Decryption of retransmitted data in an encrypted communication system

• US 5,809,148 A
• Filed: 05/17/1996
• Issued: 09/15/1998
• Est. Priority Date: 05/17/1996
• Status: Expired due to Term

• Current Assignee: Motorola Solutions, Inc.
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Doberstein, Kevin G., Pappas, Scott J., Bright, Michael W.
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): White, Carmen D.
• Application Number:
  US08/649,098
• Time in Patent Office: 851 Days
• Field of Search: 380/42, 380/37, 380/48, 380/44
• US Class Current: 713/161
• CPC Class Codes: H04L 1/1809 Selective-repeat protocols
• CPC Class Codes: H04L 9/0637 Modes of operation, e.g. cipher block chaining [CBC], electronic codebook [ECB] or Galois/counter mode [GCM]

Abstract

The method generally includes the steps of receiving (201) a message (100) comprising a message indicator (101) and a plurality of encrypted blocks (103 through 111). When at least one block of the plurality of encrypted blocks is received with error, a second message is transmitted (215), which second message requests a retransmission of the at least one block. Upon receiving the retransmission of the at least one block without error, a keystream is generated (219) from the message indicator and the at least one block is decrypted using the keystream.

Claims

1. A method comprising the steps of:

receiving a message comprising a message indicator and a plurality of encrypted blocks, wherein the plurality of encrypted blocks comprises L blocks, and wherein L is an integer greater than or equal to 2;

when at least an Nth block of the plurality of encrypted blocks is received with error, transmitting a second message requesting a retransmission of at least the Nth block, where N is an integer having a value between 1 and L;

upon receiving the retransmission of at least the Nth block without error, generating a keystream from the message indicator and decrypting the Nth block using the keystream wherein the keystream decrypts at least the Nth block but not all of the plurality of encrypted blocks.

2. The method of claim 1, further comprising the step of generating a keystream from the message indicator for use in decrypting the plurality of encrypted blocks before the step of transmitting.

3. The method of claim 2, further comprising the step of, when a first block of the plurality of encrypted blocks is received without error, decrypting the first block using the keystream.

4. The method of claim 1, wherein the step of decrypting comprises the step of decrypting only the Nth block using the keystream.

5. The method of claim 1, further comprising the step of storing an encryption state every Kth block of the plurality of encrypted blocks, where K is a positive integer.

6. The method of claim 1, further comprising the step of storing the message indicator.

7. The method of claim 6, wherein when receiving the retransmission of the at least one block without error, the step of generating includes generating the keystream from the stored encryption state and decrypting the at least one block using the keystream generated from the stored encryption state.

8. The method of claim 1, wherein the message indicator contains an initialization vector, further comprising the step of generating a keystream by executing an encryption algorithm operating in an output-feedback mode of operation N multiplied by B iterations for an N block message, where N and B are positive integers.

9. The method of claim 1, wherein the message indicator contains a linear feedback shift register vector, generating a keystream by executing an encryption algorithm operating in a counter-addressing mode of operation N multiplied by B iterations for an N block message, where N and B are positive integers.

10. A method comprising the steps of:

receiving a message comprising a message indicator and a plurality of encrypted blocks;

generating a keystream from the message indicator for use in decrypting the plurality of encrypted blocks;

when at least one block of the plurality of encrypted blocks is received with error, transmitting a second message requesting a retransmission of the at least one block;

upon receiving the retransmission of the at least one block without error, decrypting the at least one block using the keystream, wherein the plurality of encrypted blocks was encrypted sequentially, and the plurality of encrypted blocks is decrypted in an order other than sequentially.

11. The method of claim 10, further comprising the step of, when a first block of the plurality of encrypted blocks is received without error, decrypting the first block using the keystream.

12. The method of claim 10, wherein the step of decrypting comprises the step of decrypting only the Nth block using the keystream.

13. The method of claim 10, wherein the message indicator contains an initialization vector, further comprising the step of generating a keystream by executing an encryption algorithm operating in an output-feedback mode of operation N multiplied by B iterations for an N block message, where N and B are positive integers.

14. The method of claim 10, wherein the message indicator contains a linear feedback shift register vector, generating a keystream by executing an encryption algorithm operating in a counter-addressing mode of operation N multiplied by B iterations for an N block message, where N and B are positive integers.

15. A method comprising the steps of:

receiving a message comprising a message indicator and a plurality of encrypted blocks;

storing an encryption state every Kth block of the plurality of encrypted blocks, where K is a positive integer;

when at least one block of the plurality of encrypted blocks is received with error, transmitting a second message requesting a retransmission of the at least one block;

upon receiving the retransmission of the at least one block without error,

a) generating a keystream from the stored encryption state without generating

a keystream for all of the plurality of encrypted blocks;

b) decrypting the at least one block using the keystream.

16. The method of claim 15, further comprising the step of, when a first block of the plurality of encrypted blocks is received without error, decrypting the first block using the keystream.

17. The method of claim 15, further comprising the step of, when all of the plurality of encrypted blocks are received without error, decrypting the plurality of encrypted blocks using the keystream.

18. The method of claim 15, wherein the message indicator contains an initialization vector, further comprising the step of generating a keystream by executing an encryption algorithm operating in an output-feedback mode of operation N multiplied by B iterations for an N block message, where N and B are positive integers.

19. The method of claim 15, wherein the message indicator contains a linear feedback shift register vector, generating a keystream by executing an encryption algorithm operating in a counter-addressing mode of operation N multiplied by B iterations for an N block message, where N and B are positive integers.

20. A method comprising the steps of:

receiving a message comprising a message indicator and a plurality of encrypted blocks;

storing the message indicator;

when at least one block of the plurality of encrypted blocks is received with error,

transmitting a second message requesting a retransmission of the at least one block;

upon receiving the retransmission of the at least one block without error,

a) calculating an encryption state from the stored message indicator;

b) generating a keystream from the encryption state;

c) decrypting the at least one block using the keystream,

wherein the plurality of encrypted blocks was encrypted sequentially, and the plurality of encrypted blocks is decrypted in an order other than sequentially.

21. The method of claim 20, further comprising the step of, when a first block of the plurality of encrypted blocks is received without error, decrypting the first block using the keystream.

22. The method of claim 20, further comprising the step of, when all of the plurality of encrypted blocks are received without error, decrypting the plurality of encrypted blocks using the keystream.

23. The method of claim 20, wherein the step of calculating comprises the steps of:

determining a block number of the at least one block, yielding a positive integer N;

determining how many bits are in each block of the plurality of encrypted blocks, yielding a positive integer B;

multiplying N by B, yielding a number of iterations;

determining a generator polynomial for a linear feedback shift register;

combining the number of iterations with the generator polynomial, yielding the encryption state for the beginning of the Nth block.