It's a pretty primitive form of "Encryption" - there are a lot betters ones built into .NET. But it's pretty simple...
string inp = "Hello there";
byte[] data = Encoding.ASCII.GetBytes(inp);
for (int i = 0; i < data.Length; i++)
{
data[i] ^= 0xFF;
}
for (int i = 0; i < data.Length; i++)
{
data[i] ^= 0xFF;
}
string outp = System.Text.Encoding.ASCII.GetString(data);