Abstract:
The present invention provides a method performed on a computer of preventing re-use of compromised keys in a broadcast encryption system. In an exemplary embodiment, the method includes (1) incorporating a particular set of Sequence Keys assigned by a license agency into individual receivers, (2) assigning a Sequence Key Block (SKB) by the license agency to at least one distributed protected file, (3) performing incremental cryptographic testing by the individual receivers to determine if a selected Sequence Key from the set of Sequence Keys is compromised, (4) if the selected Sequence Key is not compromised, decrypting the file, and (5) if the selected Sequence Key is compromised and if a subsequent Sequence Key from the set of Sequence Keys is available, selecting the subsequent Sequence Key.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This invention is related to commonly-owned pending U.S. patent applications, each of which is hereby incorporated by reference, including: 
     U.S. Ser. No. 09/770,877, filed Jan. 26, 2001, entitled “Method for Broadcast Encryption and Key Revocation of Stateless Receivers”. 
     U.S. Ser. No. 09/771,239, filed Jan. 26, 2001, entitled “Method for Tracing Traitor Receivers in a Broadcast Encryption System”. 
     U.S. Ser. No. 09/777,506, filed Feb. 5, 2001, entitled “Method for Assigning Encryption Keys”. 
     U.S. Ser. No. 09/789,451, filed Feb. 20, 2001, entitled “Method for Assigning Encryption Keys”. 
     U.S. Ser. No. 10/042,652, filed Jan. 8, 2002, entitled “Method for Ensuring Content Protection and Subscription Compliance”. 
     U.S. Ser. No. 10/315,395, filed Dec. 9, 2002, entitled “Method for Tracing Traitors and Preventing Piracy of Digital Content in a Broadcast Encryption System”. 
     FIELD OF THE INVENTION 
     This invention relates to preventing piracy of digital content in a broadcast encryption system and more specifically to tracing traitors who may be colluding to redistribute such content and/or related decryption keys, and to renewably revoking compromised keys to prevent further use in gaining unauthorized access to content. 
     BACKGROUND OF THE INVENTION 
     The widespread transition of data from analog format to digital format has exacerbated problems relating to unauthorized copying and redistribution of protected content. Flawless copies of content can be easily produced and distributed via the Internet or on physical media. This piracy is a major concern and expense for content providers; to this end, industry consortia such as The 4C Entity (&lt;www.4centity.com&gt;) and AACSLA (&lt;www.aacsla.com&gt;) have been formed. These groups are license agencies that provide content protection tools based on Content Protection for Recordable Media (CPRM) and Advanced Access Content System (AACS), respectively. CPRM is a technology developed and licensed by the 4C group, comprising IBM, Intel, Matsushita, and Toshiba, to allow consumers to make authorized copies of commercial entertainment content where the copyright holder for such content has decided to protect it from unauthorized copying. AACS is a follow-on technology for the same purpose, under development by a group comprising IBM, Intel, Matsushita, Toshiba, Sony, Microsoft, Warner Brothers, and Disney. 
     CPRM and AACS protected files are encrypted with a key that is specific to a Media Identifier on their original storage medium (such as a DVD or CD-ROM etc.), so simply copying the content to another storage medium does not break the protection. CPRM also adds a Media Key Block (MKB) to the medium. The MKB is a file containing a very large number of keys. Each individual compliant device is assigned a set of unique Device Keys that allow it to obtain the Media Key from the MKB, that is then combined with the Media Identifier and other values to derive the keys used to decrypt the protected content. Details of the CPRM and AACS technology are provided in the applications incorporated by reference and are also available from 4C and AACS. 
     Fundamentally, the AACS protection depends on the interaction between Device Keys and the tree-based Media Key Block, which allows unlimited, precise cryptographic revocation of compromised devices without danger of collateral damage to innocent devices. Because of the inherent power of the revocation of the AACS system, it is possible that attackers may forgo building clones or non-compliant devices and instead devote themselves to attacks where they try to hide the underlying compromised device(s). These attacks are both more expensive and more legally risky for the attackers, because the attacks require them to have an active server serving either content keys or the content itself, on an instance-by-instance basis. 
     In addition to conventional CD-ROMs and DVDs, a new type of home consumer device for digital content management has been enabled by the advent of inexpensive, large-capacity hard disks. A movie rental box receives digital movies from some inexpensive source of data, usually a broadcast source (whether terrestrial or satellite-based). The movies are stored on the hard disk, so that at any moment the hard disk contains, for example, the hundred hottest movies in the rental market. The consumer selects and plays a particular movie, and the movie rental box periodically calls a clearing center and reports the consumer&#39;s content usage for billing purposes; the box may also acquire new decryption keys during this call. 
