Really simple encryption with C# and SymmetricAlgorithm

If you don’t want to handle keys yourself then let the operating system do it for your. E.g. use Windows Data Protection (DPAPI).

You can write your own, string-based, version of System.Security.Cryptography.ProtectedData.Protect and Unprotect methods by using something like:

public static string Crypt (this string text)
{
    return Convert.ToBase64String (
        ProtectedData.Protect (
            Encoding.Unicode.GetBytes (text) ) );
}

public static string Decrypt (this string text)
{
    return Encoding.Unicode.GetString (
        ProtectedData.Unprotect (
             Convert.FromBase64String (text) ) );
}