Figure 3.1: Biphase format, preambles and frame/block format
G.C. Langelaar
No universal copy protection system for all digital equipment exists yet. However, a copy protection mechanism has been defined for audio recorders. This mechanism is briefly discussed in this section.
Many different protection systems for digitally broadcasted material (analogue and digital video) are currently in use since every service provider uses his own system. A general overview of these systems is given in the next sections.
The role of the personal computer in the multimedia world becomes a major concern. To explain this, the interface of digital equipment to the personal computer is described.
Users must have the possibility to protect their own data against others
(e.g. parental control). However, in some countries it is forbidden to use
very strong cryptographic algorithms in consumer electronics. This problem
is also addressed in this section. Finally conclusions are drawn.
On October 28, 1992, President Bush signed the Audio Home Recording Act into law [1]. The Act, an historic compromise between the consumer electronics and music industries, became effective immediately. The Act confirms consumers' right to use and retailers' right to sell all analogue and digital audio recording formats. As part of this compromise, digital audio recording devices must include a system that prohibits serial copying, and manufacturers or importers must pay a modest royalty on new digital audio recording devices and media.
In the U.S. digital audio recording or interface devices must contain one of the following:
Devices or services to circumvent the SCMS or any other serial copy control system may not be distributed. The Act applies only to "digital audio recording devices," defined as devices that are designed or marketed primarily for making digital audio recordings for private use (whether or not incorporated in some other device). The following devices are not generally subject to SCMS or royalty requirements:
All digital audio recorders like the DAT, DCC and mini-disc recorders, are equipped with the SCMS (Serial Copy Management Systems) to prevent consumers from making illegal copies of copyright protected material [2]. Using this system, a consumer can make digital copies of any digital source. However, such a copy can not be duplicated further using storage devices equipped with this protection method. The copy-prohibit-bits occur frequently in the data stream at fixed intervals.
S/PDIF is a serial one-line connection in one direction for the transport of digital stereo audio with the belonging subcode and error detection. To facilitate clock recovery from the data stream biphase-mark encoding is used. Each bit to be transmitted is represented by a symbol comprising two consecutive binary states. The first state of a symbol is always different from the second state of the previous symbol. The second state of the symbol is identical to the first if the bit to be transmitted is logical "0", however it is different if the bit is logical "1" (see Figure 3.1).
Preambles are specific patterns providing synchronisation and identification of the subframes and blocks. These patterns violate the biphase mark code rules to avoid the possibility of data imitating the preambles. Three preambles are used (see Figure 3.1) to indicate the start of a sub-frame.
Each subframe contains two bits, which are part of the subcode data. The first bit is used for the user data block and is not used in most cases. The other bit (bit 30) is the same in each subframe (channel A and B) and is also responsible for the subcode block. From each frame (2 subframes) this bit is extracted to build a subcode block of 192 bits. Bit 2 (count: 0, 1, 2, ..) in this block is called the copy prohibit bit. So, by changing bit 30 and the last parity bit in frame 2, the copy protection can easily be removed [3]. The fixed position of the copy-prohibit-bit is therefore also the weakness of this protection.
Figure 3.1: Biphase format, preambles and frame/block format
Pay TV techniques rely on two independent mechanisms. On the first hand scrambling / encryption of the picture and of the sound, on the second hand management of commercial entitlements which have to be transmitted as secured messages to the descrambler box or Settop Box (control access). Encryption can easily be applied on a digital bitstream. In this case, all bits are encrypted by using for example a blockcipher like DES. Scrambling is used for analogue broadcasting. Using the latter method the signal format is changed, the synchronisation signals are suppressed and separately transmitted in an encrypted form. Sometimes, the audio signal is converted to a digital signal and encrypted. This digital encrypted audio signal can be embedded in the video signal.
The data is scrambled or encrypted using a control word (CW) or key. The control word or key will change after a short period. To send the new keys to the descrambler (STB) ECM's (Entitlement Control Messages) and EMM's (Entitlement Management Messages) are used. Those messages have a digital signature field which ensures the integrity of the message (e.g. a HASH-code). This prevents users to modify the context of the message.
An ECM is transmitted together with the scrambled signal. An ECM consists of three fields. The first field contains the access parameters. These parameters define the conditions under which access to the program is allowed. This field makes for example parental rating (additional PIN code is requested by the descrambler box) and geographical black out (a film may not be available in all European countries) possible. The second field contains the control word in encrypted form and the last field contains a data integrity check.