     The most serious attack against these new devices is likely to be the so-called “anonymous” attack, wherein a user or a group of users purchase rental movies from legitimate movie rental boxes that have been instrumented so that the protected content and/or the decryption keys can be captured and redistributed, often over the Internet. This attack is the most urgent concern of the movie studios that are investigating content protection technology. One solution to the problem is to differently watermark and differently encrypt each movie for each authorized movie rental box, so that if a movie is pirated, the watermarking and encryption information would uniquely identify the compromised box. Alas, this solution is not feasible because of the excessive computing effort and transmission bandwidth required to prepare and transmit individualized movies. The distribution system is economical only if the movies can be distributed over broadcast channels, i.e. where every receiver gets substantially the same data at the same time. 
     The approach known in the art as “tracing traitors” may be used to solve the problem. In one particular instance of this approach, an original version of each movie file is augmented before being broadcast. Specifically, the file that is actually broadcast has had at least one critical file segment replaced by a set of segment variations. Each file segment variation is differently encrypted and preferably also differently watermarked prior to encryption, although the entire file may be watermarked as well. All the variations in one segment are identical for viewing purposes though digitally different. A particular receiver is preferably given the cryptographic key to decrypt only one of the variations in each segment. All legitimate receivers with valid decryption keys can play the content, but probably through different segment combinations. If the receiver is compromised and is used to illegally rebroadcast either the keys or the segments themselves, it is possible to deduce which receiver or receivers have been compromised. 
     The tracing traitors approach has not been widely used in practice to date because previous implementations required unreasonable amounts of bandwidth in the broadcast, due to the number of segments or variations required. However, U.S. Ser. No. 10/315,395, filed Dec. 9, 2002, entitled “Method for Tracing Traitors and Preventing Piracy of Digital Content in a Broadcast Encryption System” teaches a method of distributing protected content that combats piracy and enables identification and revocation of compromised receivers in a broadcast encryption system without excessive transmission bandwidth. 
     To recap, whether dealing with DVDs or set-top boxes or other distribution means, a traitor tracing scheme has two basic steps: assigning the keys to receiver devices to enable tracing, and then identifying the traitors for revocation. Efficient traitor tracing technologies directed to both these steps enable a license agency to more quickly identify traitors and to prevent piracy even by larger groups of colluding traitors. 
     However, what happens after a traitor has been identified and a particular compromised key or set of keys is revoked? The prior art is silent as to the aftermath of a single tracing and revocation. What if a traitor repeats the attack and additional content is pirated, and/or a new key or set of keys is compromised? A system is needed that allows innocent receiver devices to still calculate a correct cryptographic answer needed to allow content to be used, while at the same time preventing traitor devices from getting to such an answer. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of this invention to provide a method, system, and program product to renewably prevent traitors in a broadcast encryption system from re-using compromised keys to make use of protected distributed files. 
     The invention employs Sequence Keys and a Sequence Key Block (SKB) to extend the previous work on broadcast encryption and traitor tracing. The Sequence Keys are assigned by a license agency to individual playback devices preferably from a key matrix. The license agency also assigns SKBs to be used on prerecorded media, in a manner similar to that of the MKBs (Media Key Blocks) used in the CPRM system. Any compliant device can process the SKB and get the right decryption key and access the content correctly. In a preferred embodiment, successful processing of the SKB enables the device to properly use the set of variations assigned to it. When a traitor device is identified and its set of Sequence Keys is to be revoked, a new SKB is formulated and distributed on new media. 
     If a device has no compromised Sequence Keys, it decrypts protected content on the new media in a straightforward manner by calculating the correct decryption key, preferably for its assigned variations. If a device has a compromised Sequence Key, then that key is not used, but instead another Sequence Key from a set (in a preferred embodiment, the next key in a linked list) is selected and used if it too has not been compromised. If it has also been compromised, then another available key in the set is selected, and so forth. Thus, innocent devices are given multiple opportunities to find an unrevoked Sequence Key to usefully decrypt the protected content. This approach provides renewability of the Sequence Keys. 
