Abstract:
Data such as a musical track is stored as a secure portable track (SPT) which can be bound to one or more players and can be bound to a particular storage medium, restricting playback of the SPT to the specific players and ensuring that playback is only from the original storage medium. The SPT is bound to a player by encrypting data of the SPT using a storage key which is unique to the player, is difficult to change, and is held in strict secrecy by the player. The SPT is bound to a particular storage medium by including data uniquely identifying the storage medium in a tamper-resistant form, e.g., cryptographically signed. The SPT can also be bound to the storage medium by embedding cryptographic logic circuitry, e.g., integrate circuitry, in the packaging of the storage medium. The SPT is bound by encrypting an encryption key using the embedded logic. By using unique cryptographic logic, only that particular storage medium can decrypt the encryption key and, therefore, the data of the SPT encrypted with the encryption key. To allow a user to playback the SPT on a number of players, players can share storage keys with one another. Such key sharing is done in a cryptographically secure manner. Before downloading an SPT to a particular external player, the ability of the external player to enforce restrictions placed upon the SPT is verified.

Description:
FIELD OF THE INVENTION 
     The present invention relates to systems for distributing and playing digitized audiovisual signals and, in particular, to a mechanism for distributing and playing such digitized audiovisual signals such that unauthorized copying of such signals is discouraged to thereby protect intellectual property rights of artists. 
     BACKGROUND OF THE INVENTION 
     Recent advances in lossless compression of digitized audio signals and storage capacity has recently led to the development of music players which play CD-quality music stored in solidstate memory. For example, a number of MP3 players are available into which a user can download compressed, CD-quality digitized audio signals into solid-state memory for subsequent playback. “MP3” generally refers to the MP3 format which is the MPEG standard for audio coding (MPEG-1 Video, Layer 3 Audio, ISO Standard #1172-3). The MP3 format provides excellent sound quality at a data rate of 128 Kbits (44 KHz, 16-bit samples, stereo). 
     While MP3 players provide very good sound quality and great convenience for the user, MP3 players provide essentially no protection whatsoever against unauthorized copying of copyrighted works. Currently, a number of computer systems provide free access to copyrighted musical works through the Internet. A user who is in possession of a digitized, copyrighted music signal in the MP3 format can, albeit most likely in violation of copyright laws, distribute unlimited identical digital copies of the music signal to friends with no compensation whatsoever to the copyright holder. Each such copy suffers no loss of quality from the original digitized music signal. 
     A few attempts have been made to thwart the unauthorized proliferation of perfect digital copies of digitized audiovisual signals. One such technique is used in minidisc and digital audio tape (DAT) devices. To allow transfer of previously purchased digitized audio signals, one digital-to-digital copy is permitted. In other words, digital copies of digital copies is prevented. Typically, a single bit in the storage medium indicates whether the stored signal is a digital copy. If content is written to the storage medium—e.g., either a minidisc or a DAT tape—through a digital port in a player/recorder, the bit is set to indicate that the content of the medium is a digital copy. Otherwise, the bit is cleared to indicate either an analog copy—content recorded through an analog port of the player/recorder—or that the content is an original recording, e.g., through a microphone. 
     This form of copy protection is insufficiently restrictive. For example, an owner of an audio DAT can distribute at least one unauthorized copy to another person. In addition, unlimited digital copies of a CD can be made onto minidiscs or DATs although each of those digital copies cannot be digitally copied. This form of copy protection can also be excessively restrictive, preventing an owner of a prerecorded audio medium to make copies for each of a number of players of the prerecorded audio owner, namely, players in the home, office, car, and for portable use. 
     As alluded to briefly above, the single-copy mechanism fails to prevent any copying of digital read-only media such as CDs. The content of such media is typically uncompressed and un-obscured such that unauthorized copying is unimpeded. 
     What is needed is a mechanism by which copyrightable content of digital storage media is protected against unauthorized copying while affording the owner of such digital storage reasonable unimpeded convenience of use and enjoyment of the content. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, data such as a musical track is stored as a secure portable track (SPT) which can be bound to one or more specific external players and can be bound to the particular storage medium in which the SPT is stored. Such restricts playback of the SPT to the specific external players and ensures that playback is only from the original storage medium. Such inhibits unauthorized copying of the SPT. 
     The SPT is bound to an external player by encrypting data representing the substantive content of the SPT using a storage key which is unique to the external player, is difficult to change (i.e., is read-only), and is held in strict secrecy by the external player. Specifically, the data is encrypted using a master media key and the master media key is encrypted using the storage key. Since only the external player knows the storage key, the master media key is passed to the external player using a secure communication session and the external player encrypts the master media key using the storage key and returns the encrypted master media key. Accordingly, only the specific external player can decrypt the master media key and, therefore, the data representing the substantive content of the SPT. 
     The SPT is bound to a particular piece of storage medium by including data uniquely identifying the storage medium in a tamper-resistant form, e.g., cryptographically signed. The medium identification data is difficult to change, i.e., read-only. Prior to playback of the SPT, the external player confirms that the media identification data has not been tampered with and properly identifies the storage medium. 
