A functional helper class for Basic authentication. For tests, check out: https://gist.github.com/mausworks/37ba3aa9d3aeb3625b2eda196a0f1796

BasicAuthentication.cs

public class BasicAuthentication
{
    public const string Scheme = "Basic";

    public const char DelimiterSymbol = ':';

    public string Username { get; }

    public string Password { get; }

    private string _base64;

    public BasicAuthentication(string username, string password)
    {
        AssertNotNull(username, password);

        Username = username;
        Password = password;
    }


    public string GetBase64(Encoding encoding)
    {
        if (_base64 == null)
        {
            var bytes = encoding.GetBytes(Combine(Username, Password));
            _base64 = Convert.ToBase64String(bytes);
        }

        return _base64;
    }

    public static BasicAuthentication FromBase64(string base64, Encoding encoding)
    {
        var auth = encoding.GetString(Base64Bytes(base64))
            .Split(DelimiterSymbol);

        return new BasicAuthentication(auth[0], auth[1])
        {
            _base64 = base64
        };
    }

    public static string Combine(string username, string password)
        => $"{username}{DelimiterSymbol}{password}";

    private void AssertNotNull(string username, string password)
    {
        if (username == null)
        {
            throw new ArgumentNullException(nameof(username));
        }
        if (password == null)
        {
            throw new ArgumentNullException(nameof(password));
        }
    }

    private static byte[] Base64Bytes(string base64)
        => Convert.FromBase64String(base64);
}

OAuth2 alternative ?