An EMM consists usually of four fields. Each EMM starts with an address field to select an individual descrambler box. There are two addressing modes, one for an individual descrambler box and one for a group of boxes. The second field contains the entitlement for the user. The third field contains the service keys in encrypted form and the last field contains a data integrity check. EMM's can also be used to send a command to the descrambler box (see VideoCipher and Videocrypt). Transmission of EMM's is generally the result from an explicit request from the user to the service provider. These messages are individual in general. Their content shall be interpreted by one descrambler box or by a limited number of descrambler boxes which are concerned by this particular entitlement.
EMM's do not have to be transmitted in a synchronous way with the program to which they apply. They have to be transmitted in advance in order to give access to the authorised consumer. Any network can be used to transmit them to the receiver: modem, mail or broadcast. Over air addressing means that the messages (EMM's) are broadcasted.
To be sure that an EMM is received by the user to renew a subscription for instance, there is no other way than to repeat the message sufficiently. EMM's are therefore organised in cycle for broadcasting. The length of the cycle is the major parameter determining the maximum time to wait to get an entitlement for a user, which has switched of his descrambler box for a long time.
Figure 3.2: Key Management in a Pay TV system
The complete key management system is represented in Figure 3.2. The audio and video are scrambled using a cycling control word or key CW. Every fixed period (e.g. 10 seconds) an ECM is transmitted together with this scrambled signal. These ECM's contain the control words encrypted with the service keys SK, which must be present in the de-scrambler box. The service keys are less frequently updated by EMM's, for example once a month. The service keys are encrypted with one or more individual unique keys, which are safely stored inside the smart card or de-scrambler box.
The main problem is that every service provider uses his own scrambling / encryption algorithm and key management system. This means that many different systems are in use today.
In September 1994, the major European television producers, broadcasters, and manufacturers agreed on a new standard for the digital broadcast of video sequences called the Digital Video Broadcast (DVB) [4 - 8]. This new technology will gradually replace the current analogue PAL and SECAM broadcast norms.
The first generation of DVB consumer receivers is expected to be a set top box called an Integrated Receiver Decoder (IRD). I.e. a small box which contains only a receiver and the above MPEG decoder. These IRDs will have the usual RF and SCART interfaces to the antenna, cable and TV/VCR. In addition IRDs are expected to have also data transmission interfaces for personal computers and other multimedia systems. One original point of the DVB system will be that the control access module (CA) will be a separated box which will be connected to the IRD using a PCM/CIA interface. A chip card slot will be optionally provided on the module.
There are two proposals for the conditional access module due to the different views of the participants. The established broadcasters, who already offer video services, would like make sure that their investment in their current de-scramblers is not lost. They will accept standardisation only up to a certain point. On the other hand, the newcomers, consisting mainly of network operators and the smaller broadcasters, would like to co-operate, since they know that not many people would buy a decoder box to watch only one or two channels. So, complete standardisation of the module would really be the solution for them. The equipment manufacturers also wish to standardise to come to cheap mass production.
In Figure 3.3 the proposed scheme for the DAVIC Conditional Access system is represented. This proposed model works with a Set Top Box implemented by a standard terminal and a detachable CA module (PCMCIA card). More information about Pay-TV systems can be found in [9].
Digital storage devices will enter the market soon like D-VHS, DVC, etc. [10] and of course our SMASH device in a later stage. To record digital signals the de-scrambler box (STB) must be equipped with a digital output. Actually, service providers are reluctant to accept digital interfaces and storage devices, but they may accept solutions in which the data is recorded in encrypted form to enable the basic time-shift function of the analogue VCR (Figure 3.4).
In this case they can still control the data, because the data must still pass the STB for de-scrambling and is nowhere in the system available in clear MPEG-2 format (only in analogue decompressed form). However, this solution has drawbacks since EMM's can change the service keys in the smart card. If a STB receives a new EMM, all recorded old data is lost. Because the keys needed to de-scramble this data are replaced.
Figure 3.3: DVB proposal for decoder and CA system
Macrovision is a videotape copy protection for analogue VHS video cassette recorders [11]. It is used on pre-recorded videotapes and in the newer STB's (see Figure 3.4). It seems to be more common in North America than in Europe. It is also used in the new Set Top Boxes to protect the outcoming signals against copying. When dubbing a protected tape, or copying a protected analogue signal, the picture that has gone through the recording VCR will get dark and then normal again periodically. This effect is caused by some new inserted false synchronisation pulses in the non-visible portion of the picture.
