BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to security systems. 
     2. Description of the Prior Art 
     It is known to use recorded audio-video material (AV) as evidence in a legal proceeding, such as a criminal prosecution. This recorded AV could be, for example, AV recorded by closed-circuit-television (CCTV) cameras located within the premises of a shop, the recorded AV being used as evidence of a theft of goods from the shop. As another example, digital AV cameras may be used to record the activities occurring in the centre of a town or city, the recorded AV being used as evidence of the occurrence of a crime. 
     However, there is always the danger that the recorded AV could be tampered with or modified. Even if it is not tampered with, the fact that this is possible means that the validity of the AV data can be challenged. There is also the danger that the AV recording systems themselves are tampered with, for example by a criminal who does not wish his actions to be successfully captured by a camera. 
     It is important, during a legal proceeding, to be satisfied of the authenticity and integrity of any recorded AV that has been submitted as evidence Otherwise, decisions based on recorded AV, the authenticity or integrity of which is in doubt, may be unsound. 
     This invention provides a media data security system for rendering a secure media data file, having associated authentication data, for output, the system comprising: 
     a data preparation unit operable with respect to a version of the media data file, having user controls for generating a list of desired render operations to be applied to the media data file; and 
     a secure data reproducer arranged to receive the list of desired render operations from the data preparation unit and to render the secure media data file for output on the basis of the list of render operations, the secure data reproducer comprising:
         a data authenticator for detecting the authenticity of the secure media data file using the authentication data; and   a data reproducer operable, if the data authenticator detects that the input data is authentic, to perform the list of desired render operations with respect to the secure media data file.       

     SUMMARY OF THE INVENTION 
     The invention allows evidence to be authenticated before it is presented in a legal proceeding, thereby providing confidence in the integrity of the evidence being presented. It is possible that only certain portions of the evidence are required for use in arguments during the legal proceeding and that these portions may need to be edited to highlight certain points. The invention allows lists of such render operations to be generated using a version of the evidence which may, for example be non-authenticatable; however, during the legal proceeding, the list of render operations is applied to the evidence that has been authenticated, thereby providing confidence in the integrity of the presented evidence. 
     This invention also provides a media data security method for rendering a secure media data file, having associated authentication data, for output, the method comprising the steps of: 
     a data preparation unit generating, with respect to a version of the media data file, a list of desired render operations to be applied to the media data file; and 
     a secure data reproducer receiving the list of desired render operations from the data preparation unit and rendering the secure media data file for output on the basis of the list of render operations, the secure data reproducer:
         detecting the authenticity of the secure media data file using the authentication data; and   if the data authenticator detects that the input data is authentic, performing the list of desired render operations with respect to the secure media data file.       

     Further respective aspects and features of the invention are defined in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings, in which: 
         FIG. 1  schematically illustrates an AV system; 
         FIG. 2  schematically illustrates the AV system of  FIG. 1  with additional security features; 
         FIG. 3  schematically illustrates the AV system of  FIG. 2  with additional security features; and 
         FIG. 4  schematically illustrates the AV system of  FIG. 3  with additional security features. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  schematically illustrates an AV system. An IP (Internet protocol) camera  10  comprises a microphone (not shown) and an AV compression unit  12  that compresses uncompressed digital AV generated by the IP camera  10  to form compressed digital AV. The AV compression unit  12  may operate according to any suitable compression algorithm, for example, the Motion Pictures Experts Group (MPEG) 2 standard (ISO/IEC-13818, see for example www.mpeg.org) and may operate to compress only the video material, leaving the audio uncompressed. The IP camera  10  is connected to a network  14  via a connection  16 . It will be appreciated that there may more than one IP camera  10  connected to the network  14  and that they may be arranged to capture different “scenes”, i.e. images and sounds at different physical locations. Each of the IP cameras  10  has its own IP address so that it can be uniquely identified. It will be appreciated that the IP camera  10  may be configured to capture only video material and not to capture any audio material. 
