Extra Credit #$5($extra$5.$c$)---$ XOR Encryption $---$ Due: on last day of class by $11$:$59$PM

Name your program: extra$5.$c

The main point of this extra credit assignment, is to get more practice with File input$/$

output processes$/$functions and to create your own little encryption program.

For this extra credit assignment, you need to create a program that reads in a text$-$file

$($specified by the user$),$ performs a $$bit$-$level Exclusive$-$OR$$ on each byte of the text$-$file

$($as a method of encryption$),$ then writes the encrypted file data back out to disk and

gives the encrypted file the same name as the input file $($i$.$e$.$ so it overwrites the original

file$).$

XOR encryption is a symmetric encryption algorithm, which means, the encrypted file

can also be decrypted with the same XOR algorithm $($as long as you use the same

encryption key, which you will, because the encryption key is hard$-$coded $--$ a variation

of this program would be to ask the user to enter the encryption key$).$

So when testing your program $($keep a backup copy of the file to be encrypted$),$ then do

the following:

$(1)$ Create a text file $($make a backup copy$)$ and encrypt it

$(2)$ Use cat or vi to view the text file to see that it$$s encrypted

$(3)$ Run the encryption again on the encrypted file, and then see if it$$s decrypted

Below is the description of the $($User$/$Application$)$ interaction:

You will need to refer to pages $603-604(5$th edition$),$ or $678-680(6$th edition$)$ of the

textbook for information on the bit$-$level Exclusive$-$OR operator $^,$ which you will need

for the encryption process.

You can also search the web for $$XOR encryption$$ to see code examples and more

information.

DESCRIPTION

$1.$ User $---$ types $./$extra$5$

$2.$ Application $---$ Asks user to enter name of file to be encrypted.

$3.$ User $----$ Enters name of file $($this file should be in the same directory as the executable file $$extra$5)$

$4.$ Application $---$ Encrypts each byte of the input file and writes encrypted data back to the same file, over

writing the original data.

$-------$ Some example code for an XOR encryption function $-------$

#include

#include

#include

int encrypt$\_$data$($FILE $*)$;

int main $($void$)$

$\{$

FILE $*$file$\_$ptr;

int return$\_$code;

$....$

$//$get file name from user, then do fopen$(...)$ for reading & writing

$....$

return$\_$code $=$ encrypt$\_$data$($ file$\_$ptr $)$;

$....$

fclose$($file$\_$ptr$)$;

return $0$;

$\}$

int encrypt$\_$data$($FILE $*$disk$\_$file$\_$ptr$)$

$\{$

int i; $//$ Used to index through file$\_$buffer

unsigned long int file$\_$size; $//$ Holds number of bytes in the file

int key$\_$length; $//$ Holds length of encryption key

char $*$file$\_$buffer $=$ NULL;

char key$[]=$ABCDEF$$; $//$default encryption key, you can change to

$//$something else if you

$//$want $($ e$.$g$.$ key$[]=12$Ygh$9$sss$$; $)$

key$\_$length $=$ strlen$($key$)$;

fseek$($disk$\_$file$\_$ptr$,0,$ SEEK$\_$END$)$; $//$ Move file$\_$pointer to end of file.

file$\_$size $=$ ftell$($disk$\_$file$\_$ptr$)$; $//$ Get current file pointer location

$//($which will be the size of the file

$//$ in bytes$)$

rewind$($disk$\_$file$\_$ptr$)$; $//$Move file$\_$pointer back to beginning of file

$//$Next step is to allocate RAM memory to hold all the bytes that are

$//$currently stored on the HardDisk

file$\_$buffer $=$ malloc$($file$\_$size$)$;

$//$Read file bytes into RAM file$\_$buffer $($which is just an array of chars$)$

if$($ fread$($file$\_$buffer, file$\_$size, $1,$ disk$\_$file$\_$ptr$)!=1)$

$\{$

printf$("$Error in reading file

$")$;

return $-1$; $//$returning error code

$\}$

for$($ i$=0$; i