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<p>Here is an example implementation of authentication logic in a microservice structure using Go:</p>
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<ol><!-- wp:list-item -->
<li>Create a new Go module:</li>
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<pre class="wp-block-preformatted">shellCopy code<code>$ mkdir authentication-microservice
$ cd authentication-microservice
$ go mod init github.com/yourusername/authentication-microservice
</code></pre>
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<ol start="2"><!-- wp:list-item -->
<li>Install required packages:</li>
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<pre class="wp-block-preformatted">shellCopy code<code>$ go get github.com/gorilla/mux
$ go get github.com/dgrijalva/jwt-go
</code></pre>
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<ol start="3"><!-- wp:list-item -->
<li>Create a main.go file:</li>
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<pre class="wp-block-preformatted">goCopy code<code>package main

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

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

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

var users = []User{
    {Username: "user1", Password: "password1"},
    {Username: "user2", Password: "password2"},
}

var jwtKey = []byte("my_secret_key")

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

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

    router.HandleFunc("/login", login).Methods("POST")
    router.HandleFunc("/protected", authenticate(protected)).Methods("GET")

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

func login(w http.ResponseWriter, r *http.Request) {
    var user User
    err := json.NewDecoder(r.Body).Decode(&amp;user)
    if err != nil {
        http.Error(w, err.Error(), http.StatusBadRequest)
        return
    }

    for _, u := range users {
        if u.Username == user.Username &amp;&amp; u.Password == user.Password {
            claims := &amp;Claims{
                Username: user.Username,
                StandardClaims: jwt.StandardClaims{
                    ExpiresAt: time.Now().Add(time.Hour * 24).Unix(),
                },
            }

            token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)

            tokenString, err := token.SignedString(jwtKey)
            if err != nil {
                http.Error(w, err.Error(), http.StatusInternalServerError)
                return
            }

            w.Write([]byte(tokenString))
            return
        }
    }

    http.Error(w, "Invalid credentials", http.StatusUnauthorized)
}

func authenticate(next http.HandlerFunc) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        authHeader := r.Header.Get("Authorization")
        tokenString := authHeader[len("Bearer "):]

        claims := &amp;Claims{}

        token, err := jwt.ParseWithClaims(tokenString, claims, func(token *jwt.Token) (interface{}, error) {
            if token.Method.Alg() != jwt.SigningMethodHS256.Alg() {
                return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
            }

            return jwtKey, nil
        })

        if err != nil {
            http.Error(w, err.Error(), http.StatusUnauthorized)
            return
        }

        if !token.Valid {
            http.Error(w, "Invalid token", http.StatusUnauthorized)
            return
        }

        next.ServeHTTP(w, r)
    }
}

func protected(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("You are authorized to access this resource"))
}
</code></pre>
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<ol start="4"><!-- wp:list-item -->
<li>Run the authentication microservice:</li>
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<pre class="wp-block-preformatted">goCopy code<code>$ go run main.go
</code></pre>
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<ol start="5"><!-- wp:list-item -->
<li>Test the authentication microservice:</li>
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<!-- wp:preformatted -->
<pre class="wp-block-preformatted">jsonCopy code<code>$ curl -X POST -H "Content-Type: application/json" -d '{"username": "user1", "password": "password1"}' http://localhost:8000/login
eyJhbGciOiJIUzI</code></pre>
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