Description

proxy object controls the creation of and access to subject object
proxy and subject objects share the same interface

Proxy vs decorator pattern

Decorator informs and empowers its client, it adds known functionalities to an object
Proxy restricts and disempowers its client, it controls access to an object with unknown functionalities like authentication

Exemple

public class User
{
    public string Name { get; set; }
}

IUserClient.cs

public interface IUserClient
{
    IReadOnlyList<User> GetUsers();
}

UserClient.cs

public class UserClient : IUserClient
{
    public IReadOnlyList<User> GetUsers()
    {
        return new[]
        {
            new User { Name = "Nicolas" },
            new User { Name = "Paul" }
        };
    }
}

ProxyUserClient

public class ProxyUserClient : IUserClient
{
    private UserClient userClient;

    public bool Authenticate(string userName, string password)
    {
        if (userName == "Nicolas" && password == "0000")
        {
            userClient = new UserClient();
            return true;
        }

        return false;
    }

    public IReadOnlyList<User> GetUsers()
    {
        if (userClient == null)
        {
            return null;
        }
        else
        {
            return userClient.GetUsers();
        }
    }
}

IUserClient userClient = new UserClient();
// call GetUsers from UserClient: no restrictions
var users = userClient.GetUsers();
Console.WriteLine(string.Join('\n', users.Select(x => $"{x.Name}")));

userClient = new ProxyUserClient();
// call GetUsers from ProxyUserClient: authentication restriction
users = userClient.GetUsers();
if (users == null)
    Console.WriteLine("You to authenticate first.");
else
    Console.WriteLine(string.Join('\n', users.Select(x => $"{x.Name}")));

// authentication
((ProxyUserClient)userClient).Authenticate("Nicolas", "0000");
users = userClient.GetUsers();
if (users == null)
    Console.WriteLine("You to authenticate first.");
else
    Console.WriteLine(string.Join('\n', users.Select(x => $"{x.Name}")));