You can use , and . to zoom in and out or Ctrl/Cmnd and the plus or minus key but there is no jump to a zoom factor of 100% (200% for a High Rez (retina) display). 100% shows you a pixel for a pixel in AE and Photoshop.
I already know the zoom keys. Their problem is that they don't centre the content/composition. So it's always going to require use of the Pan tool to bring the work area back into a centred position. This is ridiculous extraneous work for the end user after doing fine detail work. There should be a zoom to selection and a zoom to 1:1 and centred. Surely that's the most logical thing to provide for those making masks and editing them?
One thing I suppose I should be grateful for... the Middle Mouse Button actually works as a Pan tool in AE. It doesn't in Illustrator, for reasons that NOBODY knows the answer to.
WAIT... it gets better.
There's a keyboard shortcut for Zoom-to-Fit.
[Shift + /]
// which is kind of like saying the shortcut is "?", more on this in a bit.
Here's where it doesn't work. If you already have zoomed to fit, and then pan to do some work on something at an edge, but more in the middle of the view, the Zoom-to-Fit feature no longer works as a shortcut key to pull everything back to the middle of the screen, because it views the "mode" of Zoom-to-Fit to be current and active.
So to get everything centred again, requires zooming in or out, then hitting the shortcut for Zoom-to-Fit.
Fill the comp window the the comp (Zoom Factor Fit up to 100%) by pressing shift + /
It's right there in the Keyboard Shortcuts. It's amazing what you can find by typing a question in the Search Help field at the top right corner of AE.
Press / then shift + /...
BTW, I seldom if ever work at Zoom to Fit because I want to accurately position my layers on whole pixel values and you can't do that when your zoom factor is 47.8%.
The bit I forgot to add about this shortcut being "?" is that it's never certain if it's going to work