The forward slash key (/) toggles on and off the Shield (the shading of the area outside of the crop) while using the Crop tool.
To use the dimensions of one image to crop another, select the document with the desired crop (width, height, and resolution) and click the “Front Image” button in the Options bar (this will enter the height, width and resolution for the document). Then, switch to the document that needs to be cropped and use the Crop tool to make your selection. The aspect ratio will be constrained while dragging the crop and, when applied, the image will be resized to the desired width, height and resolution.
While using the Crop tool, tapping the return/enter key will automatically highlight the Width in the Options bar and allow you to enter an amount. Tab to move to the Height and Resolution fields.
To use the Crop tool to add canvas to your image, drag out a crop and release the mouse. Then, drag the crop handles outside of the image area and apply the crop. The area outside of the image will be added to the canvas. Note: to add transparency around the image (instead of filling the added space with the background color) convert the Background into a layer by selecting Layer > New > Layer From Background (or by double clicking on the word Background in the Layers panel).
To cancel a crop, tap the escape key. To apply the crop, tap the enter key, double click inside of the crop bounding box, or choose another tool from the tool bar (this last method requires an additional dialog box in order to tell Photoshop to crop or not).