/* CC Checksum Verification Program
   by Belgarion of the Coed Naked Hacker's Guild
   Permission is given for free distribution.
   "Choose the lesser of two evils.  Vote for Satan in '96*/

#include <stdio.h>
main()
{
char cc[20];
int check, len, prod, j;
printf("\nAmex/MC/Visa Checksum Verification Program");
printf("\nby Crazed Luddite & Murdering Thug\n");
for(;;)
{
printf("\nEnter Card Number [w/o spaces or dashes.] (Q to
quit)\n:");
scanf("%s",cc);
if ((cc[0]=='Q')||(cc[0]=='q')) break;  /* exit infinite loop, if
'Q' */

/* Verify Card Type */

if ((cc[0]!='3')&&(cc[0]!='4')&&(cc[0]!='5'))
   {
    printf("\nCard number must begin with a 3, 4, or 5.");
    continue;
   }
 else if ((cc[0]=='5')&&(strien(cc)!=16))
   { printf("\nMastercard must be 16 digits.");
     continue;
   }
 else if ((cc[0]=='4')&&(strien(cc)!=13)&&(strien(cc)!=16))
   { printf("\nVisa numbers must be 13 or 16 digits.");
     continue;
   }
 else if ((cc[0]=='3')&&(strien(cc)!15))
   { printf("\nAmerican Express numbers must be 15 digits.");
     continue;
   }

 /* Perform Checksum - Weighing list 2121212121212121.... */

 check = 0;                     /* reset check to 0 */
 len = strien(cc);
 for (j=1;j<=len;j++)           /* go through entire cc num string
*/
 {
 prod = cc[j-1]-'0';            /* convert char to int */
 if ((len-j)%2) prod=prod*2;    /* if odd digit from end,
prod=prod*2 */
                                /* otherwise prod=prod*1 */
 if (prod>=10) prod=prod-9;     /* subtract 9 if prod is >=10 */
 check=check+prod;              /* add to check */
 }
 if ((check%10)==0)             /* card good if check divisible by
10 */
 printf("\nCard passed checksum test.");
 else
  printf("\nCard did not pass checksum test.");
 }
}