Fix handling of replies to global requests (#112) The current code assumes that all global requests want / need a reply. This isn't always true and the request itself indicates if it wants a reply or not. It causes a specific problem with [email protected] messages. These are sent by OpenSSH after authentication to inform the client of potential other host keys for the host. This can be used to add a new type of host key or to rotate host keys. The initial information message from the server is sent as a global request, but with want_reply set to false. This means that the server doesn't expect an answer to this message. Instead the client needs to send a prove request as a reply if it wants to receive proof of ownership for the host keys. The bug doesn't cause any current problems with due to how OpenSSH treats receiving the failure message. It instead treats it as a keepalive message and further ignores it. Arguably this is a protocol violation though of Dropbear and it is only accidental that it doesn't cause a problem with OpenSSH. The bug was found when adding host keys support to libssh, which is more strict protocol wise and treats the unexpected failure message an error, also see https://gitlab.com/libssh/libssh-mirror/-/merge_requests/145 for more information. The fix here is to honor the want_reply flag in the global request and to only send a reply if the other side expects a reply.

#include "tommath_private.h"
#ifdef BN_MP_MONTGOMERY_CALC_NORMALIZATION_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */

/*
 * shifts with subtractions when the result is greater than b.
 *
 * The method is slightly modified to shift B unconditionally upto just under
 * the leading bit of b.  This saves alot of multiple precision shifting.
 */
mp_err mp_montgomery_calc_normalization(mp_int *a, const mp_int *b)
{
   int    x, bits;
   mp_err err;

   /* how many bits of last digit does b use */
   bits = mp_count_bits(b) % MP_DIGIT_BIT;

   if (b->used > 1) {
      if ((err = mp_2expt(a, ((b->used - 1) * MP_DIGIT_BIT) + bits - 1)) != MP_OKAY) {
         return err;
      }
   } else {
      mp_set(a, 1uL);
      bits = 1;
   }


   /* now compute C = A * B mod b */
   for (x = bits - 1; x < (int)MP_DIGIT_BIT; x++) {
      if ((err = mp_mul_2(a, a)) != MP_OKAY) {
         return err;
      }
      if (mp_cmp_mag(a, b) != MP_LT) {
         if ((err = s_mp_sub(a, b, a)) != MP_OKAY) {
            return err;
         }
      }
   }

   return MP_OKAY;
}
#endif