This is a simple program to encrypt plain text file.

How to use it:

run 'gcc -o encrypt encrypt.c'
If the FILE hasn't been encrypt, run './encrypt FILE', then 
input a passwd . the FILE's content will be encrypted(changed)
To decrypt, run './encrypt FILE' again and use the same 
passwd which is used to encrypt the FILE, this will decrypt 
the file.

Warning!!!

Before you run encrypt on some file, make clear whether the 
file is plaintext or ciphertext.
When you decrypt the file, MAKE SURE the passwd you input is
the same with what you input when you last encryt the file.