support implicit flow in web-identity.go example (#12600)

when a client secret is not provided,
automatically assume implicit flow
for authentication and invoke
relevant code accordingly.

docs/sts/web-identity.go

@@ -20,6 +20,7 @@
 package main
 
 import (
+	"bytes"
 	"context"
 	"crypto/rand"
 	"encoding/base64"
@@ -108,9 +109,34 @@ func init() {
 	flag.IntVar(&port, "port", 8080, "Port")
 }
 
+func implicitFlowURL(c oauth2.Config, state string) string {
+	var buf bytes.Buffer
+	buf.WriteString(c.Endpoint.AuthURL)
+	v := url.Values{
+		"response_type": {"id_token"},
+		"response_mode": {"form_post"},
+		"client_id":     {c.ClientID},
+	}
+	if c.RedirectURL != "" {
+		v.Set("redirect_uri", c.RedirectURL)
+	}
+	if len(c.Scopes) > 0 {
+		v.Set("scope", strings.Join(c.Scopes, " "))
+	}
+	v.Set("state", state)
+	v.Set("nonce", state)
+	if strings.Contains(c.Endpoint.AuthURL, "?") {
+		buf.WriteByte('&')
+	} else {
+		buf.WriteByte('?')
+	}
+	buf.WriteString(v.Encode())
+	return buf.String()
+}
+
 func main() {
 	flag.Parse()
-	if clientID == "" || clientSec == "" {
+	if clientID == "" {
 		flag.PrintDefaults()
 		return
 	}
@@ -148,29 +174,47 @@ func main() {
 			http.NotFound(w, r)
 			return
 		}
-		http.Redirect(w, r, config.AuthCodeURL(state), http.StatusFound)
+		if clientSec != "" {
+			http.Redirect(w, r, config.AuthCodeURL(state), http.StatusFound)
+		} else {
+			http.Redirect(w, r, implicitFlowURL(config, state), http.StatusFound)
+		}
 	})
 
 	http.HandleFunc("/oauth2/callback", func(w http.ResponseWriter, r *http.Request) {
 		log.Printf("%s %s", r.Method, r.RequestURI)
-		if r.URL.Query().Get("state") != state {
+
+		if err := r.ParseForm(); err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
+		}
+		if r.Form.Get("state") != state {
 			http.Error(w, "state did not match", http.StatusBadRequest)
 			return
 		}
 
-		getWebTokenExpiry := func() (*credentials.WebIdentityToken, error) {
-			oauth2Token, err := config.Exchange(ctx, r.URL.Query().Get("code"))
-			if err != nil {
-				return nil, err
-			}
-			if !oauth2Token.Valid() {
-				return nil, errors.New("invalid token")
+		var getWebTokenExpiry func() (*credentials.WebIdentityToken, error)
+		if clientSec == "" {
+			getWebTokenExpiry = func() (*credentials.WebIdentityToken, error) {
+				return &credentials.WebIdentityToken{
+					Token: r.Form.Get("id_token"),
+				}, nil
 			}
+		} else {
+			getWebTokenExpiry = func() (*credentials.WebIdentityToken, error) {
+				oauth2Token, err := config.Exchange(ctx, r.URL.Query().Get("code"))
+				if err != nil {
+					return nil, err
+				}
+				if !oauth2Token.Valid() {
+					return nil, errors.New("invalid token")
+				}
 
-			return &credentials.WebIdentityToken{
-				Token:  oauth2Token.Extra("id_token").(string),
-				Expiry: int(oauth2Token.Expiry.Sub(time.Now().UTC()).Seconds()),
-			}, nil
+				return &credentials.WebIdentityToken{
+					Token:  oauth2Token.Extra("id_token").(string),
+					Expiry: int(oauth2Token.Expiry.Sub(time.Now().UTC()).Seconds()),
+				}, nil
+			}
 		}
 
 		sts, err := credentials.NewSTSWebIdentity(stsEndpoint, getWebTokenExpiry)