     The SPT can also be bound to the storage medium by embedding logic circuitry, e.g., integrated circuitry, in the packaging of the storage medium for performing cryptographic processing. The SPT is bound by encrypting the master media key, which is used to encrypt the data representing the substantive content of the SPT, using the embedded logic. By using unique cryptographic logic in the packaging of the storage medium, only that particular storage medium can decrypt the master media key and, therefore, the substantive content of the SPT. 
     To allow a user to playback the SPT on a number of players, e.g., one in the home, one in the office, one in the car, etc., external players can share storage keys with one another. However, such key sharing must be done in a cryptographically secure manner to prevent crackers from attempting to collect storage keys from external players. 
     The two external players communicate with one another in a cryptographically secure session. One, the initiator, sends a request message which includes a certificate of the initiator and a first random number. The other, i.e., the responder, authenticates the initiator using the certificate and responds with a reply message. The reply message includes the certificate of the responder, the first random number, a second random number, and one or more storage keys of the responder encrypted with a public key of the initiator. The initiator authenticates the responder using the certificate and responds with an exchange message. The exchange message includes the first and second random numbers and one or more storage keys of the initiator encrypted with a public key of the responder. Thus, each has copies of the other&#39;s storage keys and can play SPTs bound to the other external player. 
     Before downloading an SPT to a particular external player, the ability of the external player to enforce restrictions placed upon the SPT is verified. During a registration process, the external player identifies those types of restrictions which can be enforced by the external player. Such types include a maximum number of times an SPT is played, an expiration time beyond which the SPT can no longer be played, and a number of copies of the SPT which can be made. For each type of restriction imposed upon a particular SPT, the external player is verified to be able to enforce that particular type of restriction,. If the external player is unable to enforce any of the restrictions imposed upon the SPT, downloading and/or binding of the SPT to the external player is refused. Otherwise, downloading and/or binding is permitted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a computer system which includes a player, secure portable tracks, and an interface for an external player in accordance with the present invention. 
     FIG. 2 is a block diagram of the interface and external player of FIG. 1 showing a storage medium for the secure portable track in greater detail. 
     FIG. 3 is a block diagram of the format of a secure portable track in greater detail. 
     FIG. 4 is a block diagram illustrating bindings in the header of the secure portable track of FIG. 3 in accordance with the present invention. 
     FIG. 5 is a block diagram of two external players in accordance with the present invention in greater detail. 
     FIG. 6 is a logic flow diagram of the encoding of content to bind the content to an external player and medium in accordance with the present invention. 
     FIG. 7 is a logic flow diagram of the decoding of content to enforce a binding of the content to an external player and medium in accordance with the present invention. 
     FIG. 8 is a logic flow diagram of the exchange of keys between the two external players shown in FIG. 5 in accordance with the present invention. 
     FIG. 9 is a block diagram illustrating restrictions in the header of the secure portable track of FIG. 3 in accordance with the present invention. 
     FIG. 10 is a logic flow diagram illustrating the assurance of an external player&#39;s ability to enforce restrictions in accordance with the present invention. 
     FIG. 11 is a block diagram of the interface and external player of FIG. 1 showing a storage medium for the secure portable track in greater detail. 
     FIG. 12 is a logic flow diagram of the encoding of content to bind the content to a storage medium in accordance with the present invention. 
     FIG. 13 is a logic flow diagram of the decoding of content to enforce a binding of the content to a storage medium in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     In accordance with the present invention, data such as a musical track is stored as a secure portable track (SPT) which can be bound to one or more specific external players and can be bound to the particular storage medium in which the SPT is stored. Such restricts playback of the SPT to the specific external players and ensures that playback is only from the original storage medium. Such inhibits unauthorized copying of the SPT. 
     A brief overview of the operating environment of the secure portable music playing system according to the present invention facilitates appreciation and understanding of the present invention. Computer system  100  (FIG. 1) has a typical architecture. Computer system  100  includes a processor  102  and memory  104  which is coupled to processor  102  through an interconnect  106 . Interconnect  106  can be generally any interconnect mechanism for computer system components and can be, e.g., a bus, a crossbar, a mesh, a torus, or a hypercube. Processor  102  fetches from memory  104  computer instructions and executes the fetched computer instructions. Processor  102  also reads data from and writes data to memory  104  and sends data and control signals through interconnect  106  to one or more computer display devices  120  and receives data and control signals through interconnect  106  from one or more computer user input devices  130  in accordance with fetched and executed computer instructions. 
     Memory  104  can include any type of computer memory and can include, without limitation, randomly accessible memory (RAM), read-only memory (ROM), and fixed and removable storage devices which include storage media such as magnetic and/or optical disks. Memory  104  includes a music player  110  which includes a secure portable track (SPT) interface  114  and which is all or part of one or more computer processes which in turn execute within processor  102  from memory  104 . A computer process is generally a collection of computer instructions and data which collectively define a task performed by a computer system such as computer system  100 . Thus, when a computer process, such as player  110 , takes a particular action, in reality processor  102  executes computer instructions of the computer process and execution of those computer instructions causes the particular action to be taken. 
     Each of computer display devices  120  can be any type of computer display device including without limitation a printer, a cathode ray tube (CRT), a light-emitting diode (LED) display, or a liquid crystal display (LCD). Each of computer display devices  120  receives from processor  102  control signals and data and, in response to such control signals, displays the received data. Computer display devices  120 , and the control thereof by processor  102 , are conventional. 
