A side-channel encoding method for an electronic device is provided. The side-channel encoding method includes converting an original image into a down image by downscaling the original image, converting the down image into an up image by upscaling the down image, calculating a pixel-specific luminance variation between the original image and the up image by using a local average luminance for each pixel included in the original image, converting the pixel-specific luminance variation into a first variation binary signal by binarizing the luminance variation, converting the first variation binary signal into a second variation binary signal by simplifying the first variation binary signal, and outputting the second variation binary signal by including the second variation binary signal in the down image.