How to decrypt and join these MP4 segments, pls?

[jiangxia]

The video stream is replayed in MP4 segments in order, 0000.mp4, 0002.mp4, 0004.mp4, ... which also can be downloaded freely, but it always shows black frames without sound by any media players, even MediaInfo can tell the attached MP4 file:
Format : MPEG-4 at 387 Kbps
Length : 94.9 KiB for 2s 11ms
Video #0 : AVC at 330 Kbps
Aspect : 400 x 224 (1.786) at 25.000 fps
Audio #0 : at 48.0 Kbps
Infos : 2 Channels, 44.1 KHz

I do not know how to decrypt and join these MP4 segments, help me, please!

Your assistance will be greatly appreciated.