Nowadays, digital VCRs appear on the market. Representatives of the consumer electronics and motion picture industries have agreed to seek legislation concerning digital video recorders that would protect both intellectual property and consumers' rights in the digital age [12]. A recommendation is submitted to the US Congress. This recommendation would:
Figure 3.4: D-VHS recorder connected to the STB
These provisions would apply only to digital recording devices and recordings made from digital sources with conventional analogue VCRs.
The first digital consumer recorder is the Sony DHR-1000 [13]. This device can be used to edit digital home videos without restrictions. A Serial Copy Management System is implemented to prevent illegal copying of pre-recorded tapes. It is even possible for the broadcaster to indicate whether a movie can be recorded or not. So, the broadcaster can switch off the recorders. The recorder is equipped with an analogue tuner and stores the video in a digital intra frame coded format.
For pre-recorded material on CD-I and laser disc there is still no copy protection mechanism. This is not necessary for the moment, since the interfaces to other devices are analogue. However, an interface for the CD-I to the PC exists and will be discussed in the next section.
The introduction of the Digital Video Disc (DVD) has been delayed due to
a lack of a reliable copy protection system
[12,14]. This high capacity disc (up to 18 Gigabyte)
must replace the CD-I and laser disc. The motion picture industry did not
want that the DVD can be copied, but finally agreed with the manufacturers
to allow the consumer to make one copy for own use only. The manufacturers
must now implement a copy protection system in the DVD-discs, DVD-player
/ recorder and all other digital audio and video devices (maybe even in PCs).
Currently, there are no copy protection mechanisms implemented in the PC. Software packages are not protected or have their own protection mechanisms. PCs can access data in another way than digital audio and video devices. Some software packages allow the user to extract digitally perfect copies of samples from audio CDs using a CD-ROM player. It also allows to extract MPEG streams from CD-I Digital Video CDs, and Video-CDs, and XA frames from CD-XA CDs. On every CD there is a bit which defines if copying a particular track is permitted or prohibited. The software packages usually do not care about this bit and copy the data anyway. The data can be written back to for instance a CD-recordable without any problems.
Interfaces to the PC and legal software packages exist for the DCC and mini-disc recorder. Music tracks can be copied to the hard disk and edited. So the music can also be copied from the hard disk to other tapes or discs, because the Serial Copy Management System is circumvented.
The same problem holds for the DAT-recorder. This device can also be used
as tape streamer for the PC. With some DAT-recorders it is also possible
to copy music tracks to the hard disk and back
[15].
Users want to have the possibility to protect their own data against others
(e.g. parental control). To protect the user data against theft a relative
strong encryption algorithm should be used (e.g. DES). However, in some countries
it is forbidden to use very strong cryptographic algorithms in consumer
electronics. The law-enforcement agencies wish to have access to the
communications of suspected criminals, which is threatened by secure
cryptography. Industry and individual citizens, however, want to secure their
private data, and look to cryptography to provide it. In the U.S. the Capstone
project aims to develop a technology that attempts to balance these needs
[16].
In the U.S. digital audio recording or interface devices must contain:
A recommendation for all digital recording devices is submitted to the US Congress. This recommendation would:
Service Providers are still reluctant to accept digital interfaces and storage devices, but they may accept solutions in which the data is recorded in encrypted form to enable the basic time-shift function. The service provider has still control over the broadcast data in this case. This solution only works with a Set Top Box and an analogue output to a TV-set.
Protection mechanisms exist for digital audio recorders, however a PC can easily circumvent the existing copy protection system. For video a more or less similar protection system is implemented in the first digital VCR. But the motion picture industry is still very concerned about the PC. This is also the reason for the delay of the introduction of the DVD on the market.
Only a standardised copy protection system for all digital devices including the PC can be safe.
In some countries it is forbidden to use very strong cryptographic algorithms in consumer electronics. The law-enforcement agencies wish to have access to the communications of suspected criminals, which is threatened by secure cryptography. Industry and individual citizens, however, want to secure their private data, and look to cryptography to provide it. In the U.S. the Capstone project aims to develop a technology that attempts to balance these needs.