Here's a quick way to convert a string to binary data.
var data:String = "my secret data";
var bytes:ByteArray = new ByteArray();
If you wanted the data to be stored in a format that isn't as easily readable, you could try compressing it. (By "not easily readable", I mean that they'd need to figure out that it's compressed and which algorithm that you used, so it's enough to stop most casual observers)
The ByteArray can be saved to a file (and opened again later) using FileStream.
Later, when you read the data again, you'd call inflate() to decompress it.
When you want to convert the bytes back to a string, after you decompress it, you can do this:
var data:String = bytes.readUTFBytes(bytes.bytesAvailable);