# Patent

## Title
Selective encryption to enable trick play

## Abstract

Certain embodiments consistent with the present invention involve a method of selectively encrypting and decoding digital video content. Certain of the packets are selected for encryption according to a selection criterion, wherein the selected packets exclude packets containing start of frame (SOF) headers. The selected packets are encrypted while retaining the packets containing the SOF headers unencrypted to form selectively encrypted digital video content. In decoding selectively encrypted digital video content, offsets are calculated for at least one of I frames and P frames from the SOF headers and the calculated offsets are stored as an offset table. The offset table can then be accessed to enable trick play of the selectively encrypted digital video content.

## Background
CROSS REFERENCE TO RELATED DOCUMENTS
This application is a continuation of pending U.S. patent application Ser. No. 10/319,096, filed Dec. 13, 2002 to Candelore et al. entitled “Encryption to Enable Trick Play”, and is also related to and claims priority benefit of U.S. Provisional patent application Ser. No. 60/409,675, filed Sep. 9, 2002, entitled “Generic PID Remapping for Content Replacement”, to Candelore. These applications are hereby incorporated by reference herein.


This application is also related to patent applications entitled “Critical Packet Partial Encryption” to Unger et al., Ser. No. 10/038,217; patent applications entitled “Time Division Partial Encryption” to Candelore et al., Ser. No. 10/038,032; entitled “Elementary Stream Partial Encryption”, to Candelore, Ser. No. 10/037,914; entitled “Partial Encryption and PID Mapping” to Unger et al., Ser. No. 10/037,499; entitled “Decoding and Decrypting of Partially Encrypted Information” to Unger et al., Ser. No. 10/037,498 all of which were filed on Jan. 2, 2002 and are hereby incorporated by reference herein.
This application is also related to U.S. patent applications Ser. No. 10/273,905, filed Oct. 18, 2002 to Candelore et al., entitled “Video Slice and Active Region Based Dual Partial Encryption”, Ser. No. 10/273,903, filed Oct. 18, 2002 to Candelore et al., entitled “Star Pattern Partial Encryption”, Ser. No. 10/274,084, filed Oct. 18, 2002 to Candelore et al., entitled “Slice Mask and Moat Pattern Partial Encryption”, Ser. No. 10/274,019, filed Oct. 18, 2002 to Candelore et al., entitled “Video Scene Change Detection”, which are hereby incorporated by reference.
COPYRIGHT NOTICE
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile, reproduction of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
FIELD OF THE INVENTION
This invention relates generally to the field of encryption of digital video content. More particularly, this invention relates to use of selective encryption to permit trick play with digital video content.
BACKGROUND OF THE INVENTION
When digital video such as MPEG (Moving Pictures Expert Group) digital video is played back in various trick play modes, access to I frames or P frames is generally required. I frames and P frames contain information that can be used to directly represent an image, whereas images are created with B frames by referencing and modifying preceding and/or subsequent I frames and P frames. As an example, when playback is carried out at 2× or 4× speed, the playback algorithm jumps from I frame to I frame or P frame to P frame to speed up the presentation of the video frames while skipping certain frames.
Unfortunately, when content is encrypted using traditional encryption techniques, trick play becomes difficult or impossible since content has to be decrypted in order to find the I frames and/or P frames. In the case of unencrypted content, these frames are accessed by calculating an offset from the start of frame (SOF).



BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself however, both as to organization and method of operation, together with objects and advantages thereof, may be best understood by reference to the following detailed description of the invention, which describes certain exemplary embodiments of the invention, taken in conjunction with the accompanying drawings in which:
FIG. 1 is a flow chart showing a selective encryption process and decoding process consistent with certain embodiments of the present invention.
FIG. 2 illustrates one embodiment of a selective encryption and decoding process consistent with certain embodiments of the present invention.
FIG. 3 illustrates another embodiment of a selective encryption and decoding process consistent with certain embodiments of the present invention.
FIG. 4 is a programmed processor which can represent either an encryption processor or a decoder consistent with certain embodiments of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail specific embodiments, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described. In the description below, like reference numerals are used to describe the same, similar or corresponding parts in the several views of the drawings.
The terms “scramble” and “encrypt” and variations thereof are used synonymously herein. The term “video” may be used herein to embrace not only true visual information, but also in the conversational sense (e.g., “video tape recorder”) to embrace not only video signals but associated audio and data. The present document generally uses the example of a “dual selective encryption” embodiment, but those skilled in the art will recognize that the present invention can be utilized to realize multiple partial encryption without departing from the invention. The terms “partial encryption” and “selective encryption” are used synonymously herein. Also, the terms “program” and “television program” and similar terms can be interpreted in the normal conversational sense, as well as a meaning wherein the term means any segment of A/V content that can be displayed on a television set or similar monitor device. The term “legacy” as used herein refers to existing technology used for existing cable and satellite systems. The exemplary embodiments disclosed herein are decoded by a television Set-Top Box (STB), but it is contemplated that such technology will soon be incorporated within television receivers of all types whether housed in a separate enclosure alone or in conjunction with recording and/or playback equipment or Conditional Access (CA) decryption module or within a television set itself. The present document generally uses the example of a “dual partial encryption” embodiment, but those skilled in the art will recognize that the present invention can be utilized to realize multiple partial encryption without departing from the invention.
The above-referenced commonly owned patent applications describe inventions relating to various aspects of methods generally referred to herein as partial encryption or selective encryption. More particularly, systems are described wherein selected portions of a particular selection of digital content are encrypted using two (or more) encryption techniques while other portions of the content are left unencrypted. By properly selecting the portions to be encrypted, the content can effectively be encrypted for use under multiple decryption systems without the necessity of encryption of the entire selection of content. In some embodiments, only a few percent of data overhead is needed to effectively encrypt the content using multiple encryption systems. This results in a cable or satellite system being able to utilize Set-top boxes or other implementations of conditional access (CA) receivers from multiple manufacturers in a single system—thus freeing the cable or satellite company to competitively shop for providers of Set-top boxes.
The present invention applies similar selective encryption techniques to the problem of enabling trick play with encrypted digital video content. The partial encryption processes described in the above patent applications utilize any suitable encryption method. However, these encryption techniques are selectively applied to the data stream, rather than encrypting the entire data stream, using techniques described in the above-referenced patent applications. In general, but without the intent to be limiting, the selective encryption process utilizes intelligent selection of information to encrypt so that the entire program does not have to undergo dual encryption. By appropriate selection of data to encrypt, the program material can be effectively scrambled and hidden from those who desire to hack into the system and illegally recover commercial content without paying. MPEG (or similar format) data that are used to represent the audio and video data does so using a high degree of reliance on the redundancy of information from frame to frame. Certain data can be transmitted as “anchor” data representing chrominance and luminance data. That data is then often simply moved about the screen to generate subsequent frames by sending motion vectors that describe the movement of the block. Changes in the chrominance and luminance data are also encoded as changes rather than a recoding of absolute anchor data. Thus, encryption of this anchor data, for example, or other key data can effectively render the video un-viewable.
In accordance with certain embodiments consistent with the present invention, the selected video data to be encrypted may be any individual one or combination of the following (described in greater detail in the above applications): video slice headers appearing in an active region of a video frame, data representing an active region of a video frame, data in a star pattern within the video frame, data representing scene changes, I Frame packets, packets containing motion vectors in a first P frame following an I Frame, packets having an intra_slice_flag indicator set, packets having an intra_slice indicator set, packets containing an intra_coded macroblock, data for a slice containing an intra_coded macroblock, data from a first macroblock following the video slice header,packets containing video slice headers, anchor data, and P Frame data for progressively refreshed video data, data arranged in vertical and or horizontal moat patterns on the video frame, and any other selected data that renders the video and/or audio difficult to utilize. Several such techniques as well as others are disclosed in the above-referenced patent applications, any of which (or other techniques) can be utilized with the present invention to encrypt only a portion of the content, so long as the packets containing the SOF header is not encrypted, as will be explained later.
