the invention will be detailed hereinafter with reference to the drawings . as illustrated in fig1 , a method for decrypting a data file according to a first embodiment of the invention can be applicable in data broadcast services , e . g ., an mbms service , etc ., and particularly includes the following operations 12 - 16 . in the operation 12 , a user equipment receives a toi value of a data file and a toi value of a key file to decrypt the data file in an fdt ; in the operation 14 , the corresponding data file and key file are received according to the toi value of the data file and the toi value of the key file ; and in the operation 16 , the data file is decrypted according to the key file . each key file may include one key for decrypting a corresponding data file . when the key file includes a plurality of keys , the data file including a key identifier is parsed to retrieve the corresponding key identifier , then corresponding one of the keys is extracted from the key file according to the key identifier , and next the data file is decrypted with the key . as illustrated in fig2 , a device 30 for generating description information according to the first embodiment of the invention includes : a data description information generation module 32 configured to generate description information of a data file , which includes a data file toi value ; a key description information generation module 34 configured to generate description information of a key file to decrypt the data file , which includes a key file toi value ; and a file description information generation module 38 configured to generate an fdt file according to the data file toi value and the key file toi value . taking mbms as an example , the device for generating description information resides in a content distribution server and generates the fdt file as shown below : particularly , “ key_file_toi =“ 8 ”” and “ key_file toi =“ 9 ”” are newly added statements descriptive of a key file toi value , where the toi value of the data file “ main . sdp ” is 2 and its corresponding key file toi value is 8 , and the toi value of the data file “ trailer . 3gp ” is 3 and its corresponding key file toi value is 9 . as illustrated in fig3 , a device 50 for decrypting a data file according to the first embodiment of the invention includes : a file identifier reception module 52 configured to receive a toi value of a data file and a toi value of a corresponding key file to be retrieved from a fdt ; a file reception module 54 configured to receive the corresponding data file and key file according to the data file toi value and the key file toi value ; and a decryption module 58 configured to decrypt the data file according to the key file . in the first embodiment , the data file decryption device 50 resides in a user equipment . the user equipment intended to receive the data file with the toi value of 2 locates and downloads the data file according to the toi value and also downloads the key file with the toi value of 8 corresponding to the data file according to a description in the fdt and decrypts the data file using the key in the key file . as illustrated in fig4 , a data broadcast system according to the first embodiment of the invention includes : a description information generation device 30 configured to generate file description information including an identifier of a data file and an identifier of a key file corresponding to the data file ; and a data file decryption device 50 configured to receive the data file identifier and the key file identifier , to receive the corresponding data file and key file according to the data file identifier and the key file identifier , and to decrypt the data file according to the key file . according to the first embodiment of the invention , the toi value of the data file and the toi value of the key file to decrypt the data file are carried in the fdt so that the user equipment can receive the data file and the key file rapidly and obviate both a waste of resource due to reception of a large amount of useless information and also a time delay due to decryption of the data file to thereby improve a user &# 39 ; s experience of a service . as illustrated in fig5 , a method for decrypting a data file according to a second embodiment of the invention includes the following operations 12 ′- 16 ′. in the operation 12 ′, a user equipment receives a toi value of a data file , a tot value of a key file to decrypt the data file and a key identifier corresponding to the data file in an fdt ; in the operation 14 ′, the corresponding data file and key file are received according to the toi value of the data file and the toi value of the key file ; in the operation 15 ′, a corresponding key is extracted from the key file according to the key identifier ; and in the operation 16 ′, the data file is decrypted according to the key . since each key file may include a plurality of keys corresponding to a plurality of data files , the key identifier corresponding to the data file is added in the fdt file . therefore , the key corresponding to the data file can be located more rapidly without parsing the data file . as illustrated in fig6 , a device 30 ′ for generating description information according to the second embodiment of the invention includes : a data description information generation module 32 configured to generate description information of a data file , which includes a data file toi value ; a key description information generation module 34 configured to generate description information of a key file to decrypt the data file , which includes a key file toi value ; a key identifier generation module 36 configured to generate a key identifier corresponding to the data file ; and a file description information generation module 38 configured to generate an fdt file from the data file identifier , the key file identifier and the key identifier . also taking mbms as an example , the device 30 ′ for generating