     The formulation of the new SKB by the license agency assures that all of the Sequence Keys in particular devices that have been identified as traitors will be deemed compromised when those devices try to play the content on the new media. Thus a traitor device will step through all of its Sequence Keys without finding one that will usefully decrypt the protected content. 
     The invention may be employed with broadcast encryption systems using distribution means that may include computer networks, satellite networks, cable networks, television transmissions, and physical storage media. Files may comprise any kind of digital data sequence, including but not limited to text, audio, images, video, music, movies, multimedia presentations, operating systems, video games, software applications, and cryptographic keys. 
     The foregoing objects are believed to be satisfied by the embodiment of the present invention as described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a prior art diagram of a modified distributed file. 
         FIG. 2  is a flowchart of the basic operation of a preferred embodiment of the present invention. 
         FIG. 3  is a diagram of a Sequence Key Block (SKB), according to a preferred embodiment of the present invention. 
         FIG. 4  is a diagram of a Nonce Record format, according to an embodiment of the present invention. 
         FIG. 5  is a diagram of a Calculate Variant Data Record format, according to an embodiment of the present invention. 
         FIG. 6  is a diagram of a Conditionally Calculate Variant Data Record format, according to an embodiment of the present invention. 
         FIG. 7  is a diagram of an End of Sequence Key Block Record format, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a prior art diagram of a modified or augmented distributed file  100  is shown. This file is described in detail in U.S. Ser. No. 10/315,395, filed Dec. 9, 2002, entitled “Method for Tracing Traitors and Preventing Piracy of Digital Content in a Broadcast Encryption System”, which was incorporated by reference herein. The augmented file  100  is the modified version of an original file that will actually be broadcast. The augmented file  100  includes sets of file variations that replaced critical file segments. For example, a first critical file segment has been replaced with variations  102 ,  104 ,  106 , and  108 , while a second critical file segment has been replaced with variations  110 ,  112 ,  114 , and  116 , and so forth. Each file segment variation is simply a copy of the particular corresponding critical file segment that has been differently watermarked and differently encrypted. Each entire file is also typically watermarked and encrypted in a broadcast encryption system. Each file segment variation is identified by a text designation in this application (e.g. A, B, C . . . etc.) for clarity, but in practice binary numbers are generally employed for this purpose. 
     The number of critical file segments and the number of file segment variations preferably employed depends on the properties of the file and its audience. For movies, one could select a single critical file segment and have several hundred file segment variations; however, attackers might simply choose to omit that single critical file segment in a pirated copy of the file, in hopes that viewers would not find such a glitch to be overly annoying. A pirated movie with say 15 missing critical 5-second scenes is probably going to be too annoying to any viewer for it to be of any commercial value. Thus, the illegally broadcast movies are either substantially disrupted or the attackers must incorporate some of their file segment variations, which will facilitate traitor tracing. 
     Each intended receiver of the broadcast requires variation selection information to choose a particular combination of file segment variations for each file. In terms of the movie rental box scenario, each movie rental box must know, for each movie, which set of variations to plug into the spaces where critical scenes existed in the original movie. The particular arrangement of unmodified file content and file segment variations within the augmented file  100  shown is not critical but is merely intuitive. 
     The variations facilitate traitor tracing in a commercially viable (i.e. low bandwidth overhead) manner. If a pirated version of a file is found, say on the Internet, the identity of the particular movie rental box (or boxes) that were used to create the pirated version is of keen interest to the broadcaster and/or content creator (e.g. copyright owners). The broadcaster and/or content creator may institute legal proceedings against the culprit, and would certainly want to refuse to send new decryption keys to the compromised boxes to prevent future thievery. If different boxes are assigned different combinations of file segment variations to use, an analysis of a pirated file can help determine which boxes were used as part of an anonymous attack. 
     In the event that all of the file segment variations in a redistributed version of a file match the combination of file segment variations assigned to only a single movie rental box, prior art systems would normally identify that box as being the source of the redistributed file. However, attackers are becoming increasingly sophisticated and may choose to employ a number of boxes to produce a pirated version of a file via collusion, wherein each box contributes some information or content used to produce the illicit copy after enough such information or content has been accumulated. 
