What are the two protocols used by SQL Server Windows authentication? I was asked this question in an interview, and I have no idea what the answer is. 

I know we have Windows authentication mode and SQL Server authentication. But the interviewer said that we have two protocols that Windows verify when using Windows authentication.

I just can't find anything on the Internet about this.
They were probably looking for an answer that discuss the "Kerberos" and "NTLM" Windows authentication protocol/schemes.

Once you know what to search for, there are lots of articles online about how these schemes relate to SQL Server:


Questions About Kerberos and SQL Server That You Were Too Shy to Ask
Understanding Kerberos and NTLM authentication in SQL Server Connections


  In the NTLM protocol, the client sends the user name to the server; the server generates and sends a challenge to the client; the client encrypts that challenge using the user’s password; and the client sends a response to the server.  


...and


  Kerberos authentication (protocol) provides a mechanism for mutual authentication between a client and a server on an open network.  


Comment: emphasis/meaning added
How to make sure that you are using Kerberos authentication when you create a remote connection to an instance of SQL Server 2005