Import code from previous AssetBuilder version

thirdparty/libtomcrypt/pk/dh/dh_check_pubkey.c

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+/* 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 */
+
+/* ref:         HEAD -> master, tag: v1.18.2 */
+/* git commit:  7e7eb695d581782f04b24dc444cbfde86af59853 */
+/* commit time: 2018-07-01 22:49:01 +0200 */