     Each of user input devices  130  can be any type of user input device including, without limitation, a keyboard, a numeric keypad, or a pointing device such as an electronic mouse, trackball, lightpen, touch-sensitive pad, digitizing tablet, thumb wheels, or joystick. Each of user input devices  130  generates signals in response to physical manipulation by the listener and transmits those signals through interconnect  106  to processor  102 . 
     Input/output (I/O) port  140  receives control signals from processor  102  through interconnect and, in response to the control signals, receives data from and sends data to processor  102 . In addition, I/O port  140  sends data to and receives data from a device which can be coupled to I/O port  140 . In this embodiment, a secure portable music player  150  is coupled to I/O port  140 . I/O port  140  can be, for example, a serial port or a parallel port. Secure portable music player  150  is sometimes referred to herein as portable player  150 . 
     Network access circuitry  160  couples computer system  100  to a computer network  170  which can be, for example, an intranet or internet. Network access circuitry  160  implements data transfer protocols between interconnect  106  and computer network  170  and can be, for example, a modem or ethernet circuitry. 
     Briefly, player  110  receives musical tracks  112  and associated data through computer network  170  in a manner described more completely in U.S. patent application Ser. No. 09/020,025 filed Feb. 6, 1998 entitled “Secure Online Music Distribution System” by Philip R. Wiser, Andrew R. Cherenson, Steven T. Ansell, and Susan A. Canon which is incorporated herein in its entirety by reference. Accordingly, tracks  112  are stored in an encrypted format in which only player  110  can decrypt tracks  112  for playback of the substantive content of tracks  112 . SPT interface  114  creates secure portable tracks (SPTs)  116  from tracks  112  and downloads SPTs  116  to portable player  150 . While the substantive content of tracks  112  and SPTs  116  is described in this illustrative embodiment as music, it is appreciated that many of the techniques and mechanisms described herein are equally applicable to other forms of data for which unauthorized copying is to be thwarted. Examples of such content includes, for example, still graphical images, motion video, and computer software. 
     In accordance with the present invention, SPTs  116  are bound both to storage medium  202  (FIG. 2) in which SPTs  116  are stored within portable player  150  and to one or more specific external players, e.g., portable player  150 . For example, storage medium  202  is a removable digital storage medium such as a recordable compact disc (CD-R), a minidisc, a digital video disc (DVD), digital audio tape (DAT), flash memory card, or similar removable digital storage medium. In addition, portable player  150  can include sufficient storage to store a number of SPTs  116  which can be directly downloaded into portable player  150 , obviating removable digital storage media such as storage medium  202 . However, it is desirable to permit playback of content of SPTs  116  in less-portable external players such as high-quality component players of home stereo systems and dash-mounted players installed in cars and other vehicles. Accordingly, removable storage media such as storage medium  202  is preferred to storage directly within portable player  150 . External players are playback devices which can operate while detached from computer system  100  (FIG.  1 ). 
     Binding SPTs  116  to storage medium  202  (FIG. 2) renders SPTs  116  unplayable when copied to a different storage medium. Similarly, binding SPTs  116  to a number of external players, including portable player  150 , makes SPTs  116  unplayable in external players other than the external players to which SPTs  116  are bound. Accordingly, copying of SPTs  116  is inhibited. 
     Understanding the manner in which SPTs  116  are bound to storage medium  202  and portable player  150  is facilitated by a brief description of the format of SPTs  116 . An illustrative one of SPTs  116  is shown in greater detail in FIG.  3 . SPT  116  includes a header  302  which in turn includes a number of bindings as described more completely below and a reference to a table of contents  306 . In one embodiment, table of contents  306  is the last component of SPT  116 . In such an embodiment, table of contents  306  can be formed as images  304 A-C are appended to SPT  116  during creation and can be appended to SPT  116  after all images are included in SPT  116  and table of contents  306  is complete. Each of images  304 A-C are discrete components of SPT  116  and can have a different structure. Each image of SPT  116  is represented by and is accessible through one of descriptors  308 A-D of table of contents  306 . All images of SPT  116  collectively represent the substantive content of SPT  116 , e.g., digitally represented music. 
     Header  302  includes a number of bindings  400  (FIG.  4 ), each of which binds the content of SPT  116  (FIG. 2) to both (i) storage medium  202  and (ii) a particular external player such as portable player  150 . Each of bindings  400  includes the following fields, each of which stores data representing a component of the binding: (i) media identification field  402 , (ii) media type and information field  404 , (iii) storage key identification field  406 , (iv) encrypted media master key  408 , and (v) binding message authentication code (MAC) field  410 . 
     Media identification field  402  stores data representing a read-only serial number  204  (FIG. 2) of storage medium  202 . Serial number  204  is “read-only” in that alteration of the particular value of serial number  204  is difficult. For example, serial number  204  can be stored in a portion of storage medium  202  which cannot be overwritten or can be represented in semiconductor circuitry included in storage medium  202 . It is appreciated that serial number  204  can never be completely protected from alteration by particularly industrious and persistent crackers. However, serial number  204  should not be alterable by straightforward data writing access to storage medium  202 . 
