Set-top box encryption system process ECM EMM CW SK

With the development of digital Video Broadcasting (DVB). Viewers will have to face the choice of digital TV programs more. Because of the increased cost of investment, the broadcasting industry is viewed by users who need to charge a fee.

Conditional Receive systems (Conditional Access system). Its main function is to ensure that only those who have paid or are about to pay will be able to watch the selected TV program.

Key technologies of 1 CA system

In the CA system. There are two most critical technologies: the first is the method of scrambling (scrambling) and the solution of Disturbance (descrambling). The scrambling technique is used to change or control certain characteristics of the transmitted service (program) under the control of the sending-side CA system, so that the service cannot be obtained by unauthorized users. The second is to control the disturbance. An encrypted message is provided on the sending side so that the authorized client-side spoiler can decrypt the data. This information is controlled by the CA system. and configured in the transmission stream information in an encrypted form to prevent unauthorized users from using this information directly to solve the disturbance. There is a very big difference in how different CA systems manage and transmit this information.

the common practice of scrambling is to interfere with the video, audio, or data stream by using a scrambling sequence on the sending side to disrupt the data. The scrambling sequence is generated by the pseudo-random sequence generator, and the scrambling sequence generated by the pseudo-random sequence generator can be guessed when the initial condition is known. The initial conditions of a pseudo-random sequence generator arecontrolled by control word. There is also an identical pseudo-random sequence generator at the receiving end, assuming that the control word CW is sent to the pseudo-random sequence generator, then the perturbed sequence can be obtained. The original signal is then recovered using the solution sequence. So, the core of the program's conditional reception is the transmission of the control word CW . In order to achieve confidentiality, the control word must be encrypted and transmitted after processing. The receiving end is authorized. Talent Application Decryption Program once again generate this control word.

2 encryption mechanism of control word CW

Since CW is transmitted along with scrambling information through the public network. No matter who can read and study it. Once the CW is read and cracked, the entire system is paralyzed, so the CW itself uses an encryption key , which is protected by cryptographic algorithms. In a detailed application, this key can be changed in accordance with the requirements of the network operator, usually provided by the service provider. Used to control the services it provides, so call it a business key (SK, servicekey). The use of SK is generally related to user payment conditions.

Although CW has been encrypted by SK, the key hypothesis is that it can still be read by any person, which means that the subscribers and non-subscribers of a particular service will have the same rights. It is still difficult for network operators to control specific users. There is still a problem with security. SK must be re-encrypted for protection.

This encryption process is performed entirely according to individual user characteristics, since this key is determined by personal characteristics, often referred to as the personal distribution key (PDK, Personal distribute key).

PDK generally by the CA system equipment itself to produce and strictly control, in the end of the PDK is generally by the network operator through the CA system provided by the special equipment in the prom, can not be read again. In order to provide different levels of service, a set of CA systems often allocates several PDK for each user. To meet a wide range of business needs.

In a multi-set CA system that is actually operating (mainly in Europe and America). There are many kinds of encryption authorization methods for the end users, such as manual authorization, magnetic card authorization, IC card authorization, smart card authorization, centralized address authorization (directly addressed by the control center, without card authorization), smart card and the authorization method of the central authority. The smart card authorization mode is the mainstream of the STB market, and it is also the standard accessory for China's network equipment.

Note: A smart card is a card that uses an IC to make analytical inferences, and its structure includes coprocessor, ROM, and EEPROM.

Implementation of conditional reception in 3 MPEG-2 Digital TV system

in use MPEG-2 standard digital TV system, with program flow CA system-related two data streams are: Authorization control information ( ECM, entitle Control Message ) and Authorization management information ( EMM, entitle Manage Message ). The CW, which is encrypted by SK, is transmitted in the ECM. The ECM also includes information such as program origin, time, content classification, and program price. The CW-encrypted SK is transmitted in the EMM, and the SK is encrypted by the PDK prior to transmission. The EMM also includes address, user authorization information.

In the TS (Transport stream) stream. PSI (Program specific information) is divided into four categories: the PAT (Program Association table) table, the PMT (Program Map table) table. NIT Table (Net information table) and Cat table (Conditional Access table). ,PMT and CAT are closely related to conditional acceptance .

To illustrate the problem more clearly, first introduce CA descriptor (ca descriptive narrator), as seen in table 1.

Assuming that a primitive stream is encrypted, there must be CA descriptor in the program that contains the original stream. Assuming that no CA management information (such as EMM) and raw special flow information such as ECM exist in the TS stream, there must be CA descriptor in the corresponding Pmt.

Table 1 CA Descriptor

Syntax	Number of digits	Abbreviation
Ca_descriptor () { Descriptor_tag Descriptor_length ca_system_id Reserved Ca_pid for (i=0;i<n;i++) { Private_data_byte } }	8 8 16 3 13 8	Uimsbf Uimsbf Uimsbf bslbf Uimsbf Uimsbf

Note: The value of Descriprot_tag in the description sub-CA is 9. Through this sign, the CA descriptive narrative can be distinguished from other descriptive narratives, such as video stream descriptive narration and audio stream descriptive narration.