     Referring now to  FIG. 2 , a flowchart of the basic operation of a preferred embodiment of the present invention is shown. The formulation of the current SKB by the license agency assures that all of the Sequence Keys in particular devices that have been identified as traitors will be deemed compromised when those devices try to play the content on the new media. Attackers would prefer to use already-compromised Sequence Keys if they could, so that no new forensic information could be deduced by the license agency. Therefore, it is important that compromised keys no longer be usable by the attackers. The problem is that there are many thousands of devices that may have a single compromised key. Therefore, revocation of a single key is impractical. 
     On the other hand, since no two devices have very many keys in common, even if the system has been heavily attacked and a significant fraction of the Sequence Keys is compromised, all innocent devices will have many columns in which they have uncompromised keys. Thus it is possible to revoke a set of compromised keys rather than a single key. The purpose of the Sequence Key Block is to give all innocent devices a column they can use to calculate the correct answer, while at the same time preventing traitor devices (which have compromised keys in all columns) from getting to the same answer. In an SKB there are actually many correct answers, one for each variation in the content. For the purpose of explanation, however, it is helpful to imagine that a single SKB is producing a single answer, termed the output key. However, the invention is not limited to this case. 
     In step  200 , the invention determines whether a selected Sequence Key has been compromised. In a preferred embodiment, Sequence Keys are examined one at a time, from the beginning of a given receiver&#39;s linked list of Sequence Keys to its end, though the invention is not limited to this case. If a selected Sequence Key has not been compromised, then the player is deemed not traitorous and proceeds in step  202  to usefully decrypt the protected content as an authorized device normally would, and the invention ends. However, if the selected Sequence Key has been compromised, then further processing is required to determine if the device is traitorous or is simply an innocent receiver that happens to have a Sequence Key in common with a traitor that has been identified and revoked beforehand. If the device is known to be traitorous, all its Sequence Keys will have been revoked and are thus currently identifiable by the SKB as compromised. A traitorous receiver will thus proceed through all its available Sequence Keys without finding a valid one. 
     Thus, in step  204  of a preferred embodiment, the invention checks to see if the end of the Sequence Key list has been reached (more generally, the invention checks to see if there are no additional Sequence Keys available from the assigned set). If so, then in step  208  the receiver is traitorous according to the current SKB and the protected content is not usefully decrypted, and the invention ends. However, if additional Sequence Keys exist in the list, then the invention proceeds to step  206 , where in a preferred embodiment the selected Sequence Key is deemed to be a Link Key and is used to get the next Sequence Key in the linked list of Sequence Keys. This next Sequence Key is selected as a candidate replacement for the compromised Sequence Key, and the invention returns to step  200  to check to see if it has been compromised. (Note that in the general case, the invention can select a candidate replacement for the compromised Sequence Key from a set of available Sequence Keys in any order, even randomly). Thus, an innocent receiver that happens to have a Sequence Key in common with an identified traitor is not immediately deemed traitorous but is instead allowed to employ a renewal or replacement valid Sequence Key. 
     Sets of Sequence Keys are assigned to individual devices by the license agency out of a matrix of keys. The licensing agency will generate Sequence Keys organized in a large matrix. The matrix preferably has 256 columns and not more than 65,536 rows. Each cell in the matrix is a different Sequence Key. A single receiver device has one key in each column. Thus, each device has 256 Sequence Keys in this example. In this respect, Sequence Keys are somewhat analogous to the CPRM technology Media Keys. 
     The licensing agency assigns the Sequence Key Blocks to be used with protected files. Sequence Key Blocks are similar to the CPRM Media Key Blocks, but important differences exist, arising both from the use different ciphers (preferably AES instead of C2) and from unique considerations of specific attacks that could be employed against Sequence Key lists. However, unlike MKBs, the SKBs are preferably not part of the fundamental cryptographic protection of the content. The fundamental protection of AACS is the Media Key. In a preferred embodiment of the present invention, the SKB merely allows different variants of the Media Key to be calculated by different devices. 
