contrib/ntp/libntp/authparity.c

   1 /*
   2  * auth_parity - set parity on a key/check for odd parity
   3  */
   4 #include "ntp_stdlib.h"
   5
   6 int
   7 DESauth_parity(
   8         u_int32 *key
   9         )
  10 {
  11         u_int32 mask;
  12         int parity_err;
  13         int bitcount;
  14         int half;
  15         int byte;
  16         int i;
  17
  18         /*
  19          * Go through counting bits in each byte.  Check to see if
  20          * each parity bit was set correctly.  If not, note the error
  21          * and set it right.
  22          */
  23         parity_err = 0;
  24         for (half = 0; half < 2; half++) {              /* two halves of key */
  25                 mask = 0x80000000;
  26                 for (byte = 0; byte < 4; byte++) {      /* 4 bytes per half */
  27                         bitcount = 0;
  28                         for (i = 0; i < 7; i++) {       /* 7 data bits / byte */
  29                                 if (key[half] & mask)
  30                                     bitcount++;
  31                                 mask >>= 1;
  32                         }
  33
  34                         /*
  35                          * If bitcount is even, parity must be set.  If
  36                          * bitcount is odd, parity must be clear.
  37                          */
  38                         if ((bitcount & 0x1) == 0) {
  39                                 if (!(key[half] & mask)) {
  40                                         parity_err++;
  41                                         key[half] |= mask;
  42                                 }
  43                         } else {
  44                                 if (key[half] & mask) {
  45                                         parity_err++;
  46                                         key[half] &= ~mask;
  47                                 }
  48                         }
  49                         mask >>= 1;
  50                 }
  51         }
  52
  53         /*
  54          * Return the result of the parity check.
  55          */
  56         return (parity_err == 0);
  57 }