[Libav-user] Implementing MPEG-1 Real-Time Video Encryption

[Houle, Elijah Js]

I'd like to implement the encryption algorithm from this paper (www.cs.purdue.edu/homes/bb/security99.ps), where only the sign bits for the DC/AC coefficients are selected and encrypted. This requires selecting bits in a zigzag order from each block (Y1, Y2, Y3, Y4, Cb, Cr). Not wanting to write an MPEG-1 encoder from scratch, my first thought was to modify the mpeg12enc.c file, but I can't figure out whether/where it distinguishes among the different blocks of the macroblock, especially because I'm unfamiliar with some of the terminology implicit in the variable names. Am I close to the right track, or is there a better approach?