     Referring now to  FIG. 3 , a diagram of a Sequence Key Block (SKB) according to a preferred embodiment of the present invention is shown. The SKB begins with a first column  300 , called the “unconditional” column. This column will have an encryption of the output key  302  (denoted “K” in the figure) in every uncompromised Sequence Key (to be precise, it is encrypted in a key derived from the Sequence Key, not the Sequence Key itself). Devices that do not have compromised keys in that column immediately decrypt the output key, and they are done. Devices, both innocent and otherwise, that do have compromised keys instead preferably decrypt a key called a Link Key  304  that allows them to process a further column in the SKB. To process the further column, such devices need both the Link Key and their Sequence Key in that column. Thus the subsequent columns are called “conditional” columns because they can only be processed by the device if it had been given the necessary Link Key in a previous column. 
     The conditional columns are produced the general same way as the first column, i.e. they will have an encryption of the output key in every uncompromised Sequence Key. Devices with a compromised key will get a further Link Key  304  instead of the output key. However, after some number of columns (depending on the actual number of compromised keys), the license agency will know that only compromised devices are getting the Link Key, because all innocent devices would have found the output key in this column or a previous column. At this point, rather than encrypting a Link Key, the agency simply encrypts a 0 (item  306 ), and the SKB is complete. 
     How do the devices know they have a Link Key  304  versus the output key  302 ? The short answer is they do not, at least not at first. Each conditional column preferably has a header  308  of known data (e.g. the hexademical value DEADBEEF is often used) encrypted in the Link Key  304  for that column. The device decrypts the header  308  with the key it currently has. If the header  308  decrypts correctly, the device knows it has a Link Key  304  and processes the column. If it does not decrypt correctly, the device knows it has either the output key  302  or a Link Key  304  for a further column. When it reaches the end of the SKB, it knows it must have an output key  302 . Note that this device logic allows the license agency to send different populations of devices to different columns by having more than one Link Key  304  output from a single column. For example, in the figure, column ( 1 ) links to both column ( 2 ) and column ( 5 ). This flexibility can help against certain types of attacks. 
     A unique consideration for Sequence Key lists arises from the following attack scenario. Suppose a coalition of hackers is formed that includes one identified and revoked traitor, and at least one other receiver that has not been revoked. The known traitor&#39;s first Sequence Key is used on a current SKB, and a Link Key  304  results because that Sequence Key is compromised. The invention then moves to the next column in the SKB and tries to determine if it&#39;s dealing with an innocent receiver that merely happens to have a compromised key in common with a traitor. However, instead of using the known traitor&#39;s next Sequence Key (which would lead to yet another Link Key  304  and eventually to a 0), the coalition now employs the other, unrevoked, receiver&#39;s Sequence Key along with the Link Key  304  from the previous column. In this attack and related variants, the possibility exists that the coalition would fool the system and gain access to the protected content in a way that would confound subsequent tracing of all the traitors. To guard against this scenario, the key matrix from which SKBs are generated is preferably subdivided into sub-populations small enough to allow deterministic identification of all traitors in a coalition comprising an identified revoked traitor and new “turncoats” that have not yet been identified and revoked by a given SKB. All traitor tracing schemes used in this scenario are within the scope of this invention. Similarly, SKB subdivision and population management is also employed against scenarios in which candidate Sequence Keys are not selected by proceeding through a set of Sequence Keys in any particular order. 
     Although the invention has been described above as producing a single correct cryptographic answer enabling access to protected content, in the broader case, there is not just a single output key, but multiple output keys termed Variant Data. Calculation of the Media Key Variant Data using Sequence Keys is now described. 
     Each AACS-compliant device capable of playing pre-recorded content is given a set of secret Sequence Keys when manufactured. These are in addition to the Device Keys that all AACS devices require. These Sequence Keys are provided by the license agency and are for use in processing the Sequence Key Block. The result of the calculation is Variant Data which is then combined with the Media Key from the Media Key Block to generate the Media Key Variant. Key sets may either be unique per device, or used commonly by multiple devices. 
     In a preferred embodiment, each device receives 256 64-bit Sequence Keys, which are referred to as K s   _   i (i=0, 1, . . . , 255). For each Sequence Key there is an associated Column and Row value, referred to as C s   _   i  and R s   _   i  (i=0, 1, . . . , n−1) respectively. Column and Row values start at 0. For a given device, no two Sequence Keys will have the same associated Column value (in other words, a device will have at most one Sequence Key per Column). It is possible for a device to have some Sequence Keys with the same associated Row values. 
     A device uses a Sequence Key K s   _   i  together with the Media Key K m  to calculate the Media Sequence Key K ms   _   i  as follows:
 
