options: Complete the transition to numeric toggles (`#if') For the sake of review, this commit alters only the code; the affiliated comments within the source files also need to be updated, but doing so now would obscure the operational changes that have been made here. * All on/off options have been switched to the numeric `#if' variant; that is the only way to make this `default_options.h.in' thing work in a reasonable manner. * There is now some very minor compile-time checking of the user's choice of options. * NO_FAST_EXPTMOD doesn't seem to be used, so it has been removed. * ENABLE_USER_ALGO_LIST was supposed to be renamed DROPBEAR_USER_ALGO_LIST, and this commit completes that work. * DROPBEAR_FUZZ seems to be a relatively new, as-yet undocumented option, which was added by the following commit: commit 6e0b539e9ca0b5628c6c5a3d118ad6a2e79e8039 Author: Matt Johnston <[email protected]> Date: Tue May 23 22:29:21 2017 +0800 split out checkpubkey_line() separately It has now been added to `sysoptions.h' and defined as `0' by default. * The configuration option `DROPBEAR_PASSWORD_ENV' is no longer listed in `default_options.h.in'; it is no longer meant to be set by the user, and is instead left to be defined in `sysoptions.h' (where it was already being defined) as merely the name of the environment variable in question: DROPBEAR_PASSWORD To enable or disable use of that environment variable, the user must now toggle `DROPBEAR_USE_DROPBEAR_PASSWORD'. * The sFTP support is now toggled by setting `DROPBEAR_SFTPSERVER', and the path of the sFTP server program is set independently through the usual SFTPSERVER_PATH.

/* LibTomCrypt, modular cryptographic library -- Tom St Denis
 *
 * LibTomCrypt is a library that provides various cryptographic
 * algorithms in a highly modular and flexible manner.
 *
 * The library is free for all purposes without any express
 * guarantee it works.
 */

#include "tomcrypt.h"

#ifdef LTC_MDH

/**
  Check DH public key (INTERNAL ONLY, not part of public API)
  @param key    The key you wish to test
  @return CRYPT_OK if successful
*/
int dh_check_pubkey(dh_key *key)
{
   void *p_minus1;
   ltc_mp_digit digit;
   int i, digit_count, bits_set = 0, err;

   LTC_ARGCHK(key != NULL);

   if ((err = mp_init(&p_minus1)) != CRYPT_OK) {
      return err;
   }

   /* avoid: y <= 1 OR y >= p-1 */
   if ((err = mp_sub_d(key->prime, 1, p_minus1)) != CRYPT_OK) {
      goto error;
   }
   if (mp_cmp(key->y, p_minus1) != LTC_MP_LT || mp_cmp_d(key->y, 1) != LTC_MP_GT) {
      err = CRYPT_INVALID_ARG;
      goto error;
   }

   /* public key must have more than one bit set */
   digit_count = mp_get_digit_count(key->y);
   for (i = 0; i < digit_count && bits_set < 2; i++) {
      digit = mp_get_digit(key->y, i);
      while (digit > 0) {
         if (digit & 1) bits_set++;
         digit >>= 1;
      }
   }
   if (bits_set > 1) {
      err = CRYPT_OK;
   }
   else {
      err = CRYPT_INVALID_ARG;
   }

error:
   mp_clear(p_minus1);
   return err;
}

#endif /* LTC_MDH */

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