description information resides in a content distribution server and generates the fdt file as shown below : particularly , “ key_id =“ baff ”=” is a statement in the fdt to describe the key identifier , where the toi value of the data file “ main . sdp ” is 2 and the toi value of a corresponding key file is 8 and a corresponding key identifier is “ baff =”. as illustrated in fig7 , a device 50 ′ for decrypting a data file according to the second embodiment of the invention includes : a file identifier reception module 52 configured to receive a toi value of a data file and a toi value of a corresponding key file to be retrieved from an fdt generated by the device for generating description information 30 ′; a file reception module 54 configured to receive the corresponding data file and key file according to the data file toi value and the key file tot value ; a key identifier reception module 56 configured to receive a key identifier corresponding to the data file ; and a decryption module 58 ′ configured to decrypt the data file according to the key file . the decryption module 58 ′ includes : a key extraction sub - module configured to extract a corresponding key from the key file according to the key identifier ; and a decryption sub - module configured to decrypt the data file according to the key . a user equipment intended to receive a data file with a toi value of 2 locates and downloads the corresponding data file according to the toi value and also downloads a key file with a toi value of 8 corresponding to the data file according to a description in the fdt , extracts a corresponding key from the key file with the toi value of 8 according to a received key identifier “ baff =” and decrypts the data file using the key with the key identifier “ baff =”. a data broadcast system according to the second embodiment of the invention includes : a description information generation device 30 ′ further configured to generate file description information including an identifier of a data file , an identifier of a key file corresponding to the data file and a key identifier corresponding to the data file ; and a data file decryption device 50 ′ further configured to receive the data file identifier , the key file identifier and the key identifier in the file description information , to receive the corresponding data file and key file according to the data file identifier and the key file identifier , to extract a corresponding key from the key file according to the key identifier and to decrypt the data file according to the key . according to the second embodiment , the key identifier corresponding to the data file is also added into the fdt in addition to the key file toi , and thus in the case that the key file includes a plurality of keys , the user equipment can retrieve the key directly without parsing the data file in addition to its ability to locate the key file rapidly . in a method for decrypting a data file according to a third embodiment of the invention , a user equipment can receive a corresponding key file according to a content location identifier ( content - location value ) in an fdt . however , since the user equipment shall receive a specific file from an flute stream according to a toi value , the user equipment receiving the content - location value of a key file shall further retrieve a tot value of the key file from the fdt and receive the key file according to the toi value . taking mbms as an example , the device for generating description information generates the fdt file as shown below : in the foregoing fdt file , as can be apparent from the first file section , the data file main . sdp has a toi =“ 2 ”, where a key to encrypt the file main . sdp is carried in a file identified with key_file_content - location ; and as can be apparent from the second file section , the key file key . bin has a toi =“ 8 ”. a method for decrypting a data file according to the third embodiment includes the following operations 12 ″- 18 ″. in the operation 12 ″, a user equipment receives a toi value of a data file and a content - location value of a key file to decrypt the data file in an fdt ; in the operation 14 ″, the user equipment searches the fdt table for a toi value of the key file according to the content - location value of the key file ; in the operation 16 ″, the corresponding data file and key file are received according to the toi value of the data file and the toi value of the key file ; and in the operation 18 ″, the data file is decrypted according to the key file . when the key file includes a plurality of keys , the data file including a key identifier is parsed to retrieve the corresponding key identifier , then corresponding one of the keys is extracted from the key file according to the key identifier , and next the data file is decrypted with the key . for example , a statement “ key_id =“ baff =”” descriptive of a key identifier is added in the foregoing fdt file , where the toi value of the data file “ main . sdp ” is 2 and the content - location value of the corresponding key file is “ key . bin ”, and the toi value of the key file is 8 and the corresponding key identifier is “ baff =”. according to the third embodiment of the invention , the toi value of the data file and the content - location value of the key file to decrypt the data file are carried in the fdt so that the user equipment can receive the data file and the key file rapidly and obviate both a waste of resource due to reception of a large amount of useless information and also a time delay due to decryption of the data file to thereby improve a user &# 39 ; s experience of a service . it shall be noted that the foregoing embodiments are merely illustrative but not limitative of the invention and the invention will not be limited thereto but any technical solutions and adaptations thereof without departing from the scope of the invention shall be encompassed in the scope of the claims appended to the invention .