Thread: Help please

Here's what I'm supposed to do:
Write a program (in C), which enciphers or deciphers an input file.
Assume that the input file is any type of file, at all. So you must read it in
using binary mode. Also, the output file will be written using binary mode.
The program will encipher the input file by doing the following:
1. Initialize N to the key value from the command line.
2. While not EOF:
a. Read in a byte, B, of the file.
b. Set B to (B+N) mod 256.
c. Write out B.
d. Increment N mod 256.
Notice that this is the familiar “filter” type of program. A file is deciphered by following exactly the
same procedure, but the key is subtracted from the byte in step c.
Details: The key will be a value from 0 to 255. If it is not, write out an error message and exit. Use
command line parameters for the seed and for the file names.
C:\> cipher E 221 myImage.jpg myCipheredImage.bin
C:\> cipher D -221 myCipheredImage.bin originalImage.jpg
Compute (B+N) mod 256 by doing ordinary addition (B+N) and then doing modular division % 256.
So numbers will always wrap around into the range 0..255. Compute (B-N) mod 256 by computing
(B+256-N) % 256. Test thoroughly that this part of your program is working.
Expect that input files can be anything at all, including ordinary text files, MS word files, image files, or
executables. Of course, the result after deciphering should be identical to the original file. Use
strcmp() to compare the first parameter argv[1] with “E” or “D”. Use atoi(argv[2]) for
converting the second parameter into an int. You will need to use fopen(), fgetc(),
fputc(),and fclose().
Although this cipher will leave your files unreadable by most casual interlopers, it is still what is called
a “simple cipher.” It is nowhere near commercial quality and would easily be broken by any
cryptanalyst.
Debugging: Create some test files and use them to test the operation of your program. The most
obvious test file is a file of all zero-bytes. The output of your program with this file should be an
ascending sequence of bytes, mode 256. The test file can be created by a program you write. Inspect
the output of your program using Irfanview or some other hex dump program. Also test your program
on some image files and some other files.

Here's what I have:

Code:

#include <stdlib.h>
#include <stdio.h>
/* File copy -- copy a source to a destination.
If the destination file exists it will be copied over.
*/
int main ( int argc, char** argv )
{  int N;
   printf("enter the key value(0 to 255)");
   scanf("%d", &N);
   if(N>255 || N<0)
   exit(EXIT_FAILURE);
FILE *in, *out;
int ch;
if ( argc != 'E' || argc != 'e' || argc != 'D' || argc != 'E' )
{
fprintf( stderr, "copy source dest\n");
return( EXIT_FAILURE );
}
/* Open the source file for "read binary" so that */
/* end of line characters are not translated */
if ( (in = fopen( argv[1], "rb")) == NULL )
{
perror( argv[1] );
return( EXIT_FAILURE );
}
/* Open the destination file for "write binary" */
if ( (out = fopen( argv[2], "wb")) == NULL )
{
perror( argv[2] );
return( EXIT_FAILURE );
}
/* Copy characters from input to output until end of file */
while ( (ch = fgetc(in)) != EOF )
{
 if(argc == 'e' || argc == 'E')
 {
 ch = (ch + N) % 256;
 fputc( ch, out );
 N++ % 256;
 }
 if(argc == 'D' || argc == 'd')
 {
 ch = (ch - N) % 256;
 fputc( ch, out );
 N++ % 256;
 }
}
/* Close the input and the output files */
if ( fclose( in ) == EOF )
{
perror( argv[1] );
return( EXIT_FAILURE );
}
if ( fclose( out ) == EOF )
{
perror( argv[2] );
return( EXIT_FAILURE );
}
return( EXIT_SUCCESS );
}

what am I doing wrong?

Beats me. Whats the problem? Compiler error? Logic/runtime error? Describe your problem so we know where to start (opposed to reading that entire block of your assignment, reading and understanding your code, finding the problem, fixing the problem).

Sorry, I was about to leave when i posted it, right now the problem is that it just prints "copy source dest"

Your line :

Code:

if ( argc != 'E' || argc != 'e' || argc != 'D' || argc != 'E' )

is checking 'E' twice. Should it be checking some other value??

Originally Posted by kcpilot

Your line :

Code:

if ( argc != 'E' || argc != 'e' || argc != 'D' || argc != 'E' )

is checking 'E' twice. Should it be checking some other value??

First of all argc can never equal 'e' 'E' 'd' or 'D'. Now do you see the reason why the program is only printing "copy source dest"

Originally Posted by itCbitC

First of all argc can never equal 'e' 'E' 'd' or 'D'. Now do you see the reason why the program is only printing "copy source dest"

Sure it could -- Though, why on earth someone would want 101 command line arguments is beyond me.

In truth, all are strings. You'll have to convert the input to whatever you want. Usually, this is done via switches (if you have multiple data types).

wow, i just realized a huge flaw in my logic, the if statement should look something like this
if ( argc != 5 || (*argv[1] != 'e' && *argv[1] != 'd') )

Code:

if ( argc != 5 || (*argv[1] != 'e' && *argv[1] != 'd') )

If you run the program with no arguments, youll get a segfault. As previously mentioned:

Originally Posted by MisterIO

You obviously need to check if argc is > 1 though.

You need to make sure that (at least) one argument is given, i.e. argc > 1 (argv[0] is the current filename, argv[1] is the first argument, etc).

Originally Posted by nadroj

Code:

if ( argc != 5 || (*argv[1] != 'e' && *argv[1] != 'd') )

If you run the program with no arguments, youll get a segfault. As previously mentioned:You need to make sure that (at least) one argument is given, i.e. argc > 1 (argv[0] is the current filename, argv[1] is the first argument, etc).

You didn't actually read my post.

I didnt? I was citing you as part of the solution to the op's (latest) problem). I quoted what you said, and re-stated it (equivalently).

Originally Posted by nadroj

I didnt? I was citing you as part of the solution to the op's (latest) problem). I quoted what you said, and re-stated it (equivalently).

Yes, I misunderstood, sorry.

Originally Posted by Kennedy

Sure it could -- Though, why on earth someone would want 101 command line arguments is beyond me.

Yep! that would make "complete sense" when writing obfuscated C code.