     An AV server  18  is also connected to the network  14  via a connection  20 . Compressed AV is received by the AV server  18  from the IP camera  10 . The AV server  18  may be, for example, a personal computer, and comprises a file system  22  (such as a hard disc drive or array) capable of storing received compressed AV, an operating system  24  controlling the file system  22  and an application program  26  running on the AV server. Under the control of the operating system  24  and the application  26 , compressed AV received by the AV server  18  from the IP camera  10  is stored on the file system  22 . The AV server  18  therefore acts as the main storage of the compressed AV. It will be appreciated that there may be more than one AV server  18  to store the compressed AV and that the need for a certain number of AV servers  18  may be determined by, for example, bandwidth constraints, backup requirements etc. 
     An AV client  28  is also connected to the network  14  via a connection  30 . The AV client  28  runs an application program  32  and a monitor (display)  34 . Under instruction from an operator, the application  32  may control the AV client  28  so that it receives compressed AV from the IP camera  10 . The application  32  decompresses the compressed AV that it has received before rendering the AV captured by the IP camera on the monitor  34 . In this way, the human operator of the AV client  28  can view what is currently being “seen” by the IP camera  10 . If there is more than one IP camera  10  connected to the network  14 , the application  32  may allow the operator to select a particular IP camera  10 , or to display images from two or more cameras simultaneously. 
     Alternatively, under instruction from the operator, the application  32  may control the AV client  28  so that it receives compressed AV from the AV server  18 . The AV client  28  therefore permits an operator to view both “live” AV direct from the IP camera  10  and AV recorded on the AV server  18 . 
     It will be appreciated that the AV client  28  may comprise more than one monitor  34  and that they may be controlled by the application  32  (under instruction from the operator) to display AV from different sources (either the IP cameras  10  or the AV server  18 ). It will also be appreciated that there may be more than one AV client  28  connected to the network  14 . Additionally, it will be appreciated that there may be no AV clients  28  connected to the network  14 , the system being controlled via the application  26  running on the AV server  18 . 
     The settings of the IP camera  10  (such as pan, tilt, zoom and exposure settings) may be pre-set. Alternatively, the IP camera  10  may comprise a control link  36 . Control information may be delivered to the IP camera  10  from the AV client  28  via the network  14 . The control link  36  receives the control information from the network  14  and the IP camera  10  responds to the control information accordingly (such as adjusting its zoom setting). 
     If the AV (or, more likely, a portion of the AV) stored on the AV server  18  is required as evidence in a legal proceeding (or for any other reason), then the application  26 , under instruction from an operator, controls the AV server  18  to generate an evidence copy  38  of that portion of the AV. This AV evidence copy  38  could be, for example, stored on an optical disc, such as a Compact Disc (CD) or a Digital Video Disc (DVD). The AV evidence copy  38  is then delivered (for example by courier) to the appropriate recipient, for example, a court, a barrister, a solicitor, etc. shown generally as a recipient  40  in  FIG. 1 . It will be appreciate that, for a given legal proceeding, there may be more than one recipient  40 , each of whom may request and receive one or more different AV evidence copies  38 . 
       FIG. 2  schematically illustrates the AV system of  FIG. 1  with additional security features. These additional security features relate to a first stage of securing the system that protects the compressed AV once it has been received by the AV server  18 . 
     The AV server  18  has a security module  100 , which may be (i) implemented in software (and may then form part of the application  26 ), (ii) implemented in hardware or (iii) implemented as a combined software/hardware unit. One of the roles of the security module  100  is to encrypt the compressed AV that the AV server  18  receives from the IP camera  10 . This allows the AV to be stored securely on the file system  22 . Even if an attacker (a person wishing to destroy, corrupt or gain access to the stored AV) has access to the file system  22 , the attacker will not be able to interpret the stored data since it is encrypted. The security module may operate according to any encryption algorithm, such as 3-DES and/or the Advance Encryption Standard (AES) (for both of which see http://csrc.nist.gov/CryptoToolkit/tkencryption.html). 
     The AV server  18  additionally comprises a decryption module  102  that operates to reverse the encryption provided by the security module  100 . Accordingly, when an evidence copy  38  is required or when the AV client  28  requests AV from the AV server  18 , the encrypted AV stored on the file system  22  is passed to the decryption module  102 . The decryption module  102  outputs unencrypted compressed AV which may then be used to create an evidence copy  38  or may be sent to the AV client  28 . 
