From: owner-cypherpunks@toad.com
Date: Fri, 23 Feb 1996 02:27:00 +0800
Subject: No Subject
Message-ID: <QQadzw27787.199602221110@relay3.UU.NET>
MIME-Version: 1.0
Content-Type: text/plain


> > Is there something like a checksum attached to Credit Card
> > Numbers. Or better: Is there a way to determine for a given
> > number N if
> >   -this _might_ be a valid number
> >   -this can't be a valid number

This was posted by Wayne D. Hoxsie Jr. <hoxsiew@crl.com>:

#include <stdio.h>

v(char *s)
{
  int i=0,j=0,k;

  k=!(!s[16]);
  for(;*s;*s++)
    i+=(++j%2-k)?(*s-'0')*2>9?((*s-'0')*2)-9:(*s-'0')*2:*s-'0';
  return (i%10);
}
main()
{
  char s[160];

  printf("Enter credit card number\n");
  scanf("%s",s);
  printf("Credit card number is %svalid\n",v(s)?"in":"");
}

---

Dr. Dimitri Vulis
Brighton Beach Boardwalk BBS, Forest Hills, N.Y.: +1-718-261-2013, 14.4Kbps