Traditional　image　encryption　algorithms　usually　aim　to　encrypt　the　RGB　channels　of　color　plain　images　separately.　In　this　paper,　a　novel　dimension　reduction　method　is　proposed　for　color　image　encryption,　where　a　color　plain　image　is　first　converted　into　a　grayscale　image　by　using　steganography　technique　followed　by　an　encryption　operation　on　the　resultant　grayscale　image　thereby　reducing　the　dimension　of　the　encryption.　And　the　color　information　can　be　restored　back　without　losing　any　details.　More　specifically,　a　color　plain　image　is　first　converted　into　the　YCbCr　channels,　where　the　Y　channel　is　used　as　the　steganography　carrier　and　to　hide　the　other　two　channels　that　are　down-sampled　by　a　bicubic　interpolation.　Our　algorithm　can　compress　and　encrypt　the　color　plain　image　to　generate　a　grayscale　cipher　image　so　as　to　reduce　the　repeatability　of　encryption.　Furthermore,　only　the　used　pixel　values　are　encrypted,　where　a　bridge　is　built　between　binary　image　encryption　and　traditional　grayscale　image　encryption.　The　security　and　performance　of　the　proposed　image　encryption　algorithm　are　analyzed,　and　the　unification　between　binary　image　encryption　and　grayscale　image　encryption　is　achieved.