keyboard shortcut for square

Holding down the Shift key while dragging out a shape with the Rectangle Tool creates a square for me.

Have you tried updating Photoshop and/or resetting the Preferences?

Relatively recently Adobe changed the need to hold down the Shift key to keep things square. If you prefer the old functionality, you might like to activate the "Use Legacy Free Transform" option within the General tab of the Photoshop Preferences.