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### Code Snippet for Implementing User Authentication

```python
# Import necessary libraries
from flask import Flask, request, jsonify
from werkzeug.security import generate_password_hash, check_password_hash

app = Flask(__name__)

# User data (in a real application, use a database)
users = {}

@app.route('/register', methods=['POST'])
def register():
    data = request.get_json()
    username = data['username']
    password = generate_password_hash(data['password'])
    users[username] = password
    return jsonify({'message': 'User registered successfully!'}), 201

@app.route('/login', methods=['POST'])
def login():
    data = request.get_json()
    username = data['username']
    password = data['password']
    if username in users and check_password_hash(users[username], password):
        return jsonify({'message': 'Login successful!'}), 200
    return jsonify({'message': 'Invalid credentials!'}), 401

if __name__ == '__main__':
    app.run(debug=True)
```

This example demonstrates how to create a simple user authentication system using Flask. It includes endpoints for user registration and login, utilizing hashed passwords for security.