Methods and image processing devices for encoding and decoding private data are proposed. The method for encoding private data includes to receive an original video frame, mask at least one private area in the original video frame to generate a protected video frame, generate a first encoded frame by encoding the protected video frame, and generate at least one output bitstream for streaming or storage according to the first encoded frame. The method for decoding private data includes to receive at least one input video bitstream to obtain a first encoded bitstream and a second encoded bitstream, decode the first encoded bitstream to generate a protected video frame including image data associated with at least one private area, and output the protected video frame to a display queue such that the at least one private area is displayed.