     The management of the keys used by the security module  100  during the encryption process and the decryption module  102  during the decryption process may be handled in a standard way that is well known in this field of technology. For example, passwords, user identifications and access control can all be used to ensure that only those with permission to decrypt the encrypted AV stored on the file system  22  are able to do so. 
     The security module  100  also generates one or more hash values according to the compressed AV received by the AV server  18 . The security module  100  may generate these hash values according to any hashing algorithm, for example, the Secure Hashing Algorithm (SHA)  1  (for which see http://csrc.nist.gov/CryptoToolkit/tkhash.html). Hash values are well known in this field of technology as a method of authenticating data, i.e. validating that data has not been tampered with. A hash value is a value that lies within a fixed range of values (e.g. all 40-bit numbers) that is derived from an input amount of data (which may be any number of bits long). This hash value is then said to represent the input data. A good hashing algorithm is very sensitive to the input data, so that a modification of a single bit of the input data will yield a substantially different hash value. As such, the hash value of modified data will, with a very high probability, not match the hash value of the original unmodified data. The validity of an amount of data can thus be ascertained by generating a hash value on that amount of data and comparing it with a hash value generated on the original data that is known to be correct. Additionally, with a good hashing algorithm, it is very difficult indeed to create an amount of false data which gives a hash value equal to the hash value of data known to be valid. Some hashing algorithms require the use of a secret key so that only authorised parties are able to generate the hash value. This prevents an attacker from modifying the input data, creating a modified hash value from the modified data and then attempting to pass-off the modified hash value as the original hash value in an attempt to provide evidence of authenticity for the modified data. 
     The security module  100  may derive a hash value from the received compressed AV in several ways. For example, the security module  100  may generate a hash value for each frame of video from the compression codewords used for that frame of video. These compression codewords could be, for example, variable length codewords that represent discrete cosine transform (DCT) coefficients, wavelet coefficients, header information, etc. As such, a modification to the data representing that frame can be detected. This hash value can then be embedded within the compressed data (for example by using spare user-data within the compressed data format). More preferably, in order to be able to detect the insertion or removal of frames from the compressed AV, hash values are calculated across sections of multiple frames, the sections over which the hash values are calculated being overlapping sections. This may, though, introduce extra latency in the processing of the security module  100 , if, for example, an embedded hash value is dependent on a future frame. It will be appreciated that there exist a variety of ways of generating hash values and that generation on a frame-by-frame basis or group-of-frames-by-group-of-frames basis are merely examples. Additionally, hash values may be generated on and/or embedded within the audio data. 
     As with the encryption and decryption of the compressed AV according to the security module  100  and the decryption module  102  respectively, the management of key information for the generation of hash values is carried out according to standard methods well-known in this field of technology. 
     Once the hash values have been embedded into the compressed AV, the compressed AV is encrypted by the security module  100  before being stored on the file system  22 . 
     When an evidence copy  38  is delivered to a recipient  40 , the recipient  40  can generate hash values according to the same algorithm as used by the security module  100 . If the hashing algorithm requires the use of key information, then the recipient must have access to this key information. As mentioned, this is handled by a standard key-management technique well known in this field of technology. The recipient  40  can compare the hash values that the recipient  40  has generated with the hash values that were embedded into the compressed AV by the security module  100 . If the hash values match, then the recipient  40  can be satisfied that the evidence copy  38  has not been modified during delivery from the AV server  18  to the recipient  40 ; otherwise the recipient  40  will know that a modification has occurred and may then take appropriate action (for example, re-requesting an evidence copy  38 ). 
     Furthermore, the security module  100  may insert one or more security headers into the compressed AV, the security header containing information such as camera data (e.g. IP-address) and the date/time of recording. This header may be subsequently encrypted by the security module  100 . The recipient  40  may use the information contained in these security headers to help determine the authenticity of an evidence copy  38  that has been received. 
     As a further method of authenticating the evidence copy  38  received by the recipient  40 , it is possible for another evidence copy  38 ′ to be sent to the recipient  40  (preferably by an alternative route). The recipient  40  can then compare the two evidence copies  38  and  38 ′ to determine whether one of them has been modified. It will be appreciated, of course, that any number of evidence copies  38  may be sent to the recipient for this purpose. 
