Method and apparatus for improving the security of an electronic codebook encryption scheme utilizing a data dependent encryption function

• US 5,452,358 A
• Filed: 02/08/1994
• Issued: 09/19/1995
• Est. Priority Date: 02/08/1994
• Status: Expired due to Term

• Current Assignee: Apple Computer Incorporated (Apple Inc.)
• Original Assignee: Apple Computer Incorporated (Apple Inc.)
• Inventors: Chu, Ke-Chiang, Normile, James O.
• Primary Examiner(s): Cain, David C.
• Application Number:
  US08/193,248
• Time in Patent Office: 588 Days
• Field of Search: 380/4, 380/21, 380/28, 380/29, 380/42, 380/43, 380/44, 380/45, 380/46, 380/47
• US Class Current: 380/42
• CPC Class Codes: H04L 9/0662 with particular pseudorandom sequence generator

Abstract

An apparatus and method for improving the security of an electronic codebook encryption scheme comprises a transmitter unit for encoding or encrypting data and a separate authorized receiver for decoding or decrypting data. Both the transmitter and receiver share a common secret key that has been communicated through a separate channel.

The transmitter includes a Mapping Table (MTable) that comprises a table of functions, where a first series of incrementally numbered expressions are equated with a second series of randomly generated numbers. The plaintext is input into the transmitter and applied to the MTable to provide ciphertext. The PN generator supplies a unique random sequence of binary digits. The ciphertext byte and random number byte associated with each byte of plaintext data are exchanged to change the relationships within the MTable.

An Inverse Mapping Table (IMTable) is included in the receiver and is created by reversing the relationships between the sequential numbers and the mapping numbers in the MTable. From the encrypted communication, the ciphertext is separated and the respective functions for each ciphertext byte and random number byte are again exchanged, and the associated plaintext byte and random number byte are further exchanged to produce an MTable identical to that of the transmitter and unencrypted plaintext data identical to that originally encoded.

Claims

1. An apparatus for the communication of encrypted data, the apparatus comprising a transmitter for encoding plaintext data, the transmitter comprising:

a first Pseudorandom Number (PN) generator for generating and outputting a first Pseudorandom Number (PN) sequence; and

a Mapping Table (MTable) comprising a plurality of storage positions, the MTable having inputs for receiving plaintext data and the first PN sequence and an output for communicating encrypted ciphertext, the ciphertext being produced by the data dependent mapping of plaintext data.

2. The apparatus according to claim 1 in which the transmitter MTable maps the plaintext data to produce ciphertext according to the function:

space="preserve" listing-type="equation">Ciphertext(i)=MTable[Plaintext(i)].

3. The apparatus according to claim 1 in which the transmitter MTable is initialized using a secret key and the numbers from the first PN sequence.

4. The apparatus according to claim 1 wherein the transmitter MTable substitutes values between the plaintext data and the first PN sequence to produce ciphertext.

5. The apparatus according to claim 1 wherein the transmitter MTable reorders data between storage positions following the mapping of plaintext data to produce ciphertext.

6. The apparatus according to claim 4 wherein the transmitter MTable substitutes values according to the function:

space="preserve" listing-type="equation">Swap;

MTable[Plaintext(i)] and MTable[PN Seq(i)].

7. The apparatus according to claim 5 wherein the transmitter MTable reorders data according to the function:

space="preserve" listing-type="equation">Swap;

MTable[Plaintext(i)] and MTable[PN Seq(i)].

8. The apparatus according to claim 1 further comprising a receiver for decoding the encoded plaintext produced by the transmitter, the receiver comprising:

a second PN generator for generating and outputting a second PN sequence, wherein the first and second PN sequences are identical; and

an Inverse Mapping Table (IMTable) comprising a plurality of storage positions, the IMTable having an output and having inputs for receiving the ciphertext data from the transmitter and the second PN sequence, the IMTable storing the inverse of the MTable contained in the transmitter for reversing the mapping of the ciphertext in order to produce the original plaintext data.

9. The apparatus according to claim 8 wherein the receiver further includes an MTable from which the IMTable is constructed, the MTable comprising a plurality of storage positions and having inputs for receiving the decoded ciphertext from the IMTable, the MTable reversing the mapping of the decoded ciphertext and the second PN sequence to produce the original plaintext data.

10. The apparatus according to claim 8 wherein the IMTable reverse maps the ciphertext to produce decoded plaintext data in accordance with the function:

space="preserve" listing-type="equation">Plaintext(i)=IMTable[Ciphertext(i)].