Currently there is a problem with delivering content to a personal-video-recorder (PVR) enabled set-top box is that to enable trick play it must be first descrambled. If the content is downloaded “opportunistically” from a carrousel at night, then descrambling (and locally re-scrambling) the content can cause a security problem. Keys used to locally re-scramble the content will not be as secure as those from the conditional access (CA) element. The content on the hard drive might be accessed without paying the appropriate viewing fees. Ideally, content delivered “opportunistically” or speculatively to a local drive of a set-top box would remain CA scrambled on the hard drive.
In accordance with certain embodiments consistent with the present invention, selective encryption is utilized to encrypt selective portions of content, but leaves the packets containing the SOF header in the clear (unencrypted). By leaving the SOF header unencrypted, trick play circuits and algorithms can operate properly. The remainder of the content can be encrypted fully or selectively according to any of the selective encryption schemes described in the copending applications, or any other suitable selective encryption content selection criterion.
FIG. 1 describes a process 100, in accord with certain embodiments consistent with the present invention, starting at 104. A packet selection criterion is established at 108 for determining which packets are to be encrypted. Such selection criterion can, for example, be established in accord with any of the above-referenced selective encryption selection criteria (including full encryption of all content except SOF header packets). Once the selection criterion is established, packets can be received at 112. If the packet does not meet the selection criterion at 116, then the packet is not encrypted at 120. If the packet does meet the selection criterion at 116, then control passes to 126 where the packet is examined to determine if it contains the SOF header. If so, the packet is not encrypted at 120. If not, the packet is encrypted at 130. Control passe from 120 or 130 to 134. At 134, if the last packet has not been encountered, the next packet is received at 112. If the last packet has been received at 134, the process stops at 138.
Thus, in accord with one embodiment consistent with the invention, a method of selectively encrypting digital video content, involves receiving a plurality of packets containing the digital video content; selecting certain of the packets for encryption according to a selection criterion, wherein the selected packets exclude certain packets containing start of frame (SOF) headers; encrypting the selected packets; and retaining the packets containing the SOF headers unencrypted to form selectively encrypted digital video content.
A method of decoding selectively encrypted digital video content, consistent with certain embodiments, involves receiving the selectively encrypted digital video content, wherein the selectively encrypted digital video content has unencrypted packets containing start of frame (SOF) headers; calculating offsets for at least one of I frames and P frames from the SOF headers; and storing the calculated offsets as an offset table. The offset table can be accessed to enable trick play of the selectively encrypted digital video content by identifying the location of I and/or P frames.
A selective encryption encoder for selective encryption of digital video content, consistent with certain embodiments of the present invention receives a plurality of packets containing the digital video content. A programmed processor selects certain of the packets for encryption according to a selection criterion, wherein the selected packets exclude packets containing start of frame (SOF) headers. An encrypter encrypts the selected packets while retaining the packets containing the SOF headers unencrypted to form selectively encrypted digital video content. The selectively encrypted content can then be stored on an electronic storage medium and/or transmitted to a receiver such as a television Set-top box.
In certain embodiments consistent with the present invention, all of the packets containing SOF headers can be left encrypted. However, this should not be considered limiting since in other embodiments only certain of the packets containing SOF headers might be left unencrypted, while others are encrypted. By way of example, and not limitation, I frames may generally be the more important frames containing SOF headers to have in the clear. Thus, some or all of the packets containing I frame SOF headers may be passed in the clear while other packets containing SOF headers may be encrypted. For example, B and/or P frame packets containing SOF headers, in many instances may be encrypted freely while I frame packets containing SOF headers are kept in the clear. Many variations with various advantages may be contemplated by those skilled in the art upon consideration of this teaching.
FIG. 2 depicts an embodiment of this process wherein a clear stream of data 204 is converted to a selectively encrypted stream of data 208. Clear stream 204 contains a sequence of packets each having program identifier (PID) A. Packets 212 and 214 contain SOF headers, while packets 218, 220, 222, 224 and 226 contain oilier information that forms a part of the video data stream. As a part of the encryption process of this example, all packets except those containing SOF headers are encrypted. Thus, packets 212 and 214 are transferred directly to the selectively encrypted stream 208. Packets 218, 220, 222, 224 and 226 are encrypted to produce encrypted packets 238, 240, 242, 244 and 246, respectively. At the decoder (e.g., in a television Set-top box (STB), the fact that the SOF headers are unencrypted enables calculation of offsets for trick play at 240. These offsets can then be stored in a local trick play. offset table at 254 to enable rapid access to the proper I and/or P frames, and thus produce trick play.
