Merge and decrypt webvtt

[Devil1]

From here you can get m3u8 of video file

http://9xbuddy.com/process?url=http://www.hotstar.com/movies/bindaas/1770002686

Inside the video m3u8 file, there is a subtitle m3u8 file. There are 147 segmented (split) webvtt files and a .key file ( encryption method: aes-128). It is pretty much same as a m3u8 file with bunch of .ts files and .key file.

My question is how to join and decrypt all these webvtt file? I have tried ffmpeg but it is unable to do the job. Searched google but there is no method to available to decrypt webvtt files

#Indian vpn is required to download all these files.