File CaesarShift Crypto - c

Question

I'm trying to crypt a file using ceasarshift, a new file called .enc is being created but it's empty.
Here's my code :
#include <stdio.h>
#include <string.h>
char* getFileExtension(const char*);
int main(int argc, char *argv[])
{
const int shift = (int)argv[1];
int byte;
const char *fileName = (char*)argv[2];
char *fileExtension = getFileExtension(fileName);
char *newFileName = (char*)fileName;
FILE *f_in;
FILE *f_out;
f_in = fopen(fileName, "r");
if (strcmp(fileExtension, "enc") == 0)
{
// We want to decrypt the file
strcat(newFileName, ".dec");
f_out = fopen(newFileName, "w");
while ((byte = fgetc(f_in) != EOF))
{
fputc(byte - shift, f_out);
}
}
else
{
// We want to encrypt the file
strcat(newFileName, ".enc");
f_out = fopen(newFileName, "w");
while ((byte = fgetc(f_in) != EOF))
{
fputc(byte + shift, f_out);
}
}
fclose(f_in);
fclose(f_out);
return 0;
}
char* getFileExtension(const char *fileName)
{
char *extension;
int foundExtension = 0;
while (*fileName)
{
if (foundExtension == 1)
{
*extension++ = *fileName++;
}
if (*fileName == '.')
{
foundExtension = 1;
}
fileName++;
}
return extension;
}
I've made a txt file named CryptoFile which contains the following text :
This is a crypto test file !
This is the parameters I sent in the console when running the exe :
FileCaesarShift.exe 15 CryptoFile
So the shift is 15, the file to encrypt/decrypt is called "CryptoFile"
although a file called CryptoFile.enc is being created it's simply empty.
Can someone tell me what I did wrong ?
Ok I've found out that I need to pass CryptoFile.txt including the ".txt" but I wish to remove it from the name of the new files that will be created so instead of creating CryptoFile.txt.enc I want only CryptoFile.enc so I made a removeExtension function but my program crashes , here's the new code :
#include <stdio.h>
#include <string.h>
char* getFileExtension(const char*);
void removeFileExtension(char*);
int main(int argc, char *argv[])
{
const int shift = (int)argv[1];
int byte;
const char *fileName = (char*)argv[2];
char *fileExtension = getFileExtension(fileName);
char *newFileName = (char*)fileName;
removeFileExtension(newFileName);
printf("newfilename value is %s", *newFileName);
FILE *f_in;
FILE *f_out;
f_in = fopen(fileName, "r");
if (strcmp(fileExtension, "enc") == 0)
{
// We want to decrypt the file
strcat(newFileName, ".dec");
f_out = fopen(newFileName, "w");
while ((byte = fgetc(f_in)) != EOF)
{
fputc(byte - shift, f_out);
}
}
else
{
// We want to encrypt the file
strcat(newFileName, ".enc");
f_out = fopen(newFileName, "w");
while ((byte = fgetc(f_in)) != EOF)
{
printf("byte is %d\n", byte);
fputc(byte + shift, f_out);
}
}
fclose(f_in);
fclose(f_out);
return 0;
}
char* getFileExtension(const char *fileName)
{
char *extension;
int foundExtension = 0;
while (*fileName)
{
if (foundExtension == 1)
{
*extension++ = *fileName++;
}
if (*fileName == '.')
{
foundExtension = 1;
}
fileName++;
}
return extension;
}
void removeFileExtension(char *fileName)
{
while (*fileName)
{
if (*fileName == '.')
{
*fileName == '\0';
break;
}
fileName++;
}
}
LATEST EDIT :
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void copyFileExtension(char*, char*);
int getFileNameLengthWithoutExtension(char*);
int getFileExtensionLength(char*);
void copyFileNameWithoutExtension(char*, char*);
int main(int argc, char *argv[])
{
int shift = atoi(argv[1]);
int byte;
char *fileName = (char*)argv[2];
char *fileExtension = malloc(getFileExtensionLength(fileName) + 1);
copyFileExtension(fileExtension, fileName);
char *newFileName = malloc(getFileNameLengthWithoutExtension(fileName) + 5);
copyFileNameWithoutExtension(newFileName, fileName);
FILE *f_in;
FILE *f_out;
f_in = fopen(fileName, "r");
if (strcmp(fileExtension, "enc") == 0)
{
// We want to decrypt the file
puts("dec");
strcat(newFileName, ".dec");
f_out = fopen(newFileName, "w");
while ((byte = fgetc(f_in)) != EOF)
{
fputc(byte - shift, f_out);
}
}
else
{
puts("enc");
// We want to encrypt the file
strcat(newFileName, ".enc");
f_out = fopen(newFileName, "w");
while ((byte = fgetc(f_in)) != EOF)
{
fputc(byte + shift, f_out);
}
}
fclose(f_in);
fclose(f_out);
return 0;
}
void copyFileExtension(char *fileExtension, char *fileName)
{
char *token = strtok(fileName, ".");
token = strtok(NULL, ".");
strcpy(fileExtension, token);
}
int getFileNameLengthWithoutExtension(char *fileName)
{
if (*fileName && *fileName != '.')
{
return 1 + getFileNameLengthWithoutExtension(++fileName);
}
return 0;
}
int getFileExtensionLength(char *fileName)
{
int foundExt = 0;
int len = 0;
while(*fileName)
{
if (foundExt == 1)
{
len++;
}
if (*fileName == '.')
{
foundExt = 1;
}
fileName++;
}
printf("ext len is %d\n", len);
return len;
}
void copyFileNameWithoutExtension(char* dest, char *source)
{
char *fileNameWithoutExtension = strtok(source, ".");
strcpy(dest, fileNameWithoutExtension);
}

Answer 1

Here are a few things one can notice at a quick glance:
const int shift = (int)argv[1] does not convert the input argument to the integer value 15. Rather it casts the char* pointer in argv[1] (that is the address) into an int and assigns that (typically quite large) value to shift. To actually convert the input argument into an int consider using atoi.
you are not allocating memory for newFileName, and later writing possibly past the allocated length of the "constant" input argv[2] which newFileName points to (which results in undefined behavior).
Assuming the program doesn't crash before that, the operator precedence rule between = and != makes your while loop arguments equivalent to byte = (fgetc(f_in) != EOF). So before you reach EOF, byte == 1. If you had a shift of 15 you wind up creating a file full of unreadable control characters (ascii character 16). Given the unpredictable shift from the first bullet, who knows what is actually going to be written to file.
Though these are probably not your specific errors, the following could also cause problem:
You are also not checking whether the files were opened successfully before using them
Have you consider what would happen for letters that are near the end of the valid characters?