diff --git a/cmd/owl/web/auth_test.go b/cmd/owl/web/auth_test.go
index 0bf6e76..67d443d 100644
--- a/cmd/owl/web/auth_test.go
+++ b/cmd/owl/web/auth_test.go
@@ -1,6 +1,8 @@
 package web_test
 
 import (
+	"crypto/sha256"
+	"encoding/base64"
 	"encoding/json"
 	main "h4kor/owl-blogs/cmd/owl/web"
 	"h4kor/owl-blogs/test/assertions"
@@ -73,7 +75,7 @@ func TestAuthPostCorrectPassword(t *testing.T) {
 func TestAuthPostWithIncorrectCode(t *testing.T) {
 	repo, user := getSingleUserTestRepo()
 	user.ResetPassword("testpassword")
-	user.GenerateAuthCode("http://example.com", "http://example.com/response")
+	user.GenerateAuthCode("http://example.com", "http://example.com/response", "", "")
 
 	// Create Request and Response
 	form := url.Values{}
@@ -95,7 +97,7 @@ func TestAuthPostWithIncorrectCode(t *testing.T) {
 func TestAuthPostWithCorrectCode(t *testing.T) {
 	repo, user := getSingleUserTestRepo()
 	user.ResetPassword("testpassword")
-	code, _ := user.GenerateAuthCode("http://example.com", "http://example.com/response")
+	code, _ := user.GenerateAuthCode("http://example.com", "http://example.com/response", "", "")
 
 	// Create Request and Response
 	form := url.Values{}
@@ -123,6 +125,128 @@ func TestAuthPostWithCorrectCode(t *testing.T) {
 
