# File lib/atom/http.rb, line 47
    def digest_authenticate(req, url, param_string = "")
      raise "Digest authentication requires a WWW-Authenticate header" if param_string.empty?

      params = parse_wwwauth_digest(param_string)
      qop = params[:qop]

      user, pass = username_and_password_for_realm(url, params[:realm])

      if params[:algorithm] == "MD5"
        a1 = user + ":" + params[:realm] + ":" + pass
      else
        # XXX MD5-sess
        raise "I only support MD5 digest authentication (not #{params[:algorithm].inspect})"
      end

      if qop.nil? or qop.member? "auth"
        a2 = req.method + ":" + req.path
      else
        # XXX auth-int
        raise "only 'auth' qop supported (none of: #{qop.inspect})"
      end

      if qop.nil?
        response = kd(h(a1), params[:nonce] + ":" + h(a2))
      else
        @@nonce_count += 1
        nc = ('%08x' % @@nonce_count)

        # XXX auth-int
        data = "#{params[:nonce]}:#{nc}:#{CNONCE}:#{"auth"}:#{h(a2)}"

        response = kd(h(a1), data)
      end

      header = %Q<Digest username="#{user}", uri="#{req.path}", realm="#{params[:realm]}", response="#{response}", nonce="#{params[:nonce]}">

      if params[:opaque]
        header += %Q<, opaque="#{params[:opaque]}">
      end

      if params[:algorithm] != "MD5"
        header += ", algorithm=#{algo}"
      end

      if qop
        # XXX auth-int
        header += %Q<, nc=#{nc}, cnonce="#{CNONCE}", qop=auth>
      end

      req["Authorization"] = header
    end

# File lib/atom/http.rb, line 43
    def h(data); Digest::MD5.hexdigest(data); end

# File lib/atom/http.rb, line 44
    def kd(secret, data); h(secret + ":" + data); end

# File lib/atom/http.rb, line 29
    def parse_wwwauth_digest param_string
      params = parse_quoted_wwwauth param_string
      qop = params[:qop] ? params[:qop].split(",") : nil

      param_string.gsub(/stale=([^,]*)/) do
        params[:stale] = ($1.downcase == "true")
      end

      params[:algorithm] = "MD5"
      param_string.gsub(/algorithm=([^,]*)/) { params[:algorithm] = $1 }

      params
    end