Stream cipher

/*
    Encrypt a file from stdin to stdout, to run from the command line type:
    encryption key <infile >outfile
    Compile: gcc -ansi -o encryption encryption.c
    KEEP IN MIND THAT THIS CIPHER IS WEAK! Use for educational and
    experimental purposes only!

    Copyright (C) <March 09th, 2017> Henning POLZER,
    send comments and error reports to: h underscore polzer at gmx dot de

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/


#include <stdio.h>                      /* getchar, putchar */
#include <stdlib.h>                     /* atoi, (s)rand */
#include <string.h>                     /* strlen */


int main (int argc, char *argv[])
{
    int c,i,j=0,schl_l,zuf=0;           /* 'c' nicht als char vereinbaren! */
    char *schl=argv[1];

    schl_l=strlen (schl);               /* Startwert bilden aus der */
    for (i=0; i<schl_l;i++)             /* Schluessellaenge, dem Wert jedes */
      zuf+=schl_l+schl[i]+i;            /* einzelnen Buchstabens und lfd. Nr. */
    srand (zuf);                        /* Zufallsgen. damit initialisieren */

    while ((c=getchar()) != EOF)    {   /* von stdin lesen */
      zuf=rand();                       /* Zufallszahl lesen */
      putchar (c^(zuf%256));            /* Ergebnis nach stdout schreiben */
      srand (schl[j]+zuf);              /* Zufallsgen. neu initialisieren */
      if (j<schl_l) j++; else j=0;
    } /* while */

    return 0;
} /* main */