     Naturally, in order to help ensure that an attacker is not able to gain unauthorised access to the compressed AV, the applications  26  and  32 , running on the AV server  18  and the AV client  28  respectively, are implemented according to standard security coding practices. This is particularly important when the applications  26  and  32  are being run on standard hardware (such as PCs) and the operating system  24  (and the analogous operating system of the AV client  28 , not shown) is a well-known operating system, such as Windows or Linux, as these receive a lot of attention from the attacker “community”. 
       FIG. 3  schematically illustrates the AV system of  FIG. 2  with additional security features. These additional security features relate to a second stage of securing the system in which protection for the AV begins within the IP camera  10 . 
     The data being transferred across the network  14  is encrypted by a network encryption module  200  of the IP camera  10 , a network encryption module  202  of the AV server  18  or a network encryption module  204  of the AV client  28 . The network encryption modules  202  and  204  are also arranged to decrypt encrypted data being transferred across the network  14 . The encryption algorithms and protocols used to secure the data on the network  14  may be any appropriate algorithms and protocols, for example, IPSec (for which see http://www.ietf.org/html.charters/ipsec-charter.html). The processing performed by the network encryption modules  200 ,  202  and  204  may be implemented in software, hardware, or a combination of software and hardware. In particular, if the AV server  18  receives many compressed AV streams from various IP cameras  10  and/or supplies many compressed AV streams to various AV clients  28 , then the aggregate bandwidth for the AV server  18  may be relatively high and the encryption and decryption processing by the network encryption module  202  may require hardware acceleration. 
     The flow of data across the network  14  can therefore be secured, thereby preventing an attacker from accessing the compressed AV data communicated across the network  14 . Additionally, security protocols such as IPSec make the analysis of network traffic much more difficult and helps to prevent so-called “man-in-the-middle” attacks in which data communicated on the network  14  is first received by an attacker (who may then modify, analyse etc. the data) before being distributed to the intended recipient. 
     In a similar manner to the AV server  18 , it is advantageous to provide the IP camera  10  with security features that help prevent (or at least allow detection of) modification of the compressed AV. As such the IP camera  10  has an authenticating module  206 . The authenticating module  206  may insert hash values into the compressed AV in a similar manner to the security module  100  and modification of the compressed AV can be detected as described with reference to the security module  100 . An alternative to using hash values is to use digital signatures. These are similar to hash values in that (i) they comprise a relatively small amount of data that is representative of the data to be authenticated (ii) they are sensitive to changes in the data to be authenticated and (iii) it is difficult to create data that yields a specific signature or hash value. However, digital signatures rely on public key (or asymmetric) cryptography. Digital signatures and public key cryptography are well known in this field of technology and will not be described in detail here (for more information see “The Handbook of Applied Cryptography” by A. J. Menezes et al.). As an alternative to, or in addition to, inserting hash values into the compressed AV, the authenticating module  206  may be arranged to provide one or more digital signatures for one or more of sections of the compressed AV (in a similar manner as described for the hash values). 
     In order to generate digital signatures, the IP camera  10  has a PKI (public key infrastructure) module  208  which is arranged to store certain public and private key information in a secure manner. For example, at manufacture, each IP camera  10  may be provided with (or may generate by itself) a public key/private key pair to be used for digital signatures. The private key is held securely by the IP camera  10  in the PKI module  208  and is used by the authenticating module  206  to generate digital signatures. The corresponding public key can be obtained from the IP camera  10 , for example by access over the network  14 , and may be used at a later stage for authenticating the digital signatures. As a public key/private key pair is specific to a particular IP camera  10 , it is possible not only to authenticate the compressed AV but also to authenticate that it originated with a particular IP camera  10 . 
     The use of public key cryptography is preferably supported by the use of digital certificates issued by and authenticated according to a certification authority. Digital certificates and certification authorities are well known in this field of technology and will not be described in detail here (for more details, see “The Handbook of Applied Cryptography” by A. J. Menezes et al.). The system shown in  FIG. 3  may therefore make use of digital certificates according to either a third-party certification authority or an internally-operated certification authority. Digital certificates allow the details and ownership of public keys to be authenticated. 
