Simple to implement symetric encryption

Question

I have a small app of little value and concern but the user has to identify themselves (imagine a budget book for personal use). To make life easier on the twelve colonies the user can choose to auto-login and for that I want to store the username/password in the %appdata% folder with the built-in settings file in C#.

That folder is visible to any PC admin and that's why I don't want it to store in plaintext (especially since users are prone to re-use passwords).

Is there an easy library where I can encrypt/decrypt a string with a master password that doesn't require me to code hundreds of lines?

Yes I am aware that if someone would be able to decompile the exe and get their hold on the master password that security would be breached. I know it's a bit of security-by-obscurity but I think for my use case it would fall in the category of "good enough".

Answer 1

One solution would be using System.Security.Cryptography.

In this example I use a 32 byte key to encrypt and decrypt.

public static string EncryptString(string text, string key)
{
    var aes = Aes.Create();
    var encryptor = aes.CreateEncryptor(Encoding.UTF8.GetBytes(key), aes.IV);

    using var memoryStream = new MemoryStream();
    using var cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write);
    using (var streamWriter = new StreamWriter(cryptoStream))
    {
        streamWriter.Write(text);
    }

    var iv = aes.IV;
    var encrypted = memoryStream.ToArray();

    return Convert.ToBase64String(iv.Concat(encrypted).ToArray());
}

public static string DecryptString(string cipherText, string key)
{
    var fullCipher = Convert.FromBase64String(cipherText);
    var iv = fullCipher.Take(16).ToArray();
    var cipher = fullCipher.Skip(16).ToArray();

    var aes = Aes.Create();
    var decryptor = aes.CreateDecryptor(Encoding.UTF8.GetBytes(key), iv);

    using var memoryStream = new MemoryStream(cipher);
    using var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read);
    using var streamReader = new StreamReader(cryptoStream);

    return streamReader.ReadToEnd();
}