Data Encryption Techniques

[cbenson at reed.UUCP]

     I wrote the following program to fill my need for  some
form of security for my source code on our Altair. What I am
interested in is an evaluation by some  "knowledgeable  Joe"
as  to  the  relative security of my algorithm.  Please send
your response either by mail or this news  group.   (Perhaps
the news group would be the best way since we can never fig-
ure a path that works twice.)

     I probably misspelled the name of the  French  guy  who
gave  me  the  idea,  but  I doubt that this will affect the
operation of my program.

Any, and all, advice will be greatly appreciated.

/*
Jason E. Elliott	 9 March 1984
			13 March 1984
			14 March 1984

Program uses my own modification of the Vigenere algorithm for
enciphering and deciphering files.

If one file is specified on the command line then the file is
read, a temporary file is created, and the result of the cipher-
ment is written to the temporary file.  The temporary file is
the read and the contents are written back onto the original
file.  The temporary file is then unlinked.

If two files are specified on the command line then the first
file is read and ciphered, then the ciphered data is written
on the second file.

Warning:  The contents of the output file are lost upon 
	  invocation of this program.

Usage:  cipher f1 [f2]
*/

#include <stdio.h>
#define BUFLEN 16384

main (argc, argv)
int argc;
char *argv[];
{
register int l, i, f1, f2, sr;
char key[255], buf[BUFLEN], c;
if (argc == 1 || argc >3)
	{
	printf ("Usage:  cipher f1 [f2]\n");
	exit(1);
	}
f1 = openfl (argv[1]);
if (argc == 2)
	argv[2] = "temp.out.cpr";
if (!(f2 = creat (argv[2], 0600)))
	die (argv[2]);
system ("stty cbreak");
system ("stty -echo");
printf ("Enter Key:  ");
for (l=0;(key[l]=getchar ()) != '\n';l++)
	;
printf ("\nConfirm Key:  ");
for (i=0;i<l;i++) 
	{
	c = getchar ();
	if (c != key[i])
		{
		printf ("\nKey confirmation failed, exiting cipher...\n");
		leave (1);
		}
	}
printf ("\nWorking---\n");
while (sr = read (f1, buf, BUFLEN))
	{
	for (i=0;i<sr;i++)
		{
		if (!(i%l))
			rotate (key, l);
		buf[i] = (l*(key[i%l])-buf[i])%128;
		if (!(i%512))
			putchar ('.');
		}
	putchar ('*');
	if (!(write (f2, buf, sr)))
		wrerr (argv[2]);
	}
if (argc == 2)
	{
	close (f1);
	close (f2);
	f1 = openfl (argv[2]);
	if (!(f2 = creat (argv[1], 0600)))
		die (argv[1]);
	while (sr = read (f1, buf, BUFLEN))
		{
		putchar ('*');
		if (!(write (f2, buf, sr)))
			wrerr (argv[2]);
		}
	unlink ("temp.out.cpr");
	}
putchar ('\n');
leave (0);
}

rotate (string, len)
int len;
char string[];
{
int i;
char s1[255];
for (i=0;string[i] && i<len;i++)
	s1[i] = string[(i+1)%(len)];
stcopy (s1, string);
}

stcopy (s1, s2)
char *s1, *s2;
{
while (*s2++ = *s1++)
	;
}

int openfl (file)
char *file;
{
int fd;
if (!(fd = open (file, 0)))
	die (file);
return (fd);
}

wrerr (file)
char *file;
{
printf ("cipher:  write error on %s\n", file);
leave (1);
}

die (file)
char *file;
{
printf ("cipher:  cannot open %s\n", file);
leave (1);
}

leave (n)
int n;
{
system ("stty -cbreak");
system ("stty echo");
exit (n);
}
As is often said:  thanks in advance.

               Tootil-pip!

               Jason E. Elliott