     The security provided over the network  14  by the encryption and decryption according to the encryption modules  200 ,  202  and  204  may additionally make use of asymmetric encryption in order to, for example, authenticate the identity of the communicating entities (namely the IP cameras  10 , the AV servers  18  and the AV clients  28 ), or to establish so-called “session keys” that are used throughout the lifetime of an encryption session. As such, each entity on the network  14  (namely the IP cameras  10 , the AV servers  18  and the AV clients  28 ) has (i) its own private keys held securely by the entity and (ii) public keys (preferably contained within digital certificates so that they can be authenticated) that are available to all other entities on the network  14 . For example, the PKI module  208  may be able to download and store a table of public keys via access to the network  14 . The AV server  18  and the AV client  28  may make use of a PKI module (not shown) similar to the PKI module  208 . 
     In order to provide further authentication measures, the IP camera  10  additionally has a watermarking module  210  that may apply watermarks to the AV before it is compressed.. Watermarking is well known in this field of technology and will not be described in detail (for more information, see www.watermarkingworld.org). In brief though, watermarking involves modifying the AV (preferably in an imperceptible manner) so as to embed so-called “payload data” into the AV. 
     The watermarking module  210  may be arranged to embed a so-called “fragile watermark”. Fragile watermarks possess the property that, if modifications are made to the AV, then the fragile watermark cannot be decoded. Additionally, it is known for some fragile watermarks to possess the property that, if modifications are made to only certain portions of the AV (either certain temporal portions and/or certain spatial portions) then the fragile watermark cannot be decoded from those portions, but can be decoded from the remaining portions. This enables a fragile watermark decoder to determine which portions of the AV have been modified. It will be appreciated that the watermarking module  210  may apply a fragile watermark according to any appropriate algorithm. Many fragile watermarking algorithms require the generation of either hash values or digital signatures. Accordingly, an authenticating module  206 ′ provides the watermarking module  210  with either hash values or digital signatures. The authenticating module  206 ′ may be separate from the authenticating module  206 ; alternatively, the two authenticating modules  206  and  206 ′ may be the same. However, it is possible that the PKI unit  208  has more than one public key/private key pair for the IP camera  10 , and the private key used by the authenticating module  206  may be different from the private key used by the authenticating module  206 ′. 
     An alternative form of watermarking is the so-called “robust watermarking”, which has the property that the payload data embedded can still be decoded from the watermarked AV data even if the AV data has undergone a degree of processing (such as data compression, re-sizing, cropping, etc.) The watermarking module  210  may therefore be arranged embed a robust watermark into the AV in addition to or as an alternative to embedding a fragile watermark. For example, a robust watermark embedded into the AV by the watermarking module  210  may contain information such as the identity of the IP camera  10  and/or the date/time of the AV capture. During subsequent use of the AV data, the absence of this watermark provides evidence that either the AV did not originate with the IP camera  10  or that the AV has been modified sufficiently (and is therefore not representative of the original AV) so as to render the robust watermark not decodable. 
     If the watermarking module  210  applies a fragile watermark to the AV, then it is preferable for the compression algorithm used by the AV compression unit  12  to be a so-called lossless compression algorithm (i.e. the compression afforded by the AV compression unit  12  is completely reversible) so that the fragile watermark is not destroyed. Alternatively, watermarking algorithms (both fragile and robust) that work on compressed AV (as opposed to baseband AV) are well known. The IP camera  10  may therefore have an additional watermarking module (not shown) that embeds watermarks (fragile and/or robust) into the compressed AV. 
     The IP camera  10  additionally has a control information decryption module  212  that is arranged to decrypt control information sent to it across the network  14 . It is known for some IP cameras  10  to respond to control commands only if, for example, a correct password or a user identification has been received. Additionally, some IP cameras  10  allow for the establishment of user profiles, with certain control commands only being accessible to users with a profile of a sufficient access level. However, the commands sent to the IP camera  10  should be encrypted, as otherwise an attacker may be able to analyse the network traffic and determine how to generate unauthorised control commands. As discussed earlier, the traffic on the network  14  can be encrypted according to, for example, the IPSec protocol. The control information decryption module  212  therefore performs the decryption necessary when control information is received by the IP camera  10 . If asymmetric encryption is being used, the control information decryption module  212  may make use of key information provided to it by the PKI module  208 . 