FIG. 3 depicts an embodiment of this process wherein a clear stream of data 304 is converted to a selectively encrypted stream of data 308. In this example, the selective encryption selection criterion is one that only partially encrypts packets not containing SOF headers. Clear stream 304 contains a sequence of packets each having program identifier (PID) A. Packets 312 and 314 contain SOF headers, while packets 318, 320, 322, 324 and 326 contain other information forming part of the stream of video. As a part of the encryption process of this example, only certain of the packets containing information other than the SOF header are encrypted. As illustrated, packets 312 and 314 are transferred directly to the selectively encrypted stream 308. Packets 318, 322 and 326 are encrypted to produce encrypted packets 338, 342 and 346, respectively. At the decoder (e.g., in a television STB), the fact. that the SOF headers are unencrypted enables calculation of offsets for trick play at 350. These offsets can then be stored in a local trick play offset table at 354 to enable rapid access to the proper I and/or P frames, and thus produce trick play.
When selectively encrypted digital video content is created as streams 208 or 308, the selectively encrypted digital video content can then be stored on an electronic storage medium or transmitted to one or more recipients (e.g., by downloading, streaming or broadcast by a cable or satellite content provider to subscriber's Set-top boxes serving as receivers and decoders.
Thus, certain embodiments of the present invention keep the SOF header packets in the clear. This allows commercially available decoder chips such as the model number 7030 manufactured and sold by Broadcom Corporation to calculate the byte offset to I, P and B frames. For trick play operation, the offset allows a decoder (e.g., a Set-top box) to display, for example, every second I Frame or fourth I frame without decoding other content. While the SOF headers are not encrypted, the rest of the content does not need to be completely encrypted, as described above.
The processes above can be carried out on any suitable programmed general purpose processor operating as a server/encoder such as that depicted as computer 400 of FIG. 4. Computer 400 can represent either an encryption processor or a decoder depending upon programming. Computer 400 has one or more central processor units (CPU) 410 with one or more associated buses 414 used to connect the central processor unit 410 to Random Access Memory 418 and Non-Volatile Memory 422 in a known manner. Output devices 426, such as a display and printer, may be provided in order to display and/or print output for the use of the MSO or user as well as to provide a user interface such as a Graphical User Interface (GUI). Similarly, input devices such as keyboard, mouse and removable media readers 430 may be provided for the input of information by the operator. Computer 400 also may incorporate internal and/or external attached disc or other mass storage 434 (e.g., disc and/or optical storage) for storing large amounts of information including, but not limited to, the operating system, encryption processes (for the encryption encoder), or offset calculation and local offset table if computer 400 is used as a decoder (e.g., forming a part of a television STB). The Computer system 400 also has an interface 438 for connection to the cable system if present at the MSO. While depicted as a single computer, the digital content provider may utilize multiple linked computers to carry out the functions described herein.
Those skilled in the art will recognize that the present invention has been described in terms of exemplary embodiments based upon use of a programmed processor (e.g., computer 400). However, the invention should not be so limited, since the present invention could be implemented using hardware component equivalents such as special purpose hardware and/or dedicated processors which are equivalents to the invention as described and claimed. Similarly, general purpose computers, microprocessor based computers, micro-controllers, optical computers, analog computers, dedicated processors and/or dedicated hard wired logic may be used to construct alternative equivalent embodiments of the present invention. Moreover, although the present invention has been described in terms of a general purpose personal computer providing a playback mechanism, the playback can be carried on a dedicated machine without departing from the present invention.
Those skilled in the art will appreciate that the program steps and associated data used to implement the embodiments described above can be implemented using disc storage as well as other forms of storage such as for example Read Only Memory (ROM) devices, Random Access Memory (RAM) devices; optical storage elements, magnetic storage elements, magneto-optical storage elements, flash memory, core memory and/or other equivalent storage technologies without departing from the present invention. Such alternative storage devices should be considered equivalents.
The present invention, as described in embodiments herein, is implemented using a programmed processor executing programming instructions that are broadly described above form that can be stored on any suitable electronic storage medium or transmitted over any suitable electronic communication medium or otherwise be present in any computer readable or propagation medium. However, those skilled in the art will appreciate that the processes described above can be implemented in any number of variations and in many suitable programming languages without departing from the present invention. For example, the order of certain operations carried out can often be varied, additional operations can be added or operations can be deleted without departing from the invention. Error trapping can be added and/or enhanced and variations can be made in user interface and information presentation without departing from the present invention. Such variations are contemplated and considered equivalent.
Software code and/or data embodying certain aspects of the present invention may be present in any computer readable medium, transmission medium, storage medium or propagation medium including, but not limited to, electronic storage devices such as those described above, as well as carrier waves, electronic signals, data structures (e.g., trees, linked lists, tables, packets, frames, etc.) optical signals, propagated signals, broadcast signals, transmission media (e.g., circuit connection, cable, twisted pair, fiber optic cables, waveguides, antennas, etc.) and other media that stores, carries or passes the code and/or data. Such media may either store the software code and/or data or serve to transport the code and/or data from one location to another. In the present exemplary embodiments, MPEG compliant packets, slices, tables and other data structures are used, but this should not be considered limiting since other data structures can similarly be used without departing from the present invention.
While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the scope of the appended claims.

