gistfile1.txt

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;

public static class SecretVault
{
    private static readonly string Dir =
        Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData),
            "YourCompany", "YourService");

    private static readonly string ClientIdPath = Path.Combine(Dir, "clientid.txt");
    private static readonly string SecretPath   = Path.Combine(Dir, "clientsecret.bin");

    public static void Save(string clientId, string clientSecret)
    {
        Directory.CreateDirectory(Dir);

        File.WriteAllText(ClientIdPath, clientId ?? "", Encoding.UTF8);

        var secretBytes = Encoding.UTF8.GetBytes(clientSecret ?? "");
        var protectedBytes = ProtectedData.Protect(secretBytes, null, DataProtectionScope.LocalMachine);
        File.WriteAllBytes(SecretPath, protectedBytes);
    }

    public static (string clientId, string clientSecret) Load()
    {
        if (!File.Exists(ClientIdPath) || !File.Exists(SecretPath))
            throw new InvalidOperationException("Not provisioned. Missing clientid/secret files.");

        var clientId = File.ReadAllText(ClientIdPath, Encoding.UTF8).Trim();

        var protectedBytes = File.ReadAllBytes(SecretPath);
        var secretBytes = ProtectedData.Unprotect(protectedBytes, null, DataProtectionScope.LocalMachine);
        var secret = Encoding.UTF8.GetString(secretBytes);

        return (clientId, secret);
    }
}