Idp verification via smartphone app?

[Brad Larson]

I am playing around a little bit with biometrics.  I'd like to create
a smartphone app which integrates with my Idp.

Assuming I have a custom Idp login page which launches my iOS
biometrics app, and my biometrics app can validate the user, what is a
safe way to return control to my Idp and finish the user verification?

In iOS, I can load my app using a custom URL.  However I cannot return
data from my app to the web page.  The only option I am aware of is to
have my app load a new web page when the user validation is done.  How
do I make sure the new web page which is opened was loaded by a valid
user and not somebody guessing random URLs?

Any thoughts or suggestions are appreciated!
Brad