 K   ms =AES_ G ( K   m   ,K   s   _   i ∥0000000000000000 16 )
 
     AES is the American Encryption Standard, a block cipher adopted as an encryption standard by the U.S. government. AES is described in detail in National Institute of Standards and Technology (NIST), Advanced Encryption Standard (AES), FIPS Publication 197, Nov. 26, 2001, and National Institute of Standards and Technology (NIST), Recommendation for Block Cipher Modes of Operation—Methods and Techniques, NIST Special Publication 800-38A, 2001 Edition. See also the AES common book, Advanced Access Content System: Introduction and Common Cryptographic Elements. 
     AES_G is a one-way function defined using the AES cipher. The AES-based one-way function result is calculated as:
 
AES_ G ( x   1   ,x   2 )=AES_128 D ( x   1   ,x   2 ) XORx   2 .
 
where XOR is the bitwise exclusive-OR function. AES_G is specified in the AES common book, in section 2.1.3.
 
     AES_ECBD is the AES decrypt function in electronic codebook mode (AES Electronic CodeBook Decrypt). In this mode, the cipher treats each 128-bit cipher text block as a word to be deciphered independently of any that came before or any that come after, as if it were looking it up in a codebook. When the cipher is used in this way, a change to one block of the cipher text only affects decryption of that block. Contrast this with AES in cipher block chaining mode, in which each cipher text block is combined with a value computed while deciphering the previous block in order to decipher it. When the cipher is operated in cipher block chaining mode, a change to any block of the cipher text affects decryption of all subsequent blocks in the chain. AES_ECBD (referred to as AES_128D(k, d)) is specified in the AES common book, in section 2.1.1. 
     The Sequence Keys thus serve a similar role that the Device Keys serve in CPRM, i.e. the device does not use its Sequence Key directly to decrypt, but instead it combines it with the Media Key first as shown above. That means that a given SKB is associated with a given MKB (because the SKB depends on the Media Key for correct processing). A device preferably treats its Sequence Keys as highly confidential, and their associated Row values as confidential, as defined in the AACS license agreement. 
     The SKB is generated by the license agency and allows all compliant devices, each using their set of secret Sequence Keys and the Media Key, to calculate the Variant Data, D v , which in turn allows them to calculate the Media Key Variant. If a set of Sequence Keys is compromised in a way that threatens the integrity of the system, an updated SKB can be released that causes a device with the compromised set of Sequence Keys to calculate invalid Variant Data. In this way, the compromised Sequence Keys are “revoked” by the new SKB. 
     An SKB is formatted as a sequence of contiguous Records. Each Record begins with a one-byte Record Type field, followed by a three-byte Record Length field. The Record Type field value indicates the type of the Record, and the Record Length field value indicates the number of bytes in the Record, including the Record Type and the Record Length fields themselves. Record lengths are always multiples of 4 bytes. The Record Type and Record Length fields are never encrypted. Subsequent fields in a Record may be encrypted, depending on the Record Type. 
     Using its Sequence Keys, a device calculates D v  by processing Records of the SKB one-by-one, in order, from first to last. Except where explicitly noted otherwise, a device must process every Record of the SKB. The device must not make any assumptions about the length of Records, and must instead use the Record Length field value to go from one Record to the next. If a device encounters a Record with a Record Type field value it does not recognize, it ignores that Record and skips to the next. For some Records, processing will result in the calculation of a D v  value. Processing of subsequent Records may update the D v  value that was calculated previously. After processing of the SKB is completed, the device uses the most recently calculated D v  value as the final value for D v . 
     If a device correctly processes an SKB using Sequence Keys that are revoked by that SKB, the resulting final D v  will have the special value 000000000000000000 16 . This special value will never be an SKB&#39;s correct final D v  value, and can therefore always be taken as an indication that the device&#39;s Sequence Keys are revoked. Device behavior in this situation is defined by the particular implementation. As an example, a device could exhibit a special diagnostic code, as helpful information to a service technician. 
     The remaining portion of this application describes in detail a particular implementation of the present invention, including various formats likely to be followed by the AACS license agency. However, the present invention is not limited to this particular implementation. 
     Referring now to  FIG. 4 , a Nonce Record format is shown according to an embodiment of the present invention. The nonce number X is used in the Variant Data calculation as described below. The nonce record will always precede the Calculate Variant Data Record and the Conditionally Calculate Variant Data Records in the SKB, although it may not immediately precede them. 
     Referring now to  FIG. 5 , a Calculate Variant Data Record format is shown according to an embodiment of the present invention. A properly formatted SKB will have exactly one Calculate Variant Data Record. Devices must ignore any Calculate Variant Data Records encountered after the first one in an SKB. The use of the reserved fields is currently undefined, and they are ignored. The Generation field will contain 0001 16  for the first generation. The Column field indicates the associated Column value for the Sequence Key to be used with this Record, as described below. Bytes  20  and higher contain Encrypted Key Data (possibly followed by some padding bytes at the end of the Record, not shown in  FIG. 5 ). The first ten bytes of the Encrypted Key Data correspond to Sequence Key Row  0 , the next ten bytes correspond to Sequence Key Row  1 , and so forth. 
     Before processing the Record, the device checks that both of the following conditions are true:
 