     The system according to  FIG. 3  provides many techniques for securing both the storage of and distribution across the network  14  of AV. Additionally, the system according to  FIG. 3  provides further security through authentication, which includes authentication not only of the AV itself, but also that the entities communicating across the network  14  are actually the entities they are purporting to be. It will be appreciated that some of these security measures may be excluded from the system and an associated level of security is attained. 
     It is important, though, to ensure that, once an evidence copy  38  is made and delivered to a recipient  40  (for example for use in a legal proceeding), then the AV on the evidence copy  38  is not subject to an attack. It is therefore important that, when the AV evidence copy  38  is used, it is possible to be confident that the AV presented has not been modified (for example, to exclude or remove pertinent information). 
       FIG. 4  schematically illustrates the AV system of  FIG. 3  with additional security features. These additional security features relate to a third stage of security that concerns the export of evidence copies  38 . 
     A secured evidence copy  38   a  is provided to a trusted recipient  40   a , such as a court in a legal proceeding. The secured evidence copy  38   a  contains the authentication information (such as hash values and digital signatures) and may additionally be encrypted to increase security. The trusted recipient  40   a  comprises a verified reference viewer (VRV)  300 . The purpose of this will be described later. 
     Other recipients (such a defence or prosecution lawyer)  40   b  receive evidence copies  38   b , which may be unsecured (e.g. unencrypted), full resolution AV. The evidence copies  38   b  may be used by the recipients  40   b  for various purposes, for example, for preparing arguments in a legal proceeding. The recipient  40   b  may decide that certain editing of the received evidence copy  38   b  is necessary for a specific purpose (for example, to highlight a specific point in an argument). Such editing may include, for example, AV selection and annotation, zooming (in and out), cropping, picture enhancement, noise reduction, slow-motion playback, pausing, split-screen comparisons, etc., although it will be appreciated that many more operations may be made available. The recipient  40   b  is therefore provided with a verified preparation tool (VPT)  302 . The VPT  302  is an application (either software, hardware or a combination of both) that has been designed to provide resistance to malicious attacks. The VPT  302  has a rendering and editing component (not shown) that the recipient  40   b  uses to perform the various editing/rendering operations mentioned above. The recipient  40   b  can therefore prepare AV evidence for later use. Preferably, the VPT  302  permits real-time rendering of the evidence copy  38   b  according to the edit decisions made by the recipient  40   b.    
     The VPT  302  also has an edit decision list generation component (not shown). Once the recipient  40   b  has finalised the edit decisions required on the evidence copy  38   b , the edit decision list generation component of the VPT  302  generates an evidence presentation list (EPL)  304 . The EPL  304  contains details about all of the edit instructions required by the recipient  40   b . The EPL  304  may contain one or more groups of edit instructions, each group containing one or more edit instructions that are intended to be performed sequentially. For example, the EPL  304  may consist of the following data: 
     Group of instructions 1:
         Play AV from start timecode 01:00:20:13 to end timecode 01:00:22:02;   Play AV from start time code 01:00:22:03 to end timecode 01:00:25:18 at half-speed.       

     Group of instructions 2:
         Play AV from start time code 02:13:56:10 to end timecode 02:18:00:01 with noise reduction and edge enhancement;   Play AV from start time code 02:18:00:02 to end timecode 02:19:50:22 with noise reduction and edge enhancement and zoomed in to the top left corner with a scale factor of 1.8;   Play AV from start time code 02:19:50:23 to end timecode 02:22:22:22 with noise reduction and edge enhancement and then pause at time code 02:22:22:22.       

     In the above example, the first group of instructions may be used by the recipient  40   b  during a first argument in a legal proceeding and the second group of instructions may be used by the recipient  40   b  during a subsequent argument in the legal proceedings. It will be appreciated that any number of groups of instructions may be contained in the EPL  304 . It will also be appreciated that the VPT  302  may generate more than one EPL  304 . 
     Optionally, the VPT  302  includes in the EPL  304  authentication data (such as a digital signature). As such the EPL  304  can be authenticated, both in terms of the information that it contains and its origin (i.e. originating from the recipient  40   b ). This may require the use of a trusted key management system or a PKI system (not shown) so that the trusted recipient  40   a  can independently authenticate the EPL  304 . Furthermore, the VPT  302  may encrypt the EPL  304 . 