 }
 
+func TestAuthPostWithCorrectCodeAndPKCE(t *testing.T) {
+	repo, user := getSingleUserTestRepo()
+	user.ResetPassword("testpassword")
+
+	// Create Request and Response
+	code_verifier := "test_code_verifier"
+	// create code challenge
+	h := sha256.New()
+	h.Write([]byte(code_verifier))
+	code_challenge := base64.RawURLEncoding.EncodeToString(h.Sum(nil))
+	code, _ := user.GenerateAuthCode("http://example.com", "http://example.com/response", code_challenge, "S256")
+
+	form := url.Values{}
+	form.Add("code", code)
+	form.Add("client_id", "http://example.com")
+	form.Add("redirect_uri", "http://example.com/response")
+	form.Add("grant_type", "authorization_code")
+	form.Add("code_verifier", code_verifier)
+	req, err := http.NewRequest("POST", user.AuthUrl(), strings.NewReader(form.Encode()))
+	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Add("Content-Length", strconv.Itoa(len(form.Encode())))
+	req.Header.Add("Accept", "application/json")
+	assertions.AssertNoError(t, err, "Error creating request")
+	rr := httptest.NewRecorder()
+	router := main.SingleUserRouter(&repo)
+	router.ServeHTTP(rr, req)
+
+	assertions.AssertStatus(t, rr, http.StatusOK)
+	// parse response as json
+	type responseType struct {
+		Me string `json:"me"`
+	}
+	var response responseType
+	json.Unmarshal(rr.Body.Bytes(), &response)
+	assertions.AssertEqual(t, response.Me, user.FullUrl())
+
+}
+
+func TestAuthPostWithCorrectCodeAndWrongPKCE(t *testing.T) {
+	repo, user := getSingleUserTestRepo()
+	user.ResetPassword("testpassword")
+
+	// Create Request and Response
+	code_verifier := "test_code_verifier"
+	// create code challenge
+	h := sha256.New()
+	h.Write([]byte(code_verifier + "wrong"))
+	code_challenge := base64.RawURLEncoding.EncodeToString(h.Sum(nil))
+	code, _ := user.GenerateAuthCode("http://example.com", "http://example.com/response", code_challenge, "S256")
+
+	form := url.Values{}
+	form.Add("code", code)
+	form.Add("client_id", "http://example.com")
+	form.Add("redirect_uri", "http://example.com/response")
+	form.Add("grant_type", "authorization_code")
+	form.Add("code_verifier", code_verifier)
+	req, err := http.NewRequest("POST", user.AuthUrl(), strings.NewReader(form.Encode()))
+	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Add("Content-Length", strconv.Itoa(len(form.Encode())))
+	req.Header.Add("Accept", "application/json")
+	assertions.AssertNoError(t, err, "Error creating request")
+	rr := httptest.NewRecorder()
+	router := main.SingleUserRouter(&repo)
+	router.ServeHTTP(rr, req)
+
+	assertions.AssertStatus(t, rr, http.StatusUnauthorized)
+}
+
+func TestAuthPostWithCorrectCodePKCEPlain(t *testing.T) {
+	repo, user := getSingleUserTestRepo()
+	user.ResetPassword("testpassword")
+
+	// Create Request and Response
+	code_verifier := "test_code_verifier"
+	code_challenge := code_verifier
+	code, _ := user.GenerateAuthCode("http://example.com", "http://example.com/response", code_challenge, "plain")
+
+	form := url.Values{}
+	form.Add("code", code)
+	form.Add("client_id", "http://example.com")
+	form.Add("redirect_uri", "http://example.com/response")
+	form.Add("grant_type", "authorization_code")
+	form.Add("code_verifier", code_verifier)
+	req, err := http.NewRequest("POST", user.AuthUrl(), strings.NewReader(form.Encode()))
+	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Add("Content-Length", strconv.Itoa(len(form.Encode())))
+	req.Header.Add("Accept", "application/json")
+	assertions.AssertNoError(t, err, "Error creating request")
+	rr := httptest.NewRecorder()
+	router := main.SingleUserRouter(&repo)
+	router.ServeHTTP(rr, req)
+
+	assertions.AssertStatus(t, rr, http.StatusOK)
+}
+
+func TestAuthPostWithCorrectCodePKCEPlainWrong(t *testing.T) {
+	repo, user := getSingleUserTestRepo()
+	user.ResetPassword("testpassword")
+
+	// Create Request and Response
+	code_verifier := "test_code_verifier"
+	code_challenge := code_verifier + "wrong"
+	code, _ := user.GenerateAuthCode("http://example.com", "http://example.com/response", code_challenge, "plain")
+
+	form := url.Values{}
+	form.Add("code", code)
+	form.Add("client_id", "http://example.com")
+	form.Add("redirect_uri", "http://example.com/response")
+	form.Add("grant_type", "authorization_code")
+	form.Add("code_verifier", code_verifier)
+	req, err := http.NewRequest("POST", user.AuthUrl(), strings.NewReader(form.Encode()))
+	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Add("Content-Length", strconv.Itoa(len(form.Encode())))
+	req.Header.Add("Accept", "application/json")
+	assertions.AssertNoError(t, err, "Error creating request")
+	rr := httptest.NewRecorder()
+	router := main.SingleUserRouter(&repo)
+	router.ServeHTTP(rr, req)
+
+	assertions.AssertStatus(t, rr, http.StatusUnauthorized)
+}
+
 func TestAuthRedirectUriNotSet(t *testing.T) {
 	repo, user := getSingleUserTestRepo()
 	repo.HttpClient = &mocks.MockHttpClient{}
diff --git a/cmd/owl/web/handler.go b/cmd/owl/web/handler.go
index 62ad48f..bad57c9 100644
--- a/cmd/owl/web/handler.go
+++ b/cmd/owl/web/handler.go
@@ -74,6 +74,8 @@ func userAuthHandler(repo *owl.Repository) func(http.ResponseWriter, *http.Reque
 		redirectUri := r.URL.Query().Get("redirect_uri")
 		state := r.URL.Query().Get("state")
 		responseType := r.URL.Query().Get("response_type")
+		codeChallenge := r.URL.Query().Get("code_challenge")
+		codeChallengeMethod := r.URL.Query().Get("code_challenge_method")
 