11. The apparatus according to claim 9 wherein the MTable is initialized using the second PN sequence.

12. The apparatus according to claim 11 wherein the IMTable reorders data according to the function:

space="preserve" listing-type="equation">Swap;

IMTable[Ciphertext(i)] and IMTable[MTable[PN Seq(i)]].

13. The apparatus according to claim 12 wherein following the reorder function of claim 9, the MTable further reorders according to the function:

space="preserve" listing-type="equation">Swap;

MTable[Plaintext(i)] and MTable[PN Seq(i)].

14. The apparatus according to claim 8 wherein the first PN generator and the second PN generator are both initialized using a secret key which is commonly utilized by both the transmitter and the receiver.

15. The apparatus according to claim 8 wherein the transmitter and receiver are implemented using at least one general purpose computer.

16. A data dependent encryption system comprising:

means for generating a first Pseudorandom Number (PN) sequence; and

encoding means for receiving and storing in a Mapping Table (MTable) plaintext data and the PN sequence and for encrypting the plaintext data to produce ciphertext by encoding the data according to the function;

space="preserve" listing-type="equation">Ciphertext(i)=MTable[Plaintext(i)].

17. The data dependent encryption system of claim 16 in which, following the encoding of the plaintext data, the MTable is reordered according to the function:

space="preserve" listing-type="equation">Swap;

MTable[Plaintext(i)] and MTable[PN Seq(i)].

18. The data dependent encryption system of claim 17, further comprising:

decoding means for receiving the ciphertext and the PN sequence and for reversing the encoding of the ciphertext.

19. The data dependent encryption system of claim 18, further comprising:

means for generating a second PN sequence.

20. The data dependent encryption system of claim 18 in which the ciphertext data is imported by an Inverse Mapping Table (IMTable) within the decoding means and the ciphertext is decoded according to the function:

space="preserve" listing-type="equation">Plaintext(i)=IMTable[Ciphertext(i)].

21. The data dependent encryption system of claim 18 in which, following the decoding of the ciphertext, the IMTable and MTable is reordered according to the function:

space="preserve" listing-type="equation">Swap;

IMTable[Ciphertext(i)] and IMTable[MTable[PN Seq(i)]]; and

space="preserve" listing-type="equation">Swap;

MTable[Plaintext(i)] and MTable[PN Seq(i)].

22. A computer implemented method for encoding a sequence of plaintext data, the method comprising the steps:

(i) generating a first Pseudorandom Number (PN) sequence;

(ii) initializing a Mapping Table (MTable) using the first PN sequence;

(iii) receiving plaintext data into the MTable;

(iv) encrypting plaintext data to produce ciphertext according to the function;

space="preserve" listing-type="equation">Ciphertext(i)=MTable[Plaintext(i)];

(v) reordering the MTable in accordance with the function;

space="preserve" listing-type="equation">Swap;

MTable[Plaintext(i)] and MTable[PN Seq(i)];

(vi) exporting the encrypted communication; and

(vii) repeating steps (iii) through (vi) until the sequence of plaintext data has been encrypted.

23. The method for encoding data according to claim 22 further including a method for decoding the encrypted communication comprising the steps:

(i) generating a second PN sequence;

(ii) initializing a Mapping Table (MTable) using the second PN sequence and construct and Inverse Mapping Table (IMTable) from the MTable;

(iii) receiving the encrypted communication into the IMTable;

(iv) decrypting the received encrypted communication to produce plaintext in accordance with the function;

space="preserve" listing-type="equation">Plaintext(i)=IMTable[Ciphertext(i)];

(v) reordering the MTable and IMTable in accordance with the functions;

space="preserve" listing-type="equation">Swap;

IMTable[Ciphertext(i)] and IMTable[MTable[PN Seq(i)]], and

space="preserve" listing-type="equation">Swap;

MTable[Plaintext(i)] and MTable[PN Seq(i)]; and

(vi) repeating steps (iii) through (v) for all available encrypted communication.

24. The method according to claim 23 wherein the first PN sequence and second PN sequence are identical.

25. The method according to claim 22 wherein the first PN sequence is generated by combining a Pseudorandom Number (PN) generator and a secret key.

26. The method according to claim 23 wherein the second PN sequence is generated by combining a Pseudorandom Number (PN) generator and a secret key.