C :: Create 2 Int For FOR Loop To Do Encryption

what i think i need to create 2 int for the FOR loop to do the encryption but how am i to do it meaning the actual encryption itself what is that i check with?Also i think i need more than just the #include <stdio.h> library due to the file functions that i will need or is C having them built in? Also what is that about the program doing all this from DOS?

Create 2 programs with the names encrypt and decrypt that will encrypt and decrypt files respectively.The encryption and decryption will be done through a command line of DOS and the program should be as follows: encrypt file1 file2 key and decrypt file1 file2 key

Where:
file1 is the source file
file2 is the destination file
key is the word with which we encrypt/decrypt file1

So when the user executes the command encrypt f1 f2 sunnyday the program will create a file with the name f2 which will have the contents of f1 encrypted with the key sunnyday.When the user executes the command decrypt f1 f2 sunnyday the program will create a file with the name f2 which will have the contents of f1 decrypted with the key sunnyday.

If the user gives a wrong key the program will continue to decrypt file1 but since the key is wrong the results will be unexpected.Finally the program must print the corresponding error messages in case of errors for example (There is no file 1).The program is expected to work corectly with not only plain text files but executable (exe) files etc.Also The program must be fully commented.

Note: The way the encryption will work is yours to choose but it is recommended that you encrypt every letter of the file with every letter of the key with the (start to finish and begin) technique meaning that if the key has 8 letters then the first leeter of the file will be encrypted with the first letter of the key, the second letter of the file with the second of the key, ..., the ninth letter of the source file with the first letter of the key, the tenth letter of the source file with the second letter of the key etc.