1 files changed, 76 insertions, 0 deletions
diff --git a/third_party/heimdal/lib/hcrypto/libtommath/bn_mp_exptmod.c b/third_party/heimdal/lib/hcrypto/libtommath/bn_mp_exptmod.c
new file mode 100644
index 0000000..5f811eb
--- /dev/null
+++ b/third_party/heimdal/lib/hcrypto/libtommath/bn_mp_exptmod.c
@@ -0,0 +1,76 @@
+#include "tommath_private.h"
+#ifdef BN_MP_EXPTMOD_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* this is a shell function that calls either the normal or Montgomery
+ * exptmod functions.  Originally the call to the montgomery code was
+ * embedded in the normal function but that wasted alot of stack space
+ * for nothing (since 99% of the time the Montgomery code would be called)
+ */
+mp_err mp_exptmod(const mp_int *G, const mp_int *X, const mp_int *P, mp_int *Y)
+{
+   int dr;
+
+   /* modulus P must be positive */
+   if (P->sign == MP_NEG) {
+      return MP_VAL;
+   }
+
+   /* if exponent X is negative we have to recurse */
+   if (X->sign == MP_NEG) {
+      mp_int tmpG, tmpX;
+      mp_err err;
+
+      if (!MP_HAS(MP_INVMOD)) {
+         return MP_VAL;
+      }
+
+      if ((err = mp_init_multi(&tmpG, &tmpX, NULL)) != MP_OKAY) {
+         return err;
+      }
+
+      /* first compute 1/G mod P */
+      if ((err = mp_invmod(G, P, &tmpG)) != MP_OKAY) {
+         goto LBL_ERR;
+      }
+
+      /* now get |X| */
+      if ((err = mp_abs(X, &tmpX)) != MP_OKAY) {
+         goto LBL_ERR;
+      }
+
+      /* and now compute (1/G)**|X| instead of G**X [X < 0] */
+      err = mp_exptmod(&tmpG, &tmpX, P, Y);
+LBL_ERR:
+      mp_clear_multi(&tmpG, &tmpX, NULL);
+      return err;
+   }
+
+   /* modified diminished radix reduction */
+   if (MP_HAS(MP_REDUCE_IS_2K_L) && MP_HAS(MP_REDUCE_2K_L) && MP_HAS(S_MP_EXPTMOD) &&
+       (mp_reduce_is_2k_l(P) == MP_YES)) {
+      return s_mp_exptmod(G, X, P, Y, 1);
+   }
+
+   /* is it a DR modulus? default to no */
+   dr = (MP_HAS(MP_DR_IS_MODULUS) && (mp_dr_is_modulus(P) == MP_YES)) ? 1 : 0;
+
+   /* if not, is it a unrestricted DR modulus? */
+   if (MP_HAS(MP_REDUCE_IS_2K) && (dr == 0)) {
+      dr = (mp_reduce_is_2k(P) == MP_YES) ? 2 : 0;
+   }
+
+   /* if the modulus is odd or dr != 0 use the montgomery method */
+   if (MP_HAS(S_MP_EXPTMOD_FAST) && (MP_IS_ODD(P) || (dr != 0))) {
+      return s_mp_exptmod_fast(G, X, P, Y, dr);
+   } else if (MP_HAS(S_MP_EXPTMOD)) {
+      /* otherwise use the generic Barrett reduction technique */
+      return s_mp_exptmod(G, X, P, Y, 0);
+   } else {
+      /* no exptmod for evens */
+      return MP_VAL;
+   }
+}
+
+#endif