## Claims

1. A method of selectively encrypting digital video content, comprising;
receiving a plurality of packets containing the digital video content;
selecting packets from the plurality of packets for encryption according to a selection criterion, wherein the selection criterion is explicitly based at least in part upon the presence or absence of a start of frame (SOF) header in the selected packets, and wherein the selected packets exclude certain packets containing SOF headers;
encrypting the selected packets; and
retaining the certain packets containing the SOF headers unencrypted to form selectively encrypted digital video content.

2. The method according to claim 1, further comprising storing the selectively encrypted digital video content on an electronic storage medium.

3. The method according to claim 1, further comprising transmitting the selectively encrypted digital video content to a receiver.

4. The method according to claim 1, wherein the selection criterion comprises selection of all packets except those packets containing the SOF header.

5. The method according to claim 1, wherein the selection criterion comprises selection of packets except those certain packets containing the SOF header where the selected packets represent less than all of the remaining packets excluding those certain packets containing the SOF header.

6. The method according to claim 1, wherein at least one packet containing an SOF header is not one of the certain packets containing a SOF header and is encrypted.

7. The method according to claim 1, wherein the certain packets containing the SOF headers comprise I frame packets.

8. The method according to claim 1, carried out on a programmed processor.

9. An electronic computer readable storage medium storing instructions that, when executed on a programmed processor, carry out the method of selectively encrypting digital video cement according to claim 1.