 		// check if request is valid
 		missing_params := []string{}
@@ -99,6 +101,11 @@ func userAuthHandler(repo *owl.Repository) func(http.ResponseWriter, *http.Reque
 			w.Write([]byte("Invalid response_type. Must be 'code' ('id' converted to 'code' for legacy support)."))
 			return
 		}
+		if codeChallengeMethod != "" && (codeChallengeMethod != "S256" && codeChallengeMethod != "plain") {
+			w.WriteHeader(http.StatusBadRequest)
+			w.Write([]byte("Invalid code_challenge_method. Must be 'S256' or 'plain'."))
+			return
+		}
 
 		// check if redirect_uri is registered
 		resp, _ := repo.HttpClient.Get(clientId)
@@ -127,13 +134,15 @@ func userAuthHandler(repo *owl.Repository) func(http.ResponseWriter, *http.Reque
 		http.SetCookie(w, &cookie)
 
 		reqData := owl.AuthRequestData{
-			Me:           me,
-			ClientId:     clientId,
-			RedirectUri:  redirectUri,
-			State:        state,
-			ResponseType: responseType,
-			User:         user,
-			CsrfToken:    csrfToken,
+			Me:                  me,
+			ClientId:            clientId,
+			RedirectUri:         redirectUri,
+			State:               state,
+			ResponseType:        responseType,
+			CodeChallenge:       codeChallenge,
+			CodeChallengeMethod: codeChallengeMethod,
+			User:                user,
+			CsrfToken:           csrfToken,
 		}
 
 		html, err := owl.RenderUserAuthPage(reqData)
@@ -168,9 +177,10 @@ func userAuthProfileHandler(repo *owl.Repository) func(http.ResponseWriter, *htt
 		code := r.Form.Get("code")
 		client_id := r.Form.Get("client_id")
 		redirect_uri := r.Form.Get("redirect_uri")
+		code_verifier := r.Form.Get("code_verifier")
 
 		// check if request is valid
-		valid := user.VerifyAuthCode(code, client_id, redirect_uri)
+		valid := user.VerifyAuthCode(code, client_id, redirect_uri, code_verifier)
 		if !valid {
 			w.WriteHeader(http.StatusUnauthorized)
 			w.Write([]byte("Invalid code"))
@@ -225,11 +235,12 @@ func userAuthVerifyHandler(repo *owl.Repository) func(http.ResponseWriter, *http
 			return
 		}
 		password := r.FormValue("password")
-		println("Password: ", password)
 		client_id := r.FormValue("client_id")
 		redirect_uri := r.FormValue("redirect_uri")
 		response_type := r.FormValue("response_type")
 		state := r.FormValue("state")
+		code_challenge := r.FormValue("code_challenge")
+		code_challenge_method := r.FormValue("code_challenge_method")
 
 		// CSRF check
 		formCsrfToken := r.FormValue("csrf_token")
@@ -250,17 +261,22 @@ func userAuthVerifyHandler(repo *owl.Repository) func(http.ResponseWriter, *http
 
 		password_valid := user.VerifyPassword(password)
 		if !password_valid {
+			redirect := fmt.Sprintf(
+				"%s?error=invalid_password&client_id=%s&redirect_uri=%s&response_type=%s&state=%s",
+				user.AuthUrl(), client_id, redirect_uri, response_type, state,
+			)
+			if code_challenge != "" {
+				redirect += fmt.Sprintf("&code_challenge=%s&code_challenge_method=%s", code_challenge, code_challenge_method)
+			}
 			http.Redirect(w, r,
-				fmt.Sprintf(
-					"%s?error=invalid_password&client_id=%s&redirect_uri=%s&response_type=%s&state=%s",
-					user.AuthUrl(), client_id, redirect_uri, response_type, state,
-				),
+				redirect,
 				http.StatusFound,
 			)
 			return
 		} else {
 			// password is valid, generate code
-			code, err := user.GenerateAuthCode(client_id, redirect_uri)
+			code, err := user.GenerateAuthCode(
+				client_id, redirect_uri, code_challenge, code_challenge_method)
 			if err != nil {
 				println("Error generating code: ", err.Error())
 				w.WriteHeader(http.StatusInternalServerError)
diff --git a/embed/auth.html b/embed/auth.html
index 03884ce..d2f322a 100644
--- a/embed/auth.html
+++ b/embed/auth.html
@@ -8,5 +8,7 @@
     <input type="hidden" name="response_type" value="{{.ResponseType}}">
     <input type="hidden" name="state" value="{{.State}}">
     <input type="hidden" name="csrf_token" value="{{.CsrfToken}}">
+    <input type="hidden" name="code_challenge" value="{{.CodeChallenge}}">
+    <input type="hidden" name="code_challenge_method" value="{{.CodeChallengeMethod}}">
     <input type="submit" value="Login">
 </form>
\ No newline at end of file
diff --git a/renderer.go b/renderer.go
index 03e5540..b72bf7d 100644
--- a/renderer.go
+++ b/renderer.go
@@ -21,13 +21,15 @@ type PostRenderData struct {
 }
 
