How to implement authentication and authorization using Golang

With the continuous development of Internet applications, user authentication and authorization are becoming more and more important. Authentication is the process of verifying that a user has permission to access a system, while authorization is the process of determining what actions a user can perform. In this article, we will discuss how to implement authentication and authorization using Golang.

1. Authentication using JWT

JWT (JSON Web Tokens) is a compact and self-contained way to represent a user's identity. It contains metadata and claims that can be verified and decoded. Generally speaking, JWT contains three parts: header, payload and signature.

Golang provides many useful libraries to help us implement JWT, such as: jwt-go. Generate, decode and verify JWT easily using the jwt-go library. Here is a sample code that uses the jwt-go library to generate a JWT:

import (
    "time"
    "github.com/dgrijalva/jwt-go"
)
  
func CreateToken(userId string) (string, error) {
    token := jwt.NewWithClaims(jwt.SigningMethodHS256, jwt.MapClaims{
        "user_id": userId,
        "exp":     time.Now().Add(time.Hour * 24).Unix(),
    })
  
    return token.SignedString([]byte("your-secret"))
}

In the above code, we create a JWT that contains the current user's ID and expiration time. Then sign the JWT using the jwt.SigningMethodHS256 method and the "your-secret" key. Finally, the JWT string is returned to the caller.

For each HTTP request, we can use JWT for authentication. Once the user logs in successfully, we can return the generated JWT to the client. The client can send the JWT to the server as part of the "Authorization" header on every subsequent request. The server decodes the JWT and verifies the signature to ensure it is legitimate. Below is a sample code that uses the jwt-go library to validate JWT:

import (
    "net/http"
    "github.com/dgrijalva/jwt-go"
)
  
func ValidateTokenMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        tokenHeader := r.Header.Get("Authorization")
        if tokenHeader == "" {
            w.WriteHeader(http.StatusUnauthorized)
            return
        }
  
        token, err := jwt.Parse(tokenHeader, func(token *jwt.Token) (interface{}, error) {
            return []byte("your-secret"), nil
        })
  
        if err != nil || !token.Valid {
            w.WriteHeader(http.StatusUnauthorized)
            return
        }
  
        next.ServeHTTP(w, r)
    })
}

In the above code, we define a middleware called ValidateTokenMiddleware, which is responsible for validating JWT. ValidateTokenMiddleware will check whether the HTTP header contains an Authorization token and validate it. If the JWT is invalid, it returns an HTTP 401 Unauthorized response. Otherwise, it will continue processing the request.

Using JWT for authentication ensures that the request comes from a legitimate user and provides higher security.

2. Using Roles for Authorization

In modern web applications, users often must have different roles and permissions. For example, an administrator may have permissions to perform higher-level operations that ordinary users do not. Therefore, we need to authorize different user roles separately.

In Golang, a popular approach is to use the role-based access control (RBAC) model. In the RBAC model, each role is given different permissions, and users are assigned to one or more roles. Then, before performing a specific action, the system checks whether the user has permission to perform that action.

The following is a sample code using the role-based access control model:

type Role string
  
const (
    AdminRole  Role = "admin"
    UserRole   Role = "user"
)
  
type User struct {
    ID    string `json:"id"`
    Name  string `json:"name"`
    Email string `json:"email"`
    Role  Role   `json:"role"`
}
  
// Check if the user has permission to access the resource based on the specified role.
func HasPermission(user *User, requiredRole Role) bool {
    return user.Role == requiredRole || user.Role == AdminRole
}

In the above code, we define an enumeration type named Role, which contains the List of roles used in the application. In the User structure, we assign each user to one or more roles and use the HasPermission function to check whether the user has permission to perform a specific operation. In this example, the HasPermission function will return true only if the user's role is AdminRole or requiredRole.

We can also use other methods, such as using middleware to check user roles and permissions. The following is a sample code based on middleware:

func AdminOnlyMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        user, ok := r.Context().Value("user").(*User)
        if !ok || !HasPermission(user, AdminRole) {
            w.WriteHeader(http.StatusForbidden)
            return
        }
  
        next.ServeHTTP(w, r)
    })
}

In the above code, we define a middleware named AdminOnlyMiddleware. This middleware will get the user role from the context of the HTTP request and check if the user has the necessary permissions to perform the action. If the user does not have permission, it will return an HTTP 403 Forbidden response.

If we are using an application based on a web framework, we can register the middleware directly on the route. For example, we can register AdminOnlyMiddleware for API endpoints that require administrator privileges:

router.Handle("/api/dashboard", AdminOnlyMiddleware(http.HandlerFunc(handler)))

In the above code, only users with administrator privileges can access the "/api/dashboard" endpoint.

Summary

In this article, we introduced how to implement authentication and authorization using Golang. We use JWT to verify that the user has permission to access the system and a role-based access control model to check if the user has permission to perform a specific action. These technologies can help us create web applications that are secure, scalable, and easy to maintain.

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