The challenge names Basic auth.

highlighted = computed this step

The scheme is Basic

The challenge row says the server is asking for Basic auth.

scheme=Basic\text{scheme}=\text{Basic}
Basic schemeThe Basic challenge scheme, realm, and empty response body are recomputed from pinned bytes.Basic scheme - GET /private has no Authorization; response 401 Unauthorized names Basicsectionnamevaluecheckrequest linemethodGETparsedrequest linepath/privateparsedrequest lineversionHTTP/1.1parsedheaderHostapi.exampleexactheaderAuthorizationabsentabsentstatusversionHTTP/1.1parsedstatusstatus401 UnauthorizedchallengeheaderWWW-AuthenticateBasic realm toysafe display formheaderContent-Length0empty bodybodybyte length0countedchallengeschemeBasicparsedchallengerealm displaytoyparsedcheckAuthorization headerabsentmissingcheckstatus401 Unauthorizedchallengecheckchallenge schemeBasicBasiccheckrealmtoytoychecksent body bytes0empty

This points to the next request

A later request can answer with the Basic Authorization header from the previous book.

Basic challengeBasic Authorization\text{Basic challenge} \rightarrow \text{Basic Authorization}
Basic schemeThe Basic challenge scheme, realm, and empty response body are recomputed from pinned bytes.Basic scheme - GET /private has no Authorization; response 401 Unauthorized names Basicsectionnamevaluecheckrequest linemethodGETparsedrequest linepath/privateparsedrequest lineversionHTTP/1.1parsedheaderHostapi.exampleexactheaderAuthorizationabsentabsentstatusversionHTTP/1.1parsedstatusstatus401 UnauthorizedchallengeheaderWWW-AuthenticateBasic realm toysafe display formheaderContent-Length0empty bodybodybyte length0countedchallengeschemeBasicparsedchallengerealm displaytoyparsedcheckAuthorization headerabsentmissingcheckstatus401 Unauthorizedchallengecheckchallenge schemeBasicBasiccheckrealmtoytoychecksent body bytes0empty

The scheme check passes

The check rows keep the requested scheme tied to the response bytes.

challenge scheme checked\text{challenge scheme checked}
Basic schemeThe Basic challenge scheme, realm, and empty response body are recomputed from pinned bytes.Basic scheme - GET /private has no Authorization; response 401 Unauthorized names Basicsectionnamevaluecheckrequest linemethodGETparsedrequest linepath/privateparsedrequest lineversionHTTP/1.1parsedheaderHostapi.exampleexactheaderAuthorizationabsentabsentstatusversionHTTP/1.1parsedstatusstatus401 UnauthorizedchallengeheaderWWW-AuthenticateBasic realm toysafe display formheaderContent-Length0empty bodybodybyte length0countedchallengeschemeBasicparsedchallengerealm displaytoyparsedcheckAuthorization headerabsentmissingcheckstatus401 Unauthorizedchallengecheckchallenge schemeBasicBasiccheckrealmtoytoychecksent body bytes0empty

Summary

The challenge names Basic before any Authorization header is sent.

Basic named\text{Basic named}
Basic schemeThe Basic challenge scheme, realm, and empty response body are recomputed from pinned bytes.Basic scheme - GET /private has no Authorization; response 401 Unauthorized names Basicsectionnamevaluecheckrequest linemethodGETparsedrequest linepath/privateparsedrequest lineversionHTTP/1.1parsedheaderHostapi.exampleexactheaderAuthorizationabsentabsentstatusversionHTTP/1.1parsedstatusstatus401 UnauthorizedchallengeheaderWWW-AuthenticateBasic realm toysafe display formheaderContent-Length0empty bodybodybyte length0countedchallengeschemeBasicparsedchallengerealm displaytoyparsedcheckAuthorization headerabsentmissingcheckstatus401 Unauthorizedchallengecheckchallenge schemeBasicBasiccheckrealmtoytoychecksent body bytes0empty