is it possible for a file with a customised extension to be non-readable to other app

[katy_price]

Yes.
But any other app won't be able to read the file content properly without the key for decryption.

ok, so i create a new program that encrypt the file. then when a user logs in my application to open the file my application will then decrypt it?

so il have too programs? one to encrypt and another (my login app) to decrypt

i am going to use the File::Encrypt Method to do this, do you think it will work? or is a good idea?