Generation==000001 16  
 
and
 
the device has a Sequence Key with associated Column value  C   d   _   i ==Column, for some  i.  
 
     If either of these conditions is false, the device ignores the rest of the Record. 
     Otherwise, using the value i from the condition above, the value X from the Nonce Record, and r=R d   _   i , c=C d   _   i , the device calculates:
 
 D   v =[AES_ G ( K   ms   _   i   ,XXORf ( c,r ))] msb   _   80   XORD   ke   _   r  
 
     where K ms   _   i  is the device&#39;s i th  Media Sequence Key&#39;s value and D ke   _   r  is the 80-bit value starting at byte offset r×10 within the Record&#39;s Encrypted Key Data. f(c,r) represents the 128-bit value:
 
 f ( c,r )=0000 16   ∥c∥ 0000 16   ∥r∥ 0000000000000000 16  
 
     where c and r are left-padded to lengths 16 bits, by prepending zero-valued bits to each as needed. The resulting D v  becomes the current Variant Data value. 
     It is not necessary for a first generation device to verify that Record Length is sufficient to index into the Encrypted Key Data. First generation devices are assured that the Encrypted Key Data contains a value corresponding to their Device Key&#39;s associated Row value. 
     Referring now to  FIG. 6 , a Conditionally Calculate Variant Data Record format is shown, according to an embodiment of the present invention. A properly formatted SKB may have zero or more Conditionally Calculate Media Key Records. Bytes  4  through  19  of the Record contain Encrypted Conditional Data (D ce ). If decrypted successfully, as described below, bytes  4  through  7  contain the value DEADBEEF 16 , bytes  8 - 9  contains the associated Column value for the Device Key to be used with this Record, and bytes  10 - 11  contain a Generation value of 0001 16  for the first generation. Bytes  20  and higher contain Doubly Encrypted Variant Data (possibly followed by some padding bytes at the end of the Record, not shown in  FIG. 6 ). The first ten bytes of the Doubly Encrypted Key Data correspond to Sequence Key Row  0 , the next ten bytes correspond to Sequence Key Row  1 , and so forth. 
     Upon encountering a Conditional Calculate Variant Data Record, the device first calculates its current Media Key Variant, as follows:
 