     Media type and information field  404  (FIG. 4) stores data representing the type of storage medium  202  (FIG.  2 ). Such permits comparison of the indicated type with the actual type of storage medium  202 . For example, if media type and information field  404  (FIG. 4) indicates that storage medium  202  (FIG. 2) is a DVD and portable player  150  determines that storage medium  202  is a flash memory card, portable player  150  can readily reject storage medium  202  as an invalid copy. 
     Storage key identification field  406  stores data identifying the storage key, i.e., the key with which the master media key is encrypted. The master media key is the key with which the substantive content of SPT  116  is encrypted. To bind SPT  116  to a particular external player, e.g., portable player  150 , the storage key is a key which is maintained in secrecy and is allocated to the specific external player. An example of such a storage key is read-only key  504 A (FIG. 5) of portable player  150 . Read-only key  504 A is analogous to serial number  204  (FIG. 2) of storage medium  202  in that read-only key  504 A is difficult to change, typically requiring physical deconstruction of portable player  150 . For example, read-only key  504 A can be embedded in the internal semiconductor circuitry of portably player  150 . In one embodiment, read-only key  504 A includes three (3) separate keys: one which is never shared with other external players, one which can be shared with other external players, and one which is common to all external players. By selecting a specific one of these keys as the storage key, player  110  and SPT interface  114  can select a desired level of security of the substantive content of SPT  116 . 
     Storage key identification field  406  (FIG. 4) stores a digest of the storage key to identify the storage key without recording the storage key itself within SPT  116 . 
     Encrypted media master key field  408  (FIG. 4) stores data representing an encrypted representation of the key by which the content of SPT  116  (FIG.  3 ), e.g., images  304 A-C, is encrypted. The media master key is encrypted to prevent unauthorized decryption of the content of SPT  116 . 
     Binding MAC field  410  (FIG. 4) stores data representing a message authentication code (MAC) of fields  402 - 408  and therefore provides protection against tampering with the contents of field  402 - 408  by a cracker attempting to gain unauthorized access to the content of SPT  116 . MACs are conventional and known and are not described further herein. 
     Logic flow diagram  600  (FIG. 6) illustrates the preparation of SPT  116  (FIG. 1) from one or more of tracks  110  by player  110  through SPT interface  114  for playback by portable player  150 . In step  602  (FIG.  6 ), player  110  (FIG. 1) encrypts the content of one or more of tracks  110  using, for example, symmetric key encryption. Symmetric key encryption of the content is used in this illustrative embodiment to facilitate decryption by portable player  150  with sufficient efficiency to permit uninterrupted playback of CD-quality music while simultaneously leaving sufficient processing resources within portable player  150  for decompression of compressed audio data and permitting use of relatively inexpensive components within portable player  150  with limited processing power to thereby minimize the cost of portable player  150  to consumers. 
     The master media key is encrypted using the storage key of the particular external player to which SPT  116  is to be bound. To avoid divulging the storage key to player  110 , the particular external player, rather than player  110 , encrypts the master media key. Thus, in step  604  (FIG.  6 ), player  110  (FIG. 1) encrypts the media master key using a session key formed at the onset of a secure communication session between player  110  and portable player  150  and sends the encrypted master media key to portable player  150 . Portable player  150  decrypts the master media key and re-encrypts the master media key using the storage key, e.g., read-only key  504 A and sends the encrypted master media key back to player  110 . As a result, only portable player can decrypt the encrypted master media key and therefore the content of SPT  116 . Preparation of multiple bindings is described below in greater detail. Session keys are formed using a communication key of portable player  150  which, like read-only key  504 A, is difficult to change and which is held in secrecy by portable player  150 . However, for the purposes of carrying out secure communication, portable player  150  communicates the communication key to player  110  during a one-time registration which is described more completely below. The use of a communication separate from the storage key serves to protect the secrecy of the storage key. 
     Since the master media key is encrypted using read-only key  504 A, the master media key—and therefore the content of SPT  116  which is encrypted with the master media key—can only be decrypted using read-only key  504 A. By carefully guarding the secrecy of read-only key  504 A, SPT  116  is bound to portable player  150  and can only be played back by portable player  150  or by any external player with which portable player has shared keys. A mechanism by which external players can share read-only keys in a secure manner is described below in greater detail. 
     In step  606  (FIG.  6 ), player  110  (FIG. 1) forms a digest of the storage key, e.g., read-only key  504 A (FIG.  5 ), to produce storage key identification data. 
     In step  608  (FIG.  6 ), player  110  (FIG. 1) forms SPT  116 , stores the encrypted content in SPT  116 , and forms binding  400  (FIG. 4) within header  302  of SPT  116 . Player  110  (FIG. 1) forms binding  400  (FIG. 4) by (i) storing serial number  204  (FIG. 2) in media identification field  402  (FIG.  4 ), (ii) storing data representing the type of storage medium  202  (FIG. 2) in media type and information field  404  (FIG.  4 ), (iii), storing the digest formed in step  606  (FIG. 6) in storage key identification field  406  (FIG.  4 ), (iv) storing the encrypted media master key formed in step  604  (FIG. 6) in encrypted media master key field  408  (FIG.  4 ), and (v) forming and storing in binding MAC field  410  (FIG. 4) a MAC of fields  402 - 408 . 
