[Solved] How to decrypt the c program?

It sounds like you are using Vi as your editor (:x is a bit of a give-away). From this page:

Crypt

Open the file using vi, type :XEnter, enter the key (this key will be the password to see the crypted file) and then save and exit with :wqEnter.

The file will be crypted.

You can use:

vi -x yourfile

to do the same thing.

Decrypt

To decrypt just the way you encrypted the file.

Open the file using vi, type :XEnter, enter the key and after that the file will be visible and writable. If you close and save your file by :wqEnter the file will be still crypted.