+/*-
+ * Computes the multiplicative inverse of a in GF(p), storing the result in r.
+ * If a is zero (or equivalent), you'll get a EC_R_CANNOT_INVERT error.
+ * We have a Mont structure, so SCA hardening is FLT inversion.
