Digital image scrambling for image coding systems

• US 6,505,299 B1
• Filed: 03/01/1999
• Issued: 01/07/2003
• Est. Priority Date: 03/01/1999
• Status: Expired due to Term

• Current Assignee: Rakuten, Inc. (Rakuten Group, Inc.)
• Original Assignee: Sharp Laboratories of America Incorporated (Hon Hai Precision Industry Co., Ltd.)
• Inventors: Zeng, Wenjun, Lei, Shaw-Min
• Primary Examiner(s): Peeso, Thomas R.
• Application Number:
  US09/260,193
• Time in Patent Office: 1,408 Days
• Field of Search: 713/160, 713/189, 713/200, 713/201
• US Class Current: 713/160
• CPC Class Codes: H04N 19/51 Motion estimation or motion compensation
• CPC Class Codes: H04N 19/517 by encoding
• CPC Class Codes: H04N 19/60 using transform coding
• CPC Class Codes: H04N 19/645 by grouping of coefficients into blocks after the transform
• CPC Class Codes: H04N 19/88 involving rearrangement of data among different coding units, e.g. shuffling, interleaving, scrambling or permutation of pixel data or permutation of transform coefficient data among different blocks
• CPC Class Codes: H04N 21/23476 by partially encrypting, e.g. encrypting the ending portion of a movie
• CPC Class Codes: H04N 7/1675 Providing digital key or authorisation information for generation or regeneration of the scrambling sequence pseudo-random number generators in general G06F7/58

Abstract

Methods and apparatus for encryption and decryption of digital images are disclosed. A preferred embodiment operates on an image frame after that frame has undergone a space-frequency transform operation, such as a block DCT or wavelet transform, and before the frame is passed to a bitstream coder for entropy coding. The transform coefficient map is subjected to one or more encryption operations that render a subsequently decoded (but not decrypted) image incomprehensible. These operations are designed to operate with low computational overhead and with only minor effects on compressed bit rate. They also allow secure transcoding at intermediate routers of the transmission channels without the cryptographic key.

In one operation, the sign bits of transform coefficients are scrambled. In another operation, two dimensional blocks of coefficients from a common subband are shuffled and/or rotated to pseudorandom locations and orientations. In yet another operation, coefficients occupying a common “subband”, but taken from different DCT blocks, are shuffled. Still another operation shuffles motion vectors and/or scrambles sign bits for motion vector coefficients. These operations perturb the data as it will appear visually, without greatly perturbing the entropy of the data as presented to an entropy coder.

Claims

1. A method for encrypting a digital image, comprising the steps of:

applying a space-frequency transform to an original digital image, thereby generating a transform coefficient map corresponding to the digital image; and

encrypting the transform coefficient map, using one or more encryption techniques selected from the group of techniques consisting of

scrambling the sign bits of the transform coefficients,

scrambling the refinement bits of the transform coefficients,

partitioning the transform coefficient map into a set of two-dimensional coefficient blocks, and shuffling selected blocks within the coefficient map, and

grouping a set of transform coefficients from a spatial frequency subband, and shuffling the transform coefficients within the group.

2. The method of claim 1, further comprising the step of entropy coding the encrypted transform coefficient map.

3. The method of claim 1, wherein one or more spatial frequency subbands of the original transform coefficient map bypass the encrypting step.

4. A method for encrypting a block of digital data, the method comprising the steps of:

selecting a group of bits across the block of data, the group having lower than average predicted compressibility as compared to the predicted compressibility of the block of data as a whole; and

scrambling the group of bits.

5. The method of claim 4, wherein the block of digital data comprises a space-frequency transform coefficient map.

6. The method of claim 5, wherein the group of bits comprises the sign bits from the transform coefficient map.

7. The method of claim 5, wherein the group of bits comprises refinement bits from the transform coefficient map.

8. The method of claim 4, wherein the block of digital data comprises motion-compensation data.

9. A method for encrypting a digital video stream comprising a motion-compensation data component, said method comprising the step of selectively scrambling the motion-compensation data component of the digital video stream prior to bitstream coding of the digital video stream.

10. The method of claim 9, wherein the step of selectively scrambling the motion-compensation data component comprises scrambling the sign bits of the motion-compensation data values.

11. The method of claim 9, wherein the step of selectively scrambling the motion-compensation data component comprises selecting a group of motion vectors and shuffling the vectors within the group.

12. An image encryption system comprising

an encryption buffer, and

at least one encryption subsystem operating on transform data stored in the encryption buffer, the encryption subsystem selected from the group of subsystems consisting of:

a sign bit scrambler that accepts a space-frequency transform coefficient map having signed coefficients, and scrambles the sign bits of the coefficients;

a block shuffler that accepts a space-frequency transform coefficient map and shuffles two-dimensional coefficient blocks to pseudorandom locations in the map;

a block rotator that accepts a space-frequency transform coefficient map and rotates two-dimensional coefficient blocks to pseudorandom orientations; and

a coefficient shuffler that accepts a space-frequency transform coefficient map and shuffles coefficients from a two-dimensional group of coefficients from a spatial frequency subband to pseudorandom locations within the group.

13. The encryption system of claim 12, further comprising an entropy coder that operates on encrypted transform data supplied by the encryption subsystem.

14. The image encryption system of claim 12, wherein the sign bit scrambler operates on a motion vector having signed components.

15. The image encryption system of claim 12, wherein the coefficient shuffler operates on a group of motion vector components.

16. An encrypted-image decryption system comprising

a decryption buffer, and

at least one decryption subsystem operating on encrypted transform data stored in the decryption buffer, the decryption subsystem selected from the group of subsystems consisting of:

a sign bit descrambler that accepts a space-frequency transform coefficient map and/or a motion vector array having encrypted sign bits, and applies a decryption key to the encrypted sign bits to recreate the original sign bits;

a block deshuffler that accepts a space-frequency transform coefficient map having shuffled two-dimensional coefficient blocks, and applies a decryption key to the coefficient block pattern to restore the blocks to their original locations;

a block derotator that accepts a space-frequency transform coefficient map having rotated two-dimensional coefficient blocks, and applies a decryption key to the coefficient block pattern to restore the blocks to their original orientations; and

a coefficient deshuffler that accepts a space-frequency transform coefficient map having a shuffled two-dimensional group of coefficients from a spatial frequency subband, and applies a decryption key to the coefficient pattern to restore the coefficients in the group to their original locations.

17. The encrypted-image decryption system of claim 16, further comprising an entropy decoder that operates on an input bitstream and supplies encrypted transform data to the decryption buffer.

18. The encrypted-image decryption system of claim 16, wherein the sign bit descrambler operates on a motion vector array having encrypted sign bits.

19. The encrypted-image decryption system of claim 16, wherein the coefficient deshuffler operates on a motion vector array having shuffled motion vector data.