digest auth with qop=auth is violating the RFC

Start in Network/HTTP/Auth.hs at withAuthority, where qop=auth is added to the Authorization header. Read the RFC2617 requirements in this issue and inspect the surrounding auth fields. Done means the package no longer emits a partial qop=auth response, or provides the required nc and cnonce fields; verify the resulting digest-auth behavior.

RFC2617 https://tools.ietf.org/html/rfc2617 says about qop and related fields (key bits highlighted):

qop
Indicates what "quality of protection" the client has applied to
the message. If present, its value MUST be one of the alternatives
the server indicated it supports in the WWW-Authenticate header.
These values affect the computation of the request-digest. Note
that this is a single token, not a quoted list of alternatives as
in WWW- Authenticate. This directive is optional in order to
preserve backward compatibility with a minimal implementation of
RFC 2069 [6], but SHOULD be used if the server indicated that qop
is supported by providing a qop directive in the WWW-Authenticate
header field.

cnonce
This MUST be specified if a qop directive is sent (see above), and
MUST NOT be specified if the server did not send a qop directive in
the WWW-Authenticate header field. The cnonce-value is an opaque
quoted string value provided by the client and used by both client
and server to avoid chosen plaintext attacks, to provide mutual
authentication, and to provide some message integrity protection.
See the descriptions below of the calculation of the response-
digest and request-digest values.

nonce-count
This MUST be specified if a qop directive is sent (see above), and
MUST NOT be specified if the server did not send a qop directive in
the WWW-Authenticate header field. The nc-value is the hexadecimal
count of the number of requests (including the current request)
that the client has sent with the nonce value in this request. For
example, in the first request sent in response to a given nonce
value, the client sends "nc=00000001". The purpose of this
directive is to allow the server to detect request replays by
maintaining its own copy of this count - if the same nc-value is
seen twice, then the request is a replay. See the description
below of the construction of the request-digest value.

That is, if the client sends qop=auth then it MUST also send nc=xxx and cnonce=xxx. Servers are well within their right to reject requests without these as malformed.

Currently the HTTP package does not support the nc or cnonce fields, and yet it will automatically send qop=auth if the server offered it. Either this support should be disabled or the nc/cnonce support should be added. That is it should either follow RFC2069 (which has no notion of qop) or RFC2617 (which introduces qop and its related directives) but not a partial mixture of the two.

We currently have a problem with the hackage-server https://github.com/haskell/hackage-server/pull/199 where we want to support RFC2617 style digest auth (because a certain browser only supports the newer RFC), but if we send qop=auth then the HTTP package (and thus cabal etc) return broken responses. The server is currently correctly rejecting requests that use qop=auth but without the other required fields. We will probably have to relax that and allow these incorrect requests. But other servers may not be so forgiving, and we should fix it. The easiest thing would be to simply stop sending qop=auth, e.g.

diff --git a/Network/HTTP/Auth.hs b/Network/HTTP/Auth.hs
index 4af0d67..5dad7cc 100644
--- a/Network/HTTP/Auth.hs
+++ b/Network/HTTP/Auth.hs
@@ -95,7 +95,8 @@ withAuthority a rq = case a of
                        -- plus optional stuff:
                    , fromMaybe "" (fmap (\ alg -> ",algorithm=" ++ quo (show alg)) (auAlgorithm a))
                    , fromMaybe "" (fmap (\ o   -> ",opaque=" ++ quo o) (auOpaque a))
-                   , if null (auQop a) then "" else ",qop=auth"
+                     --TODO: we currently do not support qop=auth or auth-int
+                     -- if we send qop=auth then we MUST also send 'nc' and 'cnonce'
                    ]