Many corporate/enterprise environments still have proxies that use NTLM authentication also sometimes referred to as the NT challenge/response (NTCR) scheme. As far as I can tell, this is old Microsoft products that just refuse to die. The following document explains the protocol: https://www.innovation.ch/personal/ronald/ntlm.html There does appear to be a C library implementing the protocol at: http://cntlm.sourceforge.net/ There is also a Python 1.x implementation at: http://ntlmaps.sourceforge.net/