     It will be appreciated that, if the VPT  302  is implemented at least in part in software, then this software may be either a stand-alone software application or a plug-in for an existing software application in a known manner. For example, there exist many video editing software applications (some of which rely on hardware acceleration) and the VPT  302  may be implemented as a software plug-in that (i) makes use of the video editing functionality of the video editing software application and (ii) additionally provides functionality to generate the EPL  304 , authenticate it and encrypt it. A VPT  302  implemented as a software plug-in may restrict the number of video editing/rendering operations available via the video editing software application to the recipient  40   b  so that only those operations permissible in a legal proceeding are made available to the recipient  40   b . One example of such a video editing software application is the Adobe Premier™ product. 
     The EPL  304  does not contain any AV content. 
     The EPL  304  is sent to the trusted recipient  40   a  for later use within the VRV  300 . The VRV  300 , like the VPT  302 , is an application (either software application, software plug-in, hardware or a combination of software and hardware) that has been designed to be provide resistance to malicious attacks. The VRV  300  has an authentication component that is able to determine the authenticity of the secured evidence copy  38   a  and the EPL  304 . This authentication may include the validation of hash values and digital signatures, and the reading of fragile and robust watermarks, depending on the particular form of authentication data associated with the evidence copy  38   a . The VRV  300  additionally contains a decryption component so that, if the secured evidence copy  38   a  or the EPL  304  is in encrypted form, the VRV  300  can decrypt it. The management of the keys for the decryption and authentication is, as mentioned, handled by a trusted key management system or a PKI system (not shown) in a known manner. 
     The authentication process may be done in front of a group of witnesses who can then assert that they have witnessed that the secured evidence copy  38   a  that is to be used in the legal proceedings is authentic. Additionally, if watermarking has been used to embed information into the AV, then the embedded information may be presented as additional evidence (for example, the identity of the IP camera  10  or the time/date of the capture of the AV). 
     When instructed, the VRV  300  examines the EPL  304  provided to it by the recipient  40   b  and renders the AV from the secured evidence copy  38   a  according to the edit instructions of one of the groups of instructions in the EPL  304 . As the authenticities of the secured evidence copy  38   a  and the EPL  304  have been ascertained, the AV evidence rendered by the VRV  300  can be presumed to be genuine with a high degree of certainty. In contrast to this, if the recipient  40   b  had actually prepared an edited version of the evidence copy  38   b  for use in the legal proceedings, the authenticity of this edited version may be cast in doubt as the recipient  38   b  or someone else may (either deliberately or accidentally) have made modifications. 
     Whilst this embodiment has be described by reference to providing AV evidence to a legal proceeding, it will be appreciated that it is not restricted to evidence for legal proceedings, but is, in fact, applicable to any AV system that requires AV to be recorded and delivered in a secure manner, for example, when supplying medical data. 
     Additionally, whilst this embodiment has been described by reference to AV data, it will be appreciated that it is applicable to other forms of data, such as text documents and generic data that are equally susceptible to some of the above mentioned security techniques (such as encryption, hash values, digital signatures and watermarking). 
     It will be appreciated that where features of the embodiments described above are implemented at least in part by software, such software (or components of it such as plug-ins) may be provided via a storage medium such as a disk storage medium, via a transmission medium such as a network or internet connection, or combinations of these. 
     Although particular embodiments have been described herein, it will be appreciated that the invention is not limited thereto and that many modifications and additions thereto may be made within the scope of the invention. For example, various combinations of the features of the following dependent claims can be made with the features of the independent claims without departing from the scope of the present invention.
A media data security system for rendering a secure media data file, having associated authentication data, for output comprises a data preparation unit operable with respect to a version of the media data file, having user controls for generating a list of desired render operations to be applied to the media data file; and a secure data reproducer arranged to receive the list of desired render operations from the data preparation unit and to render the secure media data file for output on the basis of the list of render operations, the secure data reproducer comprising: a data authenticator for detecting the authenticity of the secure media data file using the authentication data; and a data reproducer operable, if the data authenticator detects that the input data is authentic, to perform the list of desired render operations with respect to the secure media data file.