Custom SSH Authentication

[Alex Robbins <alexdotrobbins@gmail.com>]

I am looking to set up a custom SSH authentication system.  I have a several
RSA key pairs for my user, and I want to restrict ssh access based on which
key pair is being used (not based on user name).  On top of that, I want
to restrict keys based on time of day.  In short, a certain key can only be
used at certain times, while another key works around the clock.

I am also hoping to take it a step further and say that the restricted key
(the one that only works at certain times) also requires that a pass phrase
be provided that changes based on an arbitrary algorithm, perhaps involving
the time of day or date.

The concept is simple, so I was hoping that I could just write a bash 
script,
but I don't know that there is any way to involve a bash script in the sshd
authentication process.  Another thought was to write a PAM module.  I don't
know how difficult that would be, and I have written C# under Windows, but
I have not written C or C++.  Or, maybe I missed something, and I actually
don't have to write anything other than a configuration file?

So, my question is, does anyone have any thoughts on how I should go about
making this happen?

Thank you,
Alex Robbins