Help please! Accessing the compressed video stream

[xujyo]

My thesis is about information hiding in compressed video stream. Now I have to access the bitstream data of the compressed video, for example, the DCT coefficients, the motion vectors. Then I will modify the coefficients or motion vectors, and write back the modified data into the compressed bitstream.
Is there any tools or algorithms can do that?
Thank you!

[hrhgroup]

Dear xujyo
Did you find such a tools? I need that, too.
Thanks