The present invention will be further described in detail below in conjunction with the accompanying drawings.
 The method flow of the present invention is shown in Figure 5. The mobile TV/mobile multimedia broadcast content provider uses the CA system to generate a control word (CW). The CW is used as the content key to encrypt the content, and then the CW is issued to the mobile operator. In accordance with the BCMCS security architecture, the operator generates SK from BAK and SK_RAND to encrypt the CW, and the encrypted CW is returned to the mobile TV/mobile multimedia broadcast content provider along with the indexes BAK_ID and SK_RAND of the BAK. The mobile TV/mobile multimedia broadcast content provider broadcasts the encrypted content, BAK_ID and SK_RAND to the terminal together. On the terminal side, according to the key distribution process of BCMCS, UIM generates SK from BAK and SK_RAND, and sends SK back to ME. ME decrypts CW with SK, and then CW decrypts the content, and transmits the result to the user. The detailed process is as follows:
 Step 501, mobile phone TV/mobile multimedia broadcast content provides CW encrypted mobile broadcast content generated by a commercial CA system;
 Step 502: Send the CW as data to the mobile communication operator;
 Step 503, the BCMCS control entity in the mobile TV service platform encrypts the CW with SK;
 In step 504, the indexes BAK_ID and SK_RAND of the BAK used by the generated SK and the encrypted CW are sent to the mobile TV/mobile multimedia broadcast content provider network together;
 Step 505: Send the encrypted content, BAK_ID and SK_RAND to the terminal together through the mobile TV/mobile multimedia broadcast content provider network;
 In step 506, the terminal uses BAK_ID to obtain BAK, BAK and SK_RAND to obtain SK, decrypts with SK to obtain CW, and uses CW to decrypt mobile broadcast content.
 Step 503 includes the following specific steps:
 The first is the generation of BAK: (completed before the encrypted service stream transmission) BAK is identified by the combination of BCMCS_FLOW_ID and BAK_ID.
 UIM and SM each configure a registration key RK, which will be the basis of BCMCS authentication and key exchange;
BAKG generates BAK, and associates the value with BAK_ID identification and expiration time (BAK_Expire), BAK and corresponding BAK_ID and BAK_Expire are transmitted to SKM and BAKD together;
 When the BAK changes compared with the previous decryption operation in the terminal, the terminal applies for a new BAK: When the BAK does not change compared with the previous decryption operation in the terminal, perform the following steps:
 SKM generates SK from the random value of current BAK and SK_RAND, SKM transmits SK, SK_RAND, BAK_ID and BAK_Expire to CE;
 CS transmits CW to CE;
 CE encrypts CW with SK.
 In the step 505:
 If the BAK_ID and SK_RAND are unchanged from the last received BAK_ID and SK_RAND, the ME uses the SK value currently assigned to the encrypted CW to decrypt the encrypted CW, and uses the obtained CW to decrypt the mobile broadcast content.
 If BAK_ID or SK_RAND has changed, then SK has changed. At this time, ME requests a new SK from UIM, including BCMCS_FLOW_ID, BAK_ID and SK_RAND;
 UIM generates SK from BAK and SK_RAND, and sends SK back to ME. ME decrypts the encrypted CW, and then uses CW to decrypt the mobile broadcast content.
 BAK is the same in the above steps. If the terminal applies for a new BAK, perform the following steps:
 a. The ME sends a BAK request to BAKD. The BAK request may contain RK-based authentication information. SM can use this information to determine that the request comes from a legitimate user.
 b. In order to send BAK to UIM, BAK must be encrypted to prevent it from being received by non-target recipients. BAKD requests SM to generate a temporary key (TK). BAKD requests a new TK for each BAK.
 c. SM generates TK from the random value of TK_RAND and RK. TK_RAND can be generated by BAKD or SM. SM sends TK and TK_RAND to BAKD. Then, SM discards TK and TK_RAND.
 d. BAKD encrypts BAK with TK, and sends the encrypted BAK together with TK_RAND, BAK_ID and BAK_Expire to UIM via ME. BAKD discards TK and TK_RAND. UIM first forms TK from TK_RAND and RK, decrypts the encrypted BAK with TK, and obtains BAK. UIM discards TK and TK_RAND. BAK and its related BAK_Expire value are stored in UIM. UIM should be able to store at least two BAKs in order to obtain and store a new BAK before the BAK expires.
 In the present invention, the terminal and the mobile phone have the same meaning.
 Those skilled in the art can make various modifications to the above content without departing from the spirit and scope of the present invention as determined by the claims. Therefore, the scope of the present invention is not limited to the above description, but is determined by the scope of the claims.