part of Hypertext Transfer Protocol --
RFC 2616 Fielding, et al.
HTTP provides several OPTIONAL challenge-response authentication mechanisms which can be used by a server to challenge a client request and by a client to provide authentication information. The general framework for access authentication, and the specification of "basic" and "digest" authentication, are specified in "HTTP Authentication: Basic and Digest Access Authentication" . This specification adopts the definitions of "challenge" and "credentials" from that specification.