In this study, a new method is proposed to protect video data through partial encryption. Unlike previous methods, the bit rate of the encrypted portion can be controlled. In order to accomplish this task, a simple model for the time to break the partial encryption by a chipertext-only attack is defined. Then, the encrypted bit budget distribution strategy maximizing the time subject to the bitrate constraint is found. An algorithm to estimate the model parameters is constructed and it isthen implemented over an MPEG-4 natural video codec together with the bit budget distribution strategy. The encoder is tested with various image sequences and the output is analyzed. In addition to the developed video encryption method, a file format is defined to store encryption related side information.