Constrained Cropping in Photoshop

What is the keyboard short cut to make the crop command constrained in PhotoShop?

I am trying to crop a picture in Photoshop but I do not want to change the ratio of the picture

Isn't it the shift key?

You could also try the "crop" tool. If you are just getting the image into printable dimensions, it's better, IMO, because it color masks the part of the image outside the crop area.

Using the "crop" tool, holding the shift key before using it will constrain it to a square selection. Holding the shift key after selecting the crop area, and then dragging the corner points, will preserve the aspect ratio.

tap in the size you want in the tools options pallete and away you crop

After, you can add that proportion to a file in your crop tool.