allbery@uunet.UU.NET ( comp.sources.misc) (10/06/90)
Posting-number: Volume 15, Issue 21
Submitted-by: elevia!alain (W.A.Simon)
Archive-name: codon/part01
Several persons have requested on the net, or directly through
E-mail, simple encryption alogorithms. Here are a few simple
routines, which for all practical purposes are very secure.
They will not resist trained crypto-analysts and/or computer
powered attacks, but casual snoopers will be baffled. Just
use good keys. Here goes :
#! /bin/sh
# This file was wrapped with "dummyshar". "sh" this file to extract.
# Contents: codon.c
echo extracting 'codon.c'
if test -f 'codon.c' -a -z "$1"; then echo Not overwriting 'codon.c'; else
sed 's/^X//' << \EOF > 'codon.c'
X/*
X
X codon - filter to encode text string;
X each byte is XOR'd with a byte from a key file.
X That's one kind of One Time Pad implementation.
X codon is linked to codoff.
X
X W. A. Simon
X*/
X
X#include <stdio.h>
X#include <ctype.h>
X#include <sys/types.h>
X#include <sys/stat.h>
X
Xmain(argc, argv)
Xint argc; char **argv[];
X
X {
X FILE *in;
X register int c;
X c = 0;
X if (argc < 2)
X exit(1);
X /*
X a file name must be specified on command line,
X that's the file containing the key stream.
X for security reasons: no error message.
X */
X
X if ((in = fopen(argv[1], "r")) == NULL)
X exit(1);
X /*
X for security reasons: no error message.
X */
X
X while (( c = getchar() ) != EOF) {
X putchar( c ^ getc(in) ); /* ouput coded c */
X }
X
X exit(0);
X }
X
X
X/*
X decode - filter to decode text encoded with encode;
X see encode for details.
X
X W. A. Simon
X
X*/
X
X#include <stdio.h>
Xmain()
X {
X int c, i;
X c = 0;
X i = 153; /* seed is yours to decide - can be anything */
X while (scanf("%c", &c) != EOF) {
X c = c ^ i;
X putchar(c);
X i = c;
X }
X exit(0);
X }
X
X
X/*
X encode - filter to encode text string.
X each byte is XOR'd with previous byte.
X seed is arbitrary.
X
X W. A. Simon
X
X*/
X
X#include <stdio.h>
Xmain()
X {
X register int c, i;
X i = 153;
X c = 0;
X while ((c = getchar()) != EOF) {
X printf("%c",c ^ i);
X i = c;
X }
X exit(0);
X }
X
X ... and let's cap it with a simple way to generate healthy
X random keys :
X
X/*
X
X confuse
X
X For the purpose of cryptological use
X a good random generator
X must be unpredictable.
X A uniform distribution is not
X necessarily wanted but this
X distribution must also be unpredictable
X which in the long run is equivalent to
X being uniform.
X
X UNPREDICTABILITY is due to input file
X which can be anything at all;
X use of a binary file is advised
X for optimum results.
X
X Tendancy towards UNBIASED distribution
X is due to mixed congruential treatment of matrix
X (see Knuth's holy book).
X Any peak in the distribution would be
X smoothed out by this treatment.
X
X W. A. Simon - 1986
X
X */
X#include <stdio.h>
X#include <ctype.h>
X
Xint matrix[256];
X
Xint a, c, s;
X
Xmain()
X
X{
X
X/* initialize variables */
X
X a = 0;
X c = 0;
X s = 1;
X
X/* initialize matrix */
Xwhile ( a < 256 ) {
X c = ( ( c * 21 ) + 13 ) % 256 ;
X matrix[a] = c ;
X a++;
X }
X
X/*
X UNBIASED:
X the matrix contains one occurence only
X of each of the 256 possible 8 bit characters.
X
X UNPREDICTABLE:
X read standard input,
X index into rotated matrix,
X output entry to stdout.
X Akin to a Vigeneres table.
X
X Matrix is rotated at every turn in order to prevent
X statistically biased input from affecting the results.
X In fact, repeating the same character over and over,
X into the stdin, would result in a constant repetition
X of the matrix on stdout. A non rotated matrix would
X result in an output that has the same statistical feel
X as the input (like a substitution cipher).
X
X All of this stuff has the same statistical impact as doing
X an XOR between a data stream and a recurring string of 256
X non repeating bytes. I'll let you do the formal proof.
X
X */
X while ( ( a = getchar() ) != EOF ) {
X putchar( matrix[ ( a + s ) % 256 ] );
X s = ( s + 1 ) % 256 ;
X }
X exit(0);
X}
EOF
chars=`wc -c < 'codon.c'`
if test $chars != 3244; then echo 'codon.c' is $chars characters, should be 3244 characters!; fi
fi
exit 0
enjoy,
--
Alain
Home+Office: (514) 934 6320 UUCP: alain@elevia.UUCP