Abstract:
Content is authenticated by generating signature certificates for content that has been successfully watermark screened. When a request is received for importation of content into a secured domain, a search is made in a signature certificates cache for a signature certificate associated with that requested content. If an identified signature certificate authenticates the requested content, the content is imported into the secured domain. If no signature certificate is identified, the content is watermark screened. If the content successfully passes the watermark screening process, a new signature certificate is then added to the signature certificate cache.

Description:
BACKGROUND 
     Digital audio content is downloaded and played on a computer using an application such as an MP3 player. MP3 is short for MPEG Layer 3 (Moving Pictures Expert Group) and refers to a format for storing digital audio. 
     A Secure Digital Music Initiative (SDMI) has been formed to prevent unauthorized copying and playing of digital audio content. SDMI compliant systems are any device, software application, or any other system that conforms to the requirements of the SDMI specification. Version 1.0 of the SDMI Portable Device Specification Part 1, document No. pdwg99070802, was published on July 8, 1999. 
     Most SDMI compliant software and hardware devices being introduced onto the market today import content into an SDMI Local Environment before storing the content on the computer. The SDMI Local Environment refers to a subset of the environment where all SDMI rules and behaviors are obeyed. One SDMI rule is that unencrypted content must be watermark screened before the content may be stored in the SDMI Local Environment. Importing content directly into the SDMI Local Environment is beneficial to a user because this time-consuming watermark screening process required by SDMI is only done once at storage rather than each time the content is downloaded to a device. 
     Importing unencrypted content directly into the SDMI Local Environment requires that the content be encrypted as SDMI content to remain persistently on the user&#39;s computer after watermark screening. Storing encrypted content introduces several usability problems for the user. Once the digital content is encrypted in the SDMI Local Environment, SDMI default usage rules restrict the user&#39;s ability to copy the SDMI content to non-SDMI devices. However, a user may want to use the digital content on both SDMI compliant devices and non-SDMI compliant devices. Because of the SDMI restrictions, the user has to store one SDMI encrypted copy of the content for the SDMI compliant devices and store one unencrypted copy of the content for use with the non-SDMI compliant devices. Storing both encrypted and non-encrypted content files wastes computer disk space and prevents interoperability of any one content file with all content players. 
     The present invention addresses this and other problems associated with the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing how a computer system authenticates content using signature certificates. 
     FIG. 2 is a block diagram showing in further detail how the computer of FIG. 1 authenticates content using the signature certificates. 
     FIG. 3 is a flow diagram showing in further detail how signature certificates are used to authenticate content. 
     FIGS. 4 and 5 are flow diagrams showing different techniques for authenticating content. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a computer system  12  that includes a computer  18 , screen  14 , keyboard  22 , speakers  16  and compact disc player  20 . The computer system  12  may be connected to the Internet  30  or any other wide area or local area networks. The computer screen  14  shows an icon for a Secure Digital Music Initiative (SDMI) compliant content player  24  (SDMI application) stored in computer  18 . In one example, the SDMI content player  24  is an MP3 Jukebox player. The computer screen  14  also shows an icon for an audio content file  26  stored in computer  18 . The content in one embodiment is an MP3 audio file. However, the invention is applicable to any content that requires authentication. 
     The audio content  26 , video content or any other type of content may be downloaded onto the computer system  12  from the Internet  30 , from a compact disc loaded into compact disc player  20 , or from any other storage medium. While FIG. 1 shows a computer system  12 , the invention is applicable to any system capable of storing content and then playing or downloading that content to a user. For example, the invention may be integrated into a stereo system. 
     A third icon on screen  14  shows a signature certificate  28 . In one instance, the signature certificate  28  authenticates that the audio content  26  has been screened for an SDMI watermark. The watermark screening is done at the time audio content  26  is stored in computer  18 . But the audio content  26  is not imported into the SDMI Local Environment until the time of download to a SDMI compliant device or until time of playback by the SDMI content player  24 . This technique allows the audio content  26  to remain in the same non-encrypted format it was in prior to watermark screening. 
     The SDMI usage rules do not come into effect until the content  26  is downloaded to a device or played back by the SDMI content player  24 . The content  26  is prevented from being imported into the SDMI Local Environment until the content is first authenticated by the signature certificate  28 . Authentication means that a signature certificate was created from content that successfully passed the SDMI watermark screening process and that signature certificate was derived from the same content that is currently being requested for importation into the SDMI Local Environment. If signature certificate  28  authenticates the audio content  26 , the audio content  26  is played back through speakers  16 , or if requested, downloaded to a portable device (not shown). 
     Watermark screening may not be necessary if the content  26  is going to be played on certain content players. For example, content players that only play content directly out on the speakers  16 . The audio content  26  for these content players may be played without prior certificate authentication. 
     Usability problems are avoided because the same unencrypted audio content  26  may be used for both SDMI compliant devices, such as SDMI content player  24 , and non-SDMI complaint devices. Thus, a SDMI encrypted copy and a non-encrypted copy of the same content do not have to be stored on computer system  12 . Authentication of content  26  is also faster because the watermark screening process is only performed once at initial content download, instead of each time the content is imported into the SDMI Local Environment. This makes the watermark screening process essentially invisible to a user. 
     FIG. 2 is a more detailed block diagram of the computer  18  shown in FIG.  1 . Content files  34  and a signature certificate cache  40  are stored on a computer hard disk or other memory storage device. The SDMI content player  24  is loaded into local memory  32  and plays only SDMI compliant content in content files  34 . In other words, the SDMI content player  24  operates within the SDMI Local Environment. 