In addition, the ca_system_id in the CA descriptive narrative is used to represent the type of CA system applicable to the relevant ECM and EMM streams. DVB Project has established a dedicated organization responsible for assigning different ca_system_id values to different decoder manufacturers. Such words. A set of digital streams can transfer the keys of different CA systems to different set-top box user groups at the same time.

When the CA description is present in the PMT table, the Ca_pid points to the program groupings that contain information related to ECM. When a CA describes a narrative that is present in the cat table, Ca_pid points to the program grouping that contains the information related to the EMM .

It is thus clear that the decryption mechanism of the decoder is that when the smart card is inserted into the STB, the decoder reads the ca_system_id from it. and find the corresponding ca_pid (ie, the PID of the EMM Stream) in the CA descriptive narration in the Cat table (PID number 0x01).

The EMM code stream includes an SK that has been processed by PDK encryption. The PDK is cured in a smart card and stored in encrypted form. The user is required to provide a password to use it for decryption.

And then. The Smart card will decrypt the SK.

After completing the above steps, the decoder then looks for the corresponding ca_pid (i.e., the PID of the ECM stream) in the CA descriptive narration in the PMT table that corresponds to the above ca_system_id.

The ECM code stream includes the CW information that is processed by SK encryption. The resulting SK can be used to decrypt the ECM to get CW.

When the CW is filled into the corresponding registers of the decoding chip, the stream data can be disturbed to recover the original signal.

Understand

PDK equals private key (private key). and SK equals public key. To prevent a particular service from being received by the Subscriber, the SK should also be encrypted. But only has the PDK user ability to decrypt, thus achieves only has the subscriber ability to receive the SK the purpose.

"Mnemonic"

(1) EMM: Authorization management information, as its name implies, is information that manages authorization.

This authorization is SK, in order to prevent this authorization information from being acquired by unauthorized persons. It is necessary to manage it by encrypting it through the PDK, that is, Pdk_sk.

In addition, EMM can be used to implement EMM fingerprintand specify properties such as display content, time, interval, character color, background color, flicker, and so on.

(2) ECM: Authorization control information. Name implies. It is the information that controls authorization.

This "control" is the key to finally solve the disturbance.

"Control" is achieved by CW, and only the user who obtains the authorized information can decrypt the CW, so the SK_CW is stored in ECM.

In addition, ECM can be used to implement ECM fingerprint. The fixed fingerprint display function of the Smart card number and controls the starting and ending times of the display.

(3)The role of fingerprint: If a user will be with fingerprint program to the Internet or for other purposes, then the relevant service provider can be based on the fingerprint card number (real name) to the legal responsibility of the user.

"Flowchart"

4 scrambling

The scrambling of MPEG-2 data stream can be divided into two layers: one is the addition of PES layer, and the other is the disturbance of TS layer. The TS layer scrambling is only for the payload of the TS Data stream (payload). The PSI information in TS streams, including Pat, PMT, NIT, Cat, and private segments (including ECM,EMM) should not be scrambling. Of course, the header field of the TS stream (including the adjustment field) should not be disturbed either. After scrambling, the TS stream should define the cryptographic control word in the header field.

Selection of 5 encryption algorithm

(1) Control word encryption: for the control word encryption. Encryption algorithms that require a certain amount of strength and fast processing are mainly made up of cryptographic algorithms in the cryptographic system of private key (personal distribution key, PDK), such as DES or idea algorithm.

(2) Encryption of Business key: The change frequency of business key is much less than control word. As a result, the algorithm for its encryption can be processed slowly, but the security requirements are higher because a business key is used for a longer period of time. A number of high-strength encryption algorithms need to be selected. The encryption algorithm of public key system can be used well in this paper, because the public key system encryption algorithm is generally slower than the secret key system, although the processing rate is usually slow. However, most of them have high encryption strength, which can meet the higher security requirements of business key. In addition, using public key system encryption, the operator does not have to transfer the user's private key, just need to know the user's public key to encrypt the business key, the user can use their own private key to decrypt. This enables a pair of keys to be generated on the client side. The security of the system can be improved by simply passing the public key to the sender and verifying that it is available, while the decrypted key private key does not need to be transmitted once. In the public key system can also realize digital signature, digital certificate and other functions. It is also advantageous for the system to broaden the business, the scope of the business, to provide online payment and other functions.

At present, the basic public key encryption algorithm mainly has RSA public key encryption algorithm, knapsack public key password system, Lobin (Rabin) password, mac Riess (mceliece) public key password, Elliptic curve public key password and so on, Wide application of RSA public key encryption algorithm.

Copyright notice: This article Bo Master original article. Blog, not reproduced without consent.

Set-top box encryption system process ECM EMM CW SK