JSON Web Token example

jwt_golang_example.go

package main

import (
	"encoding/json"
	"fmt"
	"io/ioutil"
	"log"
	"net/http"
	"strings"
	"time"

	"github.com/codegangsta/negroni"
	"github.com/dgrijalva/jwt-go"
	"os"
)

const (
	privKeyPath = "path/to/keys/app.rsa"
	pubKeyPath  = "path/to/keys/app.rsa.pub"
)

var VerifyKey, SignKey []byte

func initKeys() {
	var err error

	SignKey, err = ioutil.ReadFile(privKeyPath)
	if err != nil {
		log.Fatal("Error reading private key")
		return
	}

	VerifyKey, err = ioutil.ReadFile(pubKeyPath)
	if err != nil {
		log.Fatal("Error reading public key")
		return
	}
}

type UserCredentials struct {
	Username string `json:"username"`
	Password string `json:"password"`
}

type User struct {
	ID       int    `json:"id"`
	Name     string `json:"name"`
	Username string `json:"username"`
	Password string `json:"password"`
}

type Response struct {
	Data string `json:"data"`
}

type Token struct {
	Token string `json:"token"`
}

func StartServer() {
	// public endpoints
	http.HandleFunc("/login", LoginHandler)

	// protected endpoints
	http.Handle("/resource/", negroni.New(
		negroni.HandlerFunc(ValidateTokenMiddleware),
		negroni.Wrap(http.HandlerFunc(ProtectedHandler)),
	))

	log.Println("Now listening...")
	http.ListenAndServe(":8000", nil)
}

func main() {
	initKeys()
	StartServer()
}

func ProtectedHandler(w http.ResponseWriter, r *http.Request) {
	response := Response{"Gained access to protected resource"}
	JsonResponse(response, w)

}

func LoginHandler(w http.ResponseWriter, r *http.Request) {
	var user UserCredentials

	// decode request into UserCredentials struct
	err := json.NewDecoder(r.Body).Decode(&user)
	if err != nil {
		w.WriteHeader(http.StatusForbidden)
		fmt.Fprintf(w, "Error in request")
		return
	}

	fmt.Println(user.Username, user.Password)

	// validate user credentials
	if strings.ToLower(user.Username) != "alexcons" {
		if user.Password != "kappa123" {
			w.WriteHeader(http.StatusForbidden)
			fmt.Println("Error logging in")
			fmt.Fprint(w, "Invalid credentials")
			return
		}
	}

	// create a rsa 256 signer
	signer := jwt.New(jwt.GetSigningMethod("RS256"))

	// set claims
	signer.Claims["iss"] = "admin"
	signer.Claims["exp"] = time.Now().Add(time.Minute * 20).Unix()
	signer.Claims["CustomUserInfo"] = struct {
		Name string
		Role string
	}{user.Username, "Member"}

	tokenString, err := signer.SignedString(SignKey)

	if err != nil {
		w.WriteHeader(http.StatusInternalServerError)
		fmt.Fprintln(w, "Error while signing the token")
		log.Printf("Error signing token: %v\n", err)
	}

	// create a token instance using the token string
	response := Token{tokenString}
	JsonResponse(response, w)

}

func ValidateTokenMiddleware(w http.ResponseWriter,
	r *http.Request,
	next http.HandlerFunc) {

	// validate token
	token, err := jwt.ParseFromRequest(r, func(token *jwt.Token) (interface{}, error) {
		return VerifyKey, nil
	})

	if err == nil {
		if token.Valid {
			next(w, r)
		} else {
			w.WriteHeader(http.StatusUnauthorized)
			fmt.Fprint(w, "Token is not valid")
		}
	} else {
		w.WriteHeader(http.StatusUnauthorized)
		fmt.Fprint(w, "Unauthorised access to this resource")
	}
}

func JsonResponse(response interface{}, w http.ResponseWriter) {
	json, err := json.Marshal(response)
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}

	w.WriteHeader(http.StatusOK)
	w.Header().Set("Content-Type", "application/json")
	w.Write(json)
}