10. A method of decoding selectively encrypted digital video content comprising:
receiving the selectively encrypted digital video content, wherein the selectively encrypted digital video content has certain unencrypted packets containing start of frame (SOF) headers, wherein the certain unencrypted packets were selected based upon a selection criterion that is explicitly based at least in part upon the presence or absence of a start of an SOF header in the selected packets;
calculating offsets for at least one of I frames and P frames from the SOF headers; and
storing the calculated offsets as an offset table.

11. The method according to claim 10, further comprising accessing the offset table to enable trick play of the selectively encrypted digital video content.

12. The method according to claim 10, wherein the selectively encrypted digital video content is stored on an electronic storage medium.

13. The method according to claim 10, wherein the selectively encrypted digital video content is transmitted from a content provider to a receiver.

14. The method according to claim 10, wherein the selectively encrypted digital video content is fully encrypted except for certain packets containing SOF headers.

15. The method according to claim 10, wherein the selectively encrypted digital video content comprises the certain unencrypted packets containing SOF headers, encrypted packets containing SOF headers, packets that do not contain SOF headers that are encrypted, and packets that do not contain SOF headers that are unencrypted.

16. The method according to claim 10, wherein at least one packet containing an SOF header is not one of the certain packets containing a SOF header and is encrypted.

17. The method according to claim 10, wherein the certain packets containing the SOF headers comprise I frame packets.

18. The method according to claim 10, carried out on a programmed processor.

19. An electronic storage medium storing instructions that, when executed on a programmed processor, carry out the method of decoding selectively encrypted digital video content according to claim 10.

20. A decoder for decoding selectively encrypted digital video content, comprising:
a receiver that receives the selectively encrypted digital video content, wherein the selectively encrypted digital video content has unencrypted packets containing start of frame (SOF) headers;
wherein the certain unencrypted packets were selected based upon a selection criterion that is explicitly based at least in part upon the presence or absence of a start of an SOF header in the selected packets;
an offset calculator tat calculates offsets for at least one of I frames and P frames from the SOF headers; and
means for storing the calculated offsets as an offset table.

21. The decoder according to claim 20, further comprising means for accessing the offset table to enable trick play of the selectively encrypted digital video content.

22. The decoder according to claim 20, wherein, the selectively encrypted digital video content is stored on an electronic storage medium.

23. The decoder according to claim 20, wherein the selectively encrypted digital video content is transmitted from a content provider to a receiver.

24. The decoder according to claim 20, wherein the selectively encrypted digital video content is fully encrypted except for the certain packets containing the SOF header.

25. The decoder according to claim 20, wherein the selectively encrypted digital video content comprises unencrypted packets containing SOF headers, encrypted packets containing SOF headers that are encrypted, packets that do not contain SOF headers that are encrypted, and packets that do not contain SOF headers that are unencrypted.

26. The decoder according to claim 20, wherein the offset calculator is implemented by a programmed processor.

27. The decoder according to claim 20, wherein the offset table is stored in an electronic storage medium.

28. A selective encryption encoder for selective encryption of digital video content, comprising;
means for receiving a plurality of packets containing the digital video content;
a programmed processor that selects packets for encryption according to a selection criterion, wherein the selected packets exclude certain packets containing start of frame (SOF) headers;
wherein the selection criterion is explicitly based at least in part upon the presence or absence of a SOF header in the selected packets; and
an encrypter for encrypting the selected packets while retaining the certain packets containing the SOF headers unencrypted to form selectively encrypted digital video content.

29. The encoder according to claim 28, further comprising an electronic storage medium for storing the selectively encrypted digital video content.

30. The encoder according to claim 28, further comprising a transmitter for transmitting the selectively encrypted digital video content to a receiver.

31. The encoder according to claim 28, wherein the selection criterion comprises selection of all packets except those containing the SOF header.

32. The encoder according to claim 28, wherein the selection criterion comprises selection of certain packets except those containing the SOF header where the certain packets represent less than all of the packets that do not contain the SOF header.