Canvas squaring

I need a program that will batch process a number of images (jpg, gif, png) and reshape the canvas so that they are square. This MUST NOT stretch or distort the image at all. The image should sit in the center of the new square canvas. e.g. if an image is 200px high x 400px wide the program will change it to 400 x 400 and put 100px padding on the top and bottom of the image. White background is fine for this version. Note it is possible for either the height or width to be changed. Each image in a batch will have different dimensions so the program will have to intellegently look at each image and determine the new height or width.