matlab video processing: transition from frequency to time domain

hi all,
I am working on video encryption in matlab.
I have tried an algo for encryption and decryption on images. and it gives exact image reconstruction.
But when i am trying this on video i am not able to get back the results.

Can someone please guide me about transforming the video frame, from normal time domain pixel values to the frequency domain values(or pixel values of image after applying any transfrom(dwt or fft) ) and then reconstructing the frame to time domain...

Please help.