     The SDMI content player  24  includes software  38  that authenticates and generates signature certificates for the content files  34 . A processor  42  receives user requests  46  to playback selected audio content. Pursuant to the user requests  46 , the processor  42  initiates certificate authentication/generation software  38 . If the content file  34  selected for authentication is authenticated by one of the signature certificates in cache  40 , the selected content is played back through output  44  or downloaded to a portable device. 
     The signature certificates in cache  40  may be given names that associate them with corresponding content files  34 . The signature certificate names may be the same as the content files  34  with an additional extension. For example, the content files  34  are shown with file names MP3 — 1, MP3 — 2, . . . etc. The signature certificates associated with these content files may be given the file names CER_MP3 — 1, CER_MP3 — 2, . . . etc. Alternatively, the signature certificates may be described according to the number of bits in the compressed portion of the associated content file  34 . The processor  42  searches for any signature certificates in cache  40  having the same name, or alternative identifying the same number of bits, as the selected content file  34 . Any other type of naming convention that associates the signature certificates with the content files  34  may also be used. 
     FIG. 3 shows how the signature certificates are used to authenticate watermark screening of the audio content. A user in block  60  requests importation of clear content into the SDMI Local Environment for playback on a secure SDMI compliant device or application. Clear content is content that has not been encrypted. In one case the clear content is an MP3 file. The signature certificates cache is searched in block  62  for a signature certificate having a descriptor associated with the selected content. 
     Each signature certificate in the cache is associated with a content file that has previously passed the SDMI watermark screening process. If a signature certificate is found in decision block  64 , then that signature certificate is used to authenticate the selected content. If the identified signature certificate authenticates the content in decision block  72 , the content is admitted into the SDMI Local Environment in block  76 . This means the content may be played or downloaded by the SDMI compliant application. 
     Optionally SDMI default copy restrictions may be incorporated. The SDMI rules may restrict the number of copies of a particular content file that may be checked out to portable devices. This SDMI rule may optionally be implemented in block  75  by storing a value in the signature certificate tracking how many copies of the associated content have been checked out to portable devices. If the value in the signature certificate does not violate a maximum allowable check out value in block  75 , the content is admitted to the SDMI local environment in block  76 . This allows implementation of the SDMI copy restrictions without using encrypted content. 
     If no signature certificate is found in the cache in decision block  64 , the content is submitted to the SDMI watermark screening process in block  66 . Watermarks are analog signals incorporated into uncompressed audio content. Using signal processing techniques, the watermark screening process detects the watermark audio tones in the audio content. Watermark screening decompresses the audio file into Pulse Code Modulate (PCM) data. Signal processing routines are then applied to the PCM data to detect the watermark tones. Watermark screening is known and is therefore not described in further detail. 
     If the content does not pass the watermark screening process in decision block  68 , the content is not admitted to the SDMI Local Environment in block  70 . This means the content will not be played by the SDMI content player. If the content passes the watermark screening process in decision block  68 , a signature certificate is created for the compressed content file and added to the signature certificate cache in block  74 . The content is then admitted into the SDMI Local Environment in block  76 . 
     FIG. 4 describes in further detail how the signature certificates are first generated and how selected content is authenticated with the signature certificates in block  72  (FIG.  3 ). Signature certificates are digital signatures that securely verify the content as the same unaltered content that previously passed the watermark screening process. One example of a digital signature algorithm is a Media Digest  5  (MD5) hash that generates a string of bits as a function of the source content and an encryption key. The certificate represents the string of bits output from the MD5 hash. 
     Block  88  represents the first time content is imported either into the computer or requested to be imported into the SDMI Local Environment. Since the content has never before been imported into the SDMI Local Environment, the content is watermark screened in block  90 . If the content successfully passes watermark screening, a signature certificate  94  is generated for the content in block  92 . The signature is derived from the bits of the content file as originally downloaded on the computer. This content file could be in a digitally compressed format to reduce the amount of memory needed to store the content file. 
     Any time a user requests importation of that same content into the SDMI Local Environment, the authentication process starts at block  80 . This time the content is not watermark screened. Instead the signature certificate  94  previously generated for that content is located in the signature certificate cache. That signature certificate  94  is applied to the content file in block  84  using a function that will return true or false if the content file is the same set of bits that was used to create the signature certificate  94 . 
     If the content file contains the same set of bits originally used to generate the signature certificate in decision block  86 , the content is authenticated as being the same content that previously passed the watermark screening  90 . Accordingly, the content is imported into the SDMI Local Environment in block  98 . 
     If SDMI copy rules are incorporated with the certification authentication scheme, the content will only be imported if the signature certificate also attests that the content has not already been checked out more than a predetermined number of times. 
     If the content file is not the same set of bits that generated the signature certificate, the content is not imported into the SDMI Local Environment and the session is terminated in block  96 . 
     Certificate authentication does not require decompression of the MP3 content into a Pulse Code Modulated (PCM) format and complex watermark screening of that decompressed content before every content playback. Certificate authentication also does not require persistent storage of separate SDMI and non-SDMI versions of the same content in memory. 
     FIG. 5 is another variation on content authentication. After the watermark screening process and signature certificate generation, that same content is selected again for importation into the SDMI Local Environment in block  100 . This time the same signature generation algorithm previously used for generating the signature certificate is again applied to the content in block  102 . The content is not watermark screened prior to generation of this signature. Block  104  then searches the signature certificate cache  106  for any signature certificates matching the signature that was just derived for the content. If a match is found in decision block  108 , the content is imported into the SDMI Local Environment in block  110 . If no match is found, the content is not imported into the SMDI Local Environment and the session terminated in block  112 . 
     Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention may be modified in arrangement and detail without departing from such principles. I claim all modifications and variation coming within the spirit and scope of the following claims.