 K   mv =AES_ G ( K   m   ,D   v ∥0000000000000 16 )
 
     Where D v  is its current Variant Data calculated from a previous Calculate Variant Data Record or Conditional Calculate Variant Data Record. 
     Using its current K mv  value, the device calculates Conditional Data (D c ) as:
 
 D   c =AES_ ECBD ( K   mv   ,D   ce ).
 
     Before continuing to process the Record, the device checks that all of the following conditions are true:
 
[ D   c ] msb   _   32   ==DEADBEEF   16  
 
and
 
[ D   c ] 79:64 ==0001 16  
 
and
 
the device has a Sequence Key with associated Column value  C   d   _   i   ==[D   c ]95:80 for some  i.  
 
     If any of these conditions is false, the device ignores the rest of the Record. 
     Otherwise, using the value i from the condition above, X from the Nonce Record, the device&#39;s current Variant Data D v , and r=R d   _   i , c=C d   _   i , the device calculates:
 
 D   v =[AES_ G ( K   ms   _   i   ,XXORf ( c,r )) XORD   v ] msb   _   80   XORD   kde   _   r  
 
     where D kde   _   r  is the 80-bit value starting at byte offset r×10 within the Record&#39;s Doubly Encrypted Key Data, f(c,r) represents the 128-bit value:
 
 f ( c,r )=0000 16   ∥c∥ 0000 16   ∥r∥ 0000000000000000 16  
 
     where c and r are left-padded to lengths 16 bits, by prepending zero-valued bits to each as needed. The resulting D v  becomes the current Variant Data value. This record is always a multiple of 4 bytes; if necessary, pad bytes are added on the end. 
     Referring now to  FIG. 7 , an End of Sequence Key Block Record format is shown, according to an embodiment of the present invention. A properly formatted SKB contains an End of Sequence Key Block Record. When a device encounters this Record it stops processing the SKB, using whatever D v  value it has calculated up to that point as the final D v  for that SKB. 
     The End of Sequence Key Block Record contains the license agency&#39;s signature on the data in the Sequence Key Block up to, but not including, this record. Devices may ignore the signature data. However, if any device checks the signatures and determines that the signature does not verify or is omitted, it must refuse to use the Variant Data. The length of this record is always a multiple of 4 bytes. 
     Regarding the calculation of the Media Key Variant from the Variant Data, when the device has finished processing the SKB, and if it has not been revoked, it will have an 80-bit valid Variant Data D v . The device calculates the Media Key Variant from the Variant Data as follows:
 
 K   mv =AES_ G ( K   m   ,D   v ∥000000000000 16 )
 
     In addition, the low-order 10 bits of the Variant Data identify the Variant Number for the device to use in playing the content, from 0 to 1023. This number usually denotes the particular Title Key file the device should use to decrypt the content, although the meaning and use of the Variant Number is format-specific. 
     A general purpose computer is programmed according to the inventive steps herein. The invention can also be embodied as an article of manufacture—a machine component—that is used by a digital processing apparatus to execute the present logic. This invention is realized in a critical machine component that causes a digital processing apparatus to perform the inventive method steps herein. The invention may be embodied by a computer program that is executed by a processor within a computer as a series of computer-executable instructions. These instructions may reside, for example, in RAM of a computer or on a hard drive or optical drive of the computer, or the instructions may be stored on a DASD array, magnetic tape, electronic read-only memory, or other appropriate data storage device. 
     While the invention has been described with respect to illustrative embodiments thereof, it will be understood that various changes may be made in the apparatus and means herein described without departing from the scope and teaching of the invention. Accordingly, the described embodiment is to be considered merely exemplary and the invention is not to be limited except as specified in the attached claims.