Creating a RESTful API with JWT Authentication in Go

Go has been a favorite among many developers when it comes to backend development. Its simplicity, combined with performance benefits, makes it a powerful tool for web services. In this tutorial, we'll guide you through creating a RESTful API using Go, with a focus on adding JWT (JSON Web Tokens) for authentication.

Step 1: Initialize Your Go Module

Create a new directory for your project:

mkdir go-jwt-api
cd go-jwt-api

Initialize a new module:

go mod init go-jwt-api

Step 2: Installing Required Packages

We'll use the gorilla/mux package for routing and dgrijalva/jwt-go for JWT functionality.

go get -u github.com/gorilla/mux
go get -u github.com/dgrijalva/jwt-go

Step 3: Setting Up Basic Routes

package main

import (
	"fmt"
	"net/http"
	"github.com/gorilla/mux"
)

func main() {
	r := mux.NewRouter()

	r.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		fmt.Fprintln(w, "Hello World!")
	})

	http.ListenAndServe(":8000", r)
}

Step 4: JWT Authentication

1. Create a utility for generating and validating JWTs:

package main

import (
	"fmt"
	"net/http"
	"github.com/gorilla/mux"
)

func main() {
	r := mux.NewRouter()

	r.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		fmt.Fprintln(w, "Hello World!")
	})

	http.ListenAndServe(":8000", r)
}

2. Integrate JWT authentication in your main Go file:

package jwtutil

import (
	"time"
	"github.com/dgrijalva/jwt-go"
)

var jwtKey = []byte("your_secret_key")

type Claims struct {
	Username string `json:"username"`
	jwt.StandardClaims
}

func GenerateToken(username string) (string, error) {
	expirationTime := time.Now().Add(24 * time.Hour)
	claims := &Claims{
		Username: username,
		StandardClaims: jwt.StandardClaims{
			ExpiresAt: expirationTime.Unix(),
		},
	}

	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
	return token.SignedString(jwtKey)
}

func ValidateToken(tknStr string) (*Claims, error) {
	claims := &Claims{}
	tkn, err := jwt.ParseWithClaims(tknStr, claims, func(token *jwt.Token) (interface{}, error) {
		return jwtKey, nil
	})

	if err != nil {
		return nil, err
	}
	if !tkn.Valid {
		return nil, jwt.ErrSignatureInvalid
	}
	return claims, nil
}

5. Put It All Together

package main

import (
	"fmt"
	"net/http"
	"time"

	"github.com/gorilla/mux"
	"github.com/dgrijalva/jwt-go"
)

var jwtKey = []byte("your_secret_key")

type Claims struct {
	Username string `json:"username"`
	jwt.StandardClaims
}

func GenerateToken(username string) (string, error) {
	expirationTime := time.Now().Add(24 * time.Hour)
	claims := &Claims{
		Username: username,
		StandardClaims: jwt.StandardClaims{
			ExpiresAt: expirationTime.Unix(),
		},
	}

	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
	return token.SignedString(jwtKey)
}

func ValidateToken(tknStr string) (*Claims, error) {
	claims := &Claims{}
	tkn, err := jwt.ParseWithClaims(tknStr, claims, func(token *jwt.Token) (interface{}, error) {
		return jwtKey, nil
	})

	if err != nil {
		return nil, err
	}
	if !tkn.Valid {
		return nil, jwt.ErrSignatureInvalid
	}
	return claims, nil
}

func handleLogin(w http.ResponseWriter, r *http.Request) {
	username := r.PostFormValue("username")
	password := r.PostFormValue("password")

	// For demonstration, we're assuming "admin"/"password" as valid credentials.
	if username != "admin" || password != "password" {
		http.Error(w, "Invalid credentials", http.StatusUnauthorized)
		return
	}

	token, err := GenerateToken(username)
	if err != nil {
		http.Error(w, "Failed to generate token", http.StatusInternalServerError)
		return
	}

	http.SetCookie(w, &http.Cookie{
		Name:    "token",
		Value:   token,
		Expires: time.Now().Add(24 * time.Hour),
	})
}

func handleProtectedRoute(w http.ResponseWriter, r *http.Request) {
	c, err := r.Cookie("token")
	if err != nil {
		if err == http.ErrNoCookie {
			http.Error(w, "No token", http.StatusUnauthorized)
			return
		}
		http.Error(w, "Bad Request", http.StatusBadRequest)
		return
	}

	tokenStr := c.Value
	claims, err := ValidateToken(tokenStr)
	if err != nil {
		http.Error(w, "Invalid token", http.StatusUnauthorized)
		return
	}

	w.Write([]byte(fmt.Sprintf("Hello, %s!", claims.Username)))
}

func main() {
	r := mux.NewRouter()

	r.HandleFunc("/login", handleLogin).Methods("POST")
	r.HandleFunc("/protected", handleProtectedRoute).Methods("GET")

	http.ListenAndServe(":8000", r)
}

Conclusion

With these steps, you now have a basic RESTful API in Go with JWT authentication. Remember to replace the hardcoded credentials and secret key with more secure methods in a production setting. Use bcrypt for password hashing, and consider storing your secret key securely or even rotating it regularly. Happy coding!