 type AuthRequestData struct {
-	Me           string
-	ClientId     string
-	RedirectUri  string
-	State        string
-	ResponseType string
-	User         User
-	CsrfToken    string
+	Me                  string
+	ClientId            string
+	RedirectUri         string
+	State               string
+	ResponseType        string
+	CodeChallenge       string
+	CodeChallengeMethod string
+	User                User
+	CsrfToken           string
 }
 
 func renderEmbedTemplate(templateFile string, data interface{}) (string, error) {
diff --git a/user.go b/user.go
index 4141814..8b042ef 100644
--- a/user.go
+++ b/user.go
@@ -1,6 +1,8 @@
 package owl
 
 import (
+	"crypto/sha256"
+	"encoding/base64"
 	"fmt"
 	"net/url"
 	"os"
@@ -32,10 +34,12 @@ type UserMe struct {
 }
 
 type AuthCode struct {
-	Code        string    `yaml:"code"`
-	ClientId    string    `yaml:"client_id"`
-	RedirectUri string    `yaml:"redirect_uri"`
-	Created     time.Time `yaml:"created"`
+	Code                string    `yaml:"code"`
+	ClientId            string    `yaml:"client_id"`
+	RedirectUri         string    `yaml:"redirect_uri"`
+	CodeChallenge       string    `yaml:"code_challenge"`
+	CodeChallengeMethod string    `yaml:"code_challenge_method"`
+	Created             time.Time `yaml:"created"`
 }
 
 func (user User) Dir() string {
@@ -284,21 +288,37 @@ func (user User) addAuthCode(code AuthCode) error {
 	return saveToYaml(user.AuthCodesFile(), codes)
 }
 
-func (user User) GenerateAuthCode(client_id string, redirect_uri string) (string, error) {
+func (user User) GenerateAuthCode(
+	client_id string, redirect_uri string,
+	code_challenge string, code_challenge_method string,
+) (string, error) {
 	// generate code
 	code := GenerateRandomString(32)
 	return code, user.addAuthCode(AuthCode{
-		Code:        code,
-		ClientId:    client_id,
-		RedirectUri: redirect_uri,
+		Code:                code,
+		ClientId:            client_id,
+		RedirectUri:         redirect_uri,
+		CodeChallenge:       code_challenge,
+		CodeChallengeMethod: code_challenge_method,
+		Created:             time.Now(),
 	})
 }
 
-func (user User) VerifyAuthCode(code string, client_id string, redirect_uri string) bool {
+func (user User) VerifyAuthCode(
+	code string, client_id string, redirect_uri string, code_verifier string,
+) bool {
 	codes := user.getAuthCodes()
 	for _, c := range codes {
 		if c.Code == code && c.ClientId == client_id && c.RedirectUri == redirect_uri {
-			return true
+			if c.CodeChallengeMethod == "plain" {
+				return c.CodeChallenge == code_verifier
+			} else if c.CodeChallengeMethod == "S256" {
+				// hash code_verifier
+				hash := sha256.Sum256([]byte(code_verifier))
+				return c.CodeChallenge == base64.RawURLEncoding.EncodeToString(hash[:])
+			} else if c.CodeChallengeMethod == "" {
+				return true
+			}
 		}
 	}
 	return false