Some keyboard keys not working on password input to Yosemite login screen
The fact that the keyboard works at the boot screen before user level code runs is curious.

I wonder if one of the kernel extensions or control panels that customize that keyboard are causing issues. As I don't have that hardware, it's hard to poke and see what files might be causing the issue.

You can try several things to corner this:


Make a new user account and turn off auto log in in the Users preference pane
You could hold shift down when booting to rebuild all kernel extensions and then log in safely without most third party apps running
You could uninstall any drivers and/or open a support ticket with the vendor


In each of the above cases, you can test logging in to the new account / safe boot system and see which are broken and which are working. Worst case, edit the question and/or open a new question with the details of this triage.