     Player  110  (FIG. 1) can bind SPT  116  to multiple external players by forming a separate binding  400  for each such external player. For each such binding, player  110  repeats steps  604 - 606  and step  608  except that the encrypted content is included in SPT  116  only once. Thus, there is only one media master key by which the content is encrypted but each of bindings  400  stores a different encryption of media master key. 
     The security afforded by such binding is more fully appreciated in the context of decoding for playback by portable player  150  as illustrated by logic flow diagram  700  (FIG.  7 ). In the context of logic flow diagram  700 , storage media  202  (FIG. 5) is installed in portable player  150  such that SPTs  116  are accessible to portable player  150 . Portable player  150  includes player logic  502 A which includes circuitry and/or computer software to implement the functions performed by portable player  150 . To playback a selected one of SPTs  116 , player logic  502 A reads SPT  116  and parses header  302  (FIG. 3) therefrom and parses bindings  400  (FIG. 4) from header  302 . 
     In test step  702  (FIG.  7 ), player logic  502 A (FIG. 5) retrieves read-only serial number  204  from storage media  202  and media identification data from media identification field  402  (FIG. 4) and compares read-only serial number  204  to the media identification data. If read-only serial number  204  and the media identification data are not equivalent, player logic  502 A (FIG. 5) aborts playback of SPT  116 . Accordingly, simple copying of SPT  116  from storage medium  202  to another storage media renders SPT  116  unplayable. If read-only serial number  204  and the media identification data are equivalent, processing transfers to step  704 . 
     In step  704  (FIG.  7 ), player logic  502 A (FIG. 5) selects either read-only key  504 A or a selected one of keys  506 A 1 - 4  according to the digest stored in storage key field  406  (FIG.  4 ). As described more completely below, portable player  150  can share keys with other external players. Keys  506 A 1 - 4  store read-only keys shared by other external players. The sharing of keys permits a single user to play content on a number of external players, e.g., a home player, a portable player, a player in a car, and a player at the office. In addition, read-only key  504 A can include a number of individual component keys in one embodiment. Each such component key is considered by player logic  502 A as a separate key in step  702  (FIG.  7 ). 
     To select the appropriate key, player logic  502 A forms respective digests of each component key of read-only key  504 A and each of keys  506 A 1 - 4  using the same algorithm employed by player  110  (FIG. 1) in step  606  (FIG. 6) and selects the one of keys  504 A,  506 A 1 - 4  whose digest is accurately represented in storage key identification field  406  (FIG.  4 ). If no digest is accurately represented in storage key field  406  (FIG.  4 ), player logic  502 A aborts playback and presents an error message to the user. Failure of the respective digests to be accurately represented in storage key field  406  indicates that portable player  150  (FIG. 5) does not include the storage key used by player  110  (FIG. 1) in step  604  (FIG.  6 ). Accordingly, recovery of the master media key and therefore the content of SPT  116  is not possible. 
     In step  706  (FIG.  7 ), player logic  502 A (FIG. 5) decrypts the media master key from encrypted media master key field  408  (FIG. 4) using the key selected in step  704  (FIG.  7 ). In step  708 , player logic  502 A (FIG. 5) decrypts the content of SPT  116  using the decrypted media master key. After step  708 , the content of SPT  116  is un-encrypted and is available for decompression and playback by player logic  502 A. Decompression and playback of the unencrypted content is conventional. 
     Key Sharing 
     Frequently, a user will have multiple external players—e.g., a portable player such as portable player  150 , a full-featured player as a component of a home stereo system, a dash-mounted player in a car, and perhaps a player at the user&#39;s place of work. Typically, the user would like to play a particular purchased track, e.g., SPT  116 , on all of her external players. Since SPT  116  is bound to portable player  150  according to read-only key  504 A, any external player with a copy of read-only key  504 A can also play SPT  116 . Therefore, to play SPT  116  on multiple external players, each such external player must have exchanged keys, either directly or indirectly, with portable player  150 . 
     In addition to portable player  150 , FIG. 5 shows a second external player  150 B. External player  150 B can be any of the various types of external players described above, including a second portable player. The components of portable player  150  and external player  150 B are analogous to one another as shown in FIG.  5 . Communication logic and ports  512 A-B include hardware and software to communicate with other devices such as I/O port  140  and/or other external players. In one embodiment, communication logic and ports (CLPs)  512 A-B are coupled directly to one another through a connector  520  and communicate directly with one another. Connector  502  can be, for example, a cable between communication logic and ports  512 A-B. Alternatively, connector  502  can be light signals between communication logic and ports  512 A-B which can include infrared LEDs and infrared light sensors. In an alternative embodiment, communication logic and ports  512 A-B communicate only with an I/O port of a computer such as I/O port  140  of computer system  100 . In the latter embodiment, computer system  100  includes at least two I/O ports such as I/O port  140  and both external players are coupled to computer system  100  such that SPT interface  114  acts as an intermediary to act as connector  520  between the external players. In an alternative variation of this latter embodiment, computer system  100  can have only a single I/O port  140  and SPT interface can act as a surrogate, exchanging keys with a single external player at a time and acting as a key repository. In this last embodiment, it is important that the keys stored within SPT interface  114  be stored in an encrypted form to prevent passing of the device keys to an unlimited number of external players. Such would be a serious compromise of the copy protection provided, relying more completely media binding for copy protection. 
     Logic flow diagram  800  (FIG. 8) illustrates a key exchange conducted between portable player  150  and external player  150 B. In the embodiment in which SPT interface  140  (FIG. 1) acts as a surrogate external player and a key repository, SPT  140  performs a separate key exchange with each of portable player  150  and external player  150 B in the manner described. The key exchange of logic flow diagram  800  (FIG. 8) is initiated by either of portable player  150  and external player  150 B, perhaps in response . In this illustrative embodiment, portable player  150  initiates the key exchange. 
     In step  802  (FIG.  8 ), CLP  512 A initiates the key exchange by sending a key exchange request message which includes certificate  508 A of portable player  150  and a first random number. The first random number is included to add variety to session encryption keys in a known and conventional manner to frustrate attempts of malicious and ill-tempered computer processes to masquerade as either of players  150  and  150 B having eavesdropped upon the dialogue between players  150  and  150 B in hopes of gaining unauthorized access to read-only keys  504 A and/or  504 B. Certificates are known and are not described further herein except to note that certificate  508 A can be used to authenticate portable player  150  and conveys the public key of key pair  510 A of portable player  150 . Similarly, certificate  508 B can be used to authenticate external player  150 B and conveys the public key of key pair  510 B of external player  150 B. Public/private key encryption/decryption is well-known and is not described further herein. 
     The key exchange initiate message is received by CLP  512 B in step  852  (FIG.  8 ). In step  854 , CLP  512 B (FIG. 5) encrypts read-only key  504 B and any of keys  506 B 1 - 4  which have been acquired through previous key exchanges. In the embodiment in which read-only keys  504 A-B include multiple individual keys, CLP  512 B includes only those keys of read-only key  504 B to which portable player  150  is permitted access. CLP  512 B encrypts the keys using the public key of portable player  150  parsed from the certificate in the key exchange initiate message. Accordingly, the keys can only be decrypted by CLP  512 A. CLP  512 B prepares a reply message in step  856  (FIG.  8 ). The reply message includes the encrypted keys, the first random number, a second random number, and certificate  508 B (FIG.  5 ). The second random number adds to the variety of session keys to further frustrate attempts to gain information through eavesdropping upon the dialogue between players  150  and  150 B. In step  858  (FIG.  8 ), CLP  512 B (FIG. 5) cryptographically signs the reply message using the public key of key pair  510 B and adds the signature to the reply message. 
     In step  860  (FIG.  8 ), CLP  512 B sends the reply message to CLP  512 A which receives the reply message in step  804  (FIG.  8 ). In step  806 , CLP  512 A (FIG. 5) verifies the signature of the reply message using the public key of key pair  510 B from certificate  508 B. CLP  512 A encrypts read-only key  504 A and any of keys  506 A 1 - 4  which have been acquired through previous key exchanges in step  808  (FIG.  8 ). In the embodiment in which read-only keys  504 AB include multiple individual keys, CLP  512 A includes only those keys of read-only key  504 A to which external player  150 B is permitted access. CLP  512 A (FIG. 5) encrypts the keys using the public key of external player  150 B parsed from the certificate in the reply message. Accordingly, the keys can only be decrypted by CLP  512 B. CLP  512 A prepares an exchange message in step  810  (FIG.  8 ). The exchange message includes the encrypted keys, the first random number, and the second random number. In step  812  (FIG.  8 ), CLP  512 A (FIG. 5) cryptographically signs the exchange message using the public key of key pair  510 A and adds the signature to the exchange message. 
     In step  814  (FIG.  8 ), CLP  512 A sends the exchange message to CLP  512 B which is received by CLP  512 B in step  862  (FIG.  8 ). In step  864 , CLP  512 B (FIG. 5) verifies the signature of the exchange message using the public key of key pair  510 A. The signatures of the reply and exchange messages serve to further cross-authenticate portable player  150  and external player  150 B. 
     To terminate the transaction, CLP  512 B sends a terminate message in step  866  (FIG. 8) which, in step  816 , is received by CLP  512 A (FIG.  5 ). Steps  868  (FIG. 8) and  870  are directly analogous to steps  818  and  820 , respectively. Accordingly, the following description of steps  818  and  820  is equally applicable to steps  868  and  870 , respectively. 
     In step  818 , CLP  512 A (FIG. 5) decrypts the encrypted keys using the private key of key pair  510 A. At this point, portable player  150  has all the keys of external player  150 B. In step  820  (FIG.  8 ), portable player  150  stores the decrypted keys in previously unused ones of keys  506 A 1 - 4 , discarding decrypted keys already represented in keys  506 A 1 - 4  and discarding keys when all of keys  506 A 1 - 4  are used. While only four keys  506 A 1 - 4  are shown for simplicity, more keys can be included in portable player  150 , e.g., 256 or 1,024 keys. 
     Thus, as shown in logic flow diagram  800  (FIG.  8 ), portable player  150  and external player  150 B exchange keys such that any SPT, e.g., SPT  116 , bound to either of portable player  150  and external player  150 B can be played by the other. Such only requires a one-time key exchange when a new external player is acquired by a particular user. 
     Enforcement of Restrictions on SPT  116   
     Tracks  112  can have restrictions placed upon them by player  110  (FIG. 1) and, indirectly, by a server from which player  110  acquires tracks  112 . Any such restrictions are included in SPTs  116 . Such restrictions are represented in header  302  which is shown in greater detail in FIG.  9 . Header can include a number of restrictions  902 , each of which includes a restriction type field  904 , a restriction data field  906 , and a restriction state  908 . 
     Restriction type field  904  stores data specifying a type of restriction on playback of SPT  116  (FIG.  3 ). Such restriction types can include, for example, the number of times SPT  116  can be played back, an expiration time beyond which SPT  116  cannot be played back, a number of storage media such as storage medium  202  (FIG. 2) on which SPT  116  can be fixed, and the number of devices to which SPT  116  can be bound. 
     Restriction data field  906  (FIG. 9) stores data specifying type-specific data to specify more particularly the restriction placed upon SPT  116 . For example, if the restriction type is a number of times SPT  116  can be played back, restriction data field  906  specifies the number. If the restriction type is an expiration time beyond which SPT  116  cannot be played back, restriction data field  906  specifies the time. If the restriction type is a number of storage media such as storage medium  202  (FIG. 2) on which SPT  116  can be fixed, restriction data field  906  specifies the number. And, if the restriction type is a number of devices to which SPT  116  can be bound, restriction data field  906  specifies the number. 
     Restriction state field  908  (FIG. 9) stores data specifying the current state of the restriction. For example, if the restriction type is a number of times SPT  116  can be played back, restriction state field  908  stores the number of times SPT  116  has been played back to date. Restriction state  908  allows SPT  116  to be passed between a couple of external players which can both enforce restriction  902 . 
     Player  110  (FIG. 1) and SPT interface  114  rely largely upon portable player  150 , and player logic  502 A (FIG. 5) in particular, for enforcement of restrictions  902  (FIG.  9 ). 
     Accordingly, SPT interface  114  (FIG. 9) requires assurance from portable player  150  than all restrictions can be enforced by portably player  150  as a precondition to downloading SPT  116  to portable player  150 . Such downloading can include, for example, binding SPT  116  to portable player and copying SPT  116  as bound to a removable storage medium. 
     Logic flow diagram  1000  (FIG. 10) illustrates the conditional downloading of SPT  116  (FIG. 1) by SPT interface  114  contingent upon assurance by portable player  150  that restrictions  902  (FIG. 9) can be enforced by portable player  150 . In step  1002  (FIG.  10 ), SPT interface  114  receives from portable player  150  a list of restriction types which can be enforced within portable player  150  during registration. Player  110  maintains this restriction enforceability information along with the communication key of player  110 . Accordingly, step  1002  is performed only once for each external player while the following steps are performed as a precondition of downloading each SPT to an external player. 
     In step  1004  (FIG.  10 ), SPT interface  114  (FIG. 1) determines which restrictions are imposed upon SPT  116  by reference to restrictions  902  (FIG.  9 ). Loop step  1006  and next step  1014  define a loop in which each of restrictions  906  is processed according to steps  1008 - 1012 . During each iteration of this loop, the particular one of restrictions  902  processed by SPT interface  114  is referred to as the subject restriction. 
     For each of restrictions  902 , processing transfers to test step  1008  (FIG. 10) in which SPT interface  114  (FIG. 1) determines whether the subject restriction is of a type enforceable by portable player  150 . If not, processing transfers to step  1010  (FIG. 10) in which SPT interface  114  refuses to download SPT  116  for portable player  150  and processing terminates in step  1012 . Conversely, if the subject restriction is of a type enforceable by portable player  150 , processing transfers through next step  1014  to loop step  1006  and the next of restrictions  902  (FIG. 9) is processed according to the loop of steps  1006 - 1014 . 
     When all restrictions  902  (FIG. 9) have been processed in the loop of steps  1006 - 1014 , SPT interface  114  has determined that portable player  150  can enforce all restrictions  902  and processing transfers to step  1016  in which SPT interface  114  proceeds with downloading SPT  116  for portable player  150 . Thus, SPT interface  114  ensures that portable player  150  can enforce all restrictions placed upon SPT  116  prior to making SPT  116  available to portable player  150 . 
     Smart Media 
     In one embodiment, storage medium  202  (FIG. 2) is replaced with smart medium  1102  (FIG.  11 ). Smart medium  1102  replaces read-only serial number  204  (FIG. 2) with cryptographic logic  1104 . Cryptographic logic  1104  is embedded in the packaging of smart medium  1102  in a manner which is analogous to the embedding of logic in any currently available smart card, e.g., a plastic card of the approximate dimensions of a credit card with embedded integrated circuitry. Cryptographic logic  1104  performs encryption and decryption using an encryption algorithm and key which are both kept entirely secret within cryptographic logic. 
     Logic flow diagram  1200  (FIG. 12) illustrates the preparation of SPT  116  (FIG. 1) from one or more of tracks  110  by SPT interface  114  for playback by portable player  150 . In step  1202  (FIG.  12 ), SPT interface  114  (FIG. 1) encrypts the content of one or more of tracks  110  using, for example, symmetric key encryption. 
     In step  1204  (FIG.  12 ), SPT interface  114  (FIG. 11) sends the master media key to cryptographic logic  1104  for encryption. Cryptographic logic  1104  returns the master media key in an encrypted form. The particular manner in which the master media key is encrypted by cryptographic logic  1104  is not known by, and is of no concern to, SPT interface  114  so long as cryptographic logic  1104  can later decrypt the master media key. 
     Since the master media key is encrypted using cryptographic logic  1104 , the master media key—and therefore the content of SPT  116  which is encrypted with the master media key—can only be decrypted using cryptographic logic  1104 . By embedding cryptographic logic  1104  in the packaging of smart medium  1102  thereby carefully guarding the secrecy of cryptographic logic  1104 , SPT  116  is bound to smart medium  1102  and can only be played back from smart medium  1102 . SPT  116  cannot be played back from any other storage medium unless cryptographic logic  1104  is accurately replicated. Replication of such embedded logic is particularly difficult, especially for casual listeners of music. 
     In step  1206  (FIG.  12 ), SPT interface  114  (FIG. 11) forms SPT  116  and stores the encrypted content in SPT  116 . SPT interface  114  stores the encrypted master media key in the header of SPT  116 . SPT  116  is therefore bound to smart medium  1102 . 
     The security afforded by such binding is more fully appreciated in the context of decoding for playback by portable player  150  as illustrated by logic flow diagram  1300  (FIG.  13 ). In the context of logic flow diagram  1300 , storage media  1102  (FIG. 11) is installed in portable player  150  such that SPTs  116  are accessible to portable player  150 . To playback a selected one of SPTs  116 , player logic  502 A (FIG. 5) reads SPT  116  and parses header  302  (FIG. 3) therefrom and parses the encrypted master media key from header  302  in step  1302  (FIG.  13 ). 
     In step  1304  (FIG.  13 ), player logic  502 A (FIG. 5) sends the encrypted master media key to cryptographic logic  1104  (FIG. 11) for decryption. Cryptographic logic  1104  returns the master media key in an un-encrypted form. The particular manner in which the master media key is decrypted by cryptographic logic  1104  is not known by, and is of no concern to, player logic  502 A (FIG.  5 ). Since player  110  (FIG.  1 ), SPT interface  114 , and player  150  do not know the particular encryption/decryption algorithm implemented by cryptographic logic  1104  (FIG.  11 ), the secrecy of that algorithm is more easily protected. 
     In step  1306  (FIG.  13 ), player logic  502 A (FIG. 5) decrypts the content of SPT  116  using the decrypted media master key. After step  1306  (FIG.  13 ), the content of SPT  116  is un-encrypted and is available for decompression and playback by player logic  502 A. Decompression and playback of the un-encrypted content is conventional. 
     External Player Registration 
     As described above, player  110  (FIG. 1) requires device identification data such as read-only key  504 A (FIG. 5) to bind SPTs  116  to a particular external player such as portable player  150 . To register portable player  150  (FIG.  1 ), portable player  150  communicates with player  110 , e.g., through I/O port  140  and SPT interface  114 . Portable player  150  can be coupled to I/O port  140  using a convenient cradle such as those used in conjunction with currently available portable MP3 players and with the Palm series of personal digital assistants (PDAs) available from 3Com Corp. of Santa Clara, Calif. For external players which are somewhat less portable, e.g., components of a home stereo system, CLP  512 A (FIG.  5 ), certificate  508 A, key pair  510 A, and keys  504 A and  506 A 1 - 4  can be included on a smart card such as those used in conjunction with currently available digital satellite system (DSS) receivers. Such smart cards can be inserted into a reader coupled to I/O port  140  (FIG. 1) to carry out registration and key exchange and re-inserted in the stereo system component external player for playback of SPTs  116 . Dash-mounted external players in a car can include CLP  512 A (FIG.  5 ), certificate  508 A, key pair  510 A, and keys  504 A and  506 A 1 - 4  in a detachable face plate such as those commonly used for theft deterrence. The detachable face plate can be coupled to I/O port  140  (FIG. 1) through a cradle similar to those described above except that the form of the cradle fits the detachable face and include electrical contacts to meet contacts included in the detachable face plate. 
     Once portable player  150  is in communication with SPT interface  114 , and therethrough with player  110 , portable player  150  and player  110  conduct a key exchange in the manner described above. As a result, player  110  has a copy of read-only key  504 A (FIG. 5) and can bind SPTs  116  to portable player  150 . To allow the user of portable player  150  to acquire music products at locations other than computer system  100  (FIG.  1 ), player  100  can upload read-only key  504 A to a server computer system through computer network  170  in a cryptographically secure manner. In an embodiment in which computer network  170  is the Internet, the user can purchase content at any of a great multitude of computer systems all over the world and, in addition, at specially designated kiosks at various retail locations. Upon proper authentication of the user at any such site, the user can purchase and encode SPTs  116  for portable player  150  and, indirectly, for any external player with which portable player  150  has exchanged keys. 
     The above description is illustrative only and is not limiting. The present invention is limited only by the claims which follow.