrypted message.

10. The method of claim 3, wherein the encrypted message comprises a first portion and a second portion, the first portion encoding the original message, a generator, and a random key, and the second portion encoding the public key corresponding to the grantor and the random key.

11. The method of claim 10, wherein the applying step operates on the second portion of the encrypted message.

12. The method of claim 4, wherein the encrypted message comprises a first portion and a second portion, the first portion encoding the original message, a generator, and a random key, and the second portion encoding the public key corresponding to the grantor and the random key.

13. The method of claim 12, wherein the applying step operates on the second portion of the encrypted message.

14. The method of claim 1, wherein the original message is passed to a recipient through at least one additional intermediate grantor by repeating the generating and applying steps for each additional intermediate grantor.

24. The method of claim 1, wherein the encrypted message has been encrypted with a Cramer-Shoup encryption scheme.

25. The method of claim 1, wherein it is computationally difficult to recover the grantor'"'"'s private key from the public proxy key.

26. The method of claim 1, wherein said public encryption scheme is a discrete-logarithm-based encryption scheme, wherein said combination of said private keys comprises using the modular difference of both private keys as an exponent in a modular exponentiation.

27. The method of claim 4, further comprising a step of obtaining the recipient'"'"'s private key by the grantor.

28. The method of claim 1, further comprising implementing the method with one or more hardware or software devices configured to perform the method.

29. The method of claim 1, further comprising implementing the method with one or more computer-readable instructions embedded on a computer-readable medium and configured to cause one or more computer processors to perform the method.

15. A method for encrypting an original message to be passed to a recipient by way of a grantor, the method comprising the steps of:

obtaining an encrypted message representative of the original message, the encrypted message having been encrypted with a public key corresponding to the grantor according to a public key encryption scheme;

generating a public proxy key based on a public key corresponding to the recipient and on the private key corresponding to the public key of said grantor, wherein said grantor'"'"'s private key and said recipient'"'"'s public key are combined, and the combination of said grantor'"'"'s private key and said recipient'"'"'s public key is based on said public key encryption scheme; and

applying the public proxy key to the encrypted message to transform the encrypted message into a transformed message, wherein the transformed message is decryptable by the recipient using information selected from the private key corresponding to the recipient'"'"'s public key and the available public key information, and wherein the encrypted message remains in an encrypted state while being transformed into the transformed message and is not decrypted to the original message and re-encrypted at any point during the transformation.

16. The method of claim 15, wherein the encrypted message has been encrypted with a Cramer-Shoup encryption scheme.

17. The method of claim 15 wherein the transformed message is decryptable by the recipient using a private key corresponding to the recipient.

18. The method of claim 15, wherein the original message is passed to a recipient through at least one additional intermediate grantor by repeating the steps of generating and applying for each additional intermediate grantor.

19. The method of claim 15 wherein it is computationally difficult to recover the grantor'"'"'s private key from the public proxy key.

20. The method of claim 15, wherein the encrypted message has been encrypted with an ElGamal encryption scheme.

21. The method of claim 15, wherein the encrypted message has been encrypted with a modified ElGamal encryption scheme.

22. The method of claim 15, wherein the receiving, generating, and applying steps are performed by the grantor.

23. The method of claim 22, further comprising a step of obtaining said recipient'"'"'s private key by said grantor.

30. The method of claim 15, further comprising implementing the method with one or more hardware or software devices configured to perform the method.

31. The method of claim 15, further comprising implementing the method with one or more computer-readable instructions embedded on a computer-readable medium and configured to cause one or more computer processors to perform the method.