Raoul Duke Raoul Duke - 4 months ago 15
C Question

How to encrypt a text file using bit-wise XOR?

I am trying to encrypt a message from a text file by using the bit-wise XOR operation on the left and right characters with two specific keys from another file(keys.txt), but I am receiving unreadable code in front of the original text file(nothing changed), which is not right. I am using two text files:

1) Input.txt - containing the message that is to be encrypted

2) Keys.txt - This contains two characters that do the XOR operation to each character in the input.txt (character 1 is key 1 and character 2 is key 2)

The following code in my program:

str[i]=str[i]^str2[2];
str[++i]=str[i]^str2[1];
break;


is the line of code that is suppose to be performing the XOR operation

Note My desired output should look similar to this:


m@#EmI(>9S(@)H#FmN# XGmmmmU,H!Gmr(DmI"VmD,F(S!XmU%DmM"C>U(S>,O)9I(9T?U!D>,M!,E;@#B(Gmu%D4,S(:@$U$O*"OmU%DmR%H#F!D`V$M!4N8.N Dm@#EmK"H#9I(+mmmm)@#B(f


Can someone clarify the issue that I am running into?

The user should enter:

gcc myProgram.c
./a.out e input.txt keys.txt


(The e just stands for encryption)

#include<stdio.h>
#include<stdlib.h>
#include<string.h>


int main(int args, char *argc[]){

int i;
int len=0;
char str[1024];
char str2[2];
FILE *finp;
FILE *keyFile;

if ( strcmp(argc[1], "e") == 0 )
{
if ( (finp = fopen(argc[2],"r")) == NULL )
{
printf("Could Not Open file %s\n", argc[2]);
exit(1);
}

if ( (keyFile = fopen(argc[3],"r")) == NULL )
{
printf("Could Not Open file %s\n", argc[3]);
exit(1);
}


while((fgets(str,1024,finp)!=NULL)&(fgets(str2,2,keyFile)!=NULL))
{
printf("%c\n %c",str2[1],str2[2]);


/* *** START CODE THAT USES INPUT.TXT FILE & KEYS.TXT *** */
len = strlen(str);
for(i=0;i<len;i++)
{
str[i]=str[i]^str2[2];
str[++i]=str[i]^str2[1];
break;
}
}

printf("%s\n", str);
fclose(finp);
return 0;

}
else
{
printf("SORRY!");
}

Answer

C arrays indexes are zero based, then you should use

str2[0],str2[1]

instead of

str2[1],str2[2]

In this fragment

for(i=0;i<len;i++)
{
  str[i]=str[i]^str2[2];
  str[++i]=str[i]^str2[1];
  break;
}

break statement stop loop after first iteration. You should remove it. Then you get

for(i=0;i<len;i++)
{
  str[i]^=str2[1];
  str[++i]^=str2[0];
}

In line

while((fgets(str,1024,finp)!=NULL)&(fgets(str2,2,keyFile)!=NULL))

you need logical AND instead of bitwise

while((fgets(str,1024,finp)!=NULL)&&(fgets(str2,2,keyFile)!=NULL))

And if your input.txt file contain more the 1024 byte to show all results you need to move printf("%s\n", str); into the while loop

while((fgets(str,1024,finp)!=NULL)&&(fgets(str2,2,keyFile)!=NULL))
{
    ...
    printf("%s\n", str);
}