So from time to time I am asked about how to add support for a new authentication method for Windows Logon, the answer to that question is buried in what authentication methods Windows natively supports.
For the purpose of this blog post lets scope that question to the Windows Kerberos Implementation as this is the “modern” authentication platform in Windows (in other words Kerberos as they do not want you to be using NTLM any further), additionally it supports most authentication methods.
So what are those methods? Essentially there are two passwords and public/private keys. What about the Windows Native implementation of Biometrics and Picture Passwords? These like nearly every other modality of authentication in Windows is simply a layer on top of the native support of passwords that is built into Windows.
I should note I was the Lead Program Manager for the Windows Biometric Framework and I do think it is a good solution for what it was designed for.
Let me explain; let’s use Biometric as an example. In this solution there is a service that works with the biometric sensors to perform the biometric match. When the match occurs this service releases a clear text copy of your password which in turn is stuffed into the Windows logon path just as if the user entered the password by hand.
This means that somewhere in the file-system there is a clear text copy of the password stored in a reversibly encrypted form. Specifically the encryption done in this case is performed using Windows subsystem called the Data Protection API (DPAPI). DPAPI is used by applications when they need keep a secret — secret, you see this is a Catch 22 scenario unless a password (or key) is entered there is no secret to encrypt with which means ultimately there is a secret that is stored in the clear. This means if you can get at the root secret you can get at the clear text values encrypted with DPAPI.
There are ways to mitigate some of the associated risks; one example being the use of Bitlocker and a TPM protector but unfortunately this is not broadly deployed and doesn’t address the full risk profile; this is why Microsoft positions the Biometric feature as one designed Windows as a convince. That is not to say it cannot be used in a secure way or to suggest it can not be used in an enterprise but unless thought is given to the threat model and the right mitigations are deployed it actually weakens your security.
So what about those enterprise OTP and challenge response solutions that we keep hearing about, surely they must be different right? Unfortunately no they are not. In fact they are probably worse because unlike the Windows Biometric solution they create a central repository of all clear text passwords.
You see for them to work they introduce a client (once called a GINA, now called a Logon Provider) that collects the challenge and likely has a service component or SSPI provider that implements a protocol that interacts with their product’s service which then validates the challenge and returns a clear text password which they then stuff on the users behalf.
No matter how good the design and implementation of this system is they are reliant on that singular repository of clear text / reversibly encrypted passwords. Things get even worse when you look at how these systems have to deal with offline and local logons, you know the scenario where a user takes their laptop on a plane and still needs to do work? For this to work they need to store the clear text password on the clear on the local machine just like the Biometrics solution.
What about the public / private key approach? There are essentially two variants of this in windows PKINIT and PKU2U. The most common example is the use of a smart card, this is what is used by many Fortune 500s and governments to secure their logons. It is also possible to add these other modalities into Windows by emulating this approach with a software virtual smartcard but they too fall prey to the same attack vectors, namely what key do you use to keep the private key protected (the answer is you use DPAPI).
So does this mean OTP is a bad solution? No in fact it’s much better than passwords if a system was designed to use it and it makes a great additional factor, the thing is Windows was not.