Add digest authentication

- Adhere to RFC 7616 ("HTTP Digest Access Authentication")
- Added SetDigestAuth methods for Client and Request
- Currently not supporting auth-int Quality of Protection

fixes #467

client.go

@@ -55,6 +55,8 @@ var (
 	hdrContentLengthKey   = http.CanonicalHeaderKey("Content-Length")
 	hdrContentEncodingKey = http.CanonicalHeaderKey("Content-Encoding")
 	hdrLocationKey        = http.CanonicalHeaderKey("Location")
+	hdrAuthorizationKey   = http.CanonicalHeaderKey("Authorization")
+	hdrWwwAuthenticateKey = http.CanonicalHeaderKey("WWW-Authenticate")
 
 	plainTextType   = "text/plain; charset=utf-8"
 	jsonContentType = "application/json"
@@ -125,6 +127,7 @@ type Client struct {
 	// value when `SetAuthToken` option is used.
 	HeaderAuthorizationKey string
 
+	digestCredentials  *digestCredentials
 	jsonEscapeHTML     bool
 	setContentLength   bool
 	closeConnection    bool
@@ -396,6 +399,21 @@ func (c *Client) SetAuthScheme(scheme string) *Client {
 	return c
 }
 
+// SetDigestAuth method sets the Digest Access auth scheme for the client. If a server responds with 401 and sends
+// a Digest challenge in the WWW-Authenticate Header, requests will be resent with the appropriate Authorization Header.
+//
+// For Example: To set the Digest scheme with user "Mufasa" and password "Circle Of Life"
+// 		client.SetDigestAuth("Mufasa", "Circle Of Life")
+//
+// Information about Digest Access Authentication can be found in RFC7616:
+//     https://datatracker.ietf.org/doc/html/rfc7616
+//
+// See `Request.SetDigestAuth`.
+func (c *Client) SetDigestAuth(username, password string) *Client {
+	c.digestCredentials = &digestCredentials{username: username, password: password}
+	return c
+}
+
 // R method creates a new request instance, its used for Get, Post, Put, Delete, Patch, Head, Options, etc.
 func (c *Client) R() *Request {
 	r := &Request{
@@ -1003,6 +1021,16 @@ func (c *Client) execute(req *Request) (*Response, error) {
 
 	req.RawRequest.Body = newRequestBodyReleaser(req.RawRequest.Body, req.bodyBuf)
 
+	transport := c.httpClient.Transport
+	if req.digestCredentials != nil {
+		c.httpClient.Transport = &digestTransport{digestCredentials: *req.digestCredentials, transport: transport}
+	} else if c.digestCredentials != nil {
+		c.httpClient.Transport = &digestTransport{digestCredentials: *c.digestCredentials, transport: transport}
+	}
+	defer func() {
+		c.httpClient.Transport = transport
+	}()
+
 	req.Time = time.Now()
 	resp, err := c.httpClient.Do(req.RawRequest)
 

client_test.go

@@ -80,6 +80,76 @@ func TestClientAuthScheme(t *testing.T) {
 
 }
 
+func TestClientDigestAuth(t *testing.T) {
+	conf := defaultDigestServerConf()
+	ts := createDigestServer(t, conf)
+	defer ts.Close()
+
+	c := dc().
+		SetBaseURL(ts.URL+"/").
+		SetDigestAuth(conf.username, conf.password)
+
+	resp, err := c.R().
+		SetResult(&AuthSuccess{}).
+		Get(conf.uri)
+	assertError(t, err)
+	assertEqual(t, http.StatusOK, resp.StatusCode())
+
+	t.Logf("Result Success: %q", resp.Result().(*AuthSuccess))
+	logResponse(t, resp)
+}
+
+func TestClientDigestSession(t *testing.T) {
+	conf := defaultDigestServerConf()
+	conf.algo = "MD5-sess"
+	ts := createDigestServer(t, conf)
+	defer ts.Close()
+
+	c := dc().
+		SetBaseURL(ts.URL+"/").
+		SetDigestAuth(conf.username, conf.password)
+
+	resp, err := c.R().
+		SetResult(&AuthSuccess{}).
+		Get(conf.uri)
+	assertError(t, err)
+	assertEqual(t, http.StatusOK, resp.StatusCode())
+
+	t.Logf("Result Success: %q", resp.Result().(*AuthSuccess))
+	logResponse(t, resp)
+}
+
+func TestClientDigestErrors(t *testing.T) {
+	type test struct {
+		mutateConf func(*digestServerConfig)
+		expect     error
+	}
+	tests := []test{
+		{mutateConf: func(c *digestServerConfig) { c.algo = "BAD_ALGO" }, expect: ErrAlgNotSupported},
+		{mutateConf: func(c *digestServerConfig) { c.qop = "bad-qop" }, expect: ErrQopNotSupported},
+		{mutateConf: func(c *digestServerConfig) { c.qop = "" }, expect: ErrNoQop},
+		{mutateConf: func(c *digestServerConfig) { c.charset = "utf-16" }, expect: ErrCharset},
+		{mutateConf: func(c *digestServerConfig) { c.uri = "/bad" }, expect: ErrBadChallenge},
+		{mutateConf: func(c *digestServerConfig) { c.uri = "/unknown_param" }, expect: ErrBadChallenge},
+		{mutateConf: func(c *digestServerConfig) { c.uri = "/no_challenge" }, expect: ErrBadChallenge},
+		{mutateConf: func(c *digestServerConfig) { c.uri = "/status_500" }, expect: nil},
+	}
+
+	for _, tc := range tests {
+		conf := defaultDigestServerConf()
+		tc.mutateConf(conf)
+		ts := createDigestServer(t, conf)
+
+		c := dc().
+			SetBaseURL(ts.URL+"/").
+			SetDigestAuth(conf.username, conf.password)
+
+		_, err := c.R().Get(conf.uri)
+		assertErrorIs(t, tc.expect, err)
+		ts.Close()
+	}
+}
+
 func TestOnAfterMiddleware(t *testing.T) {
 	ts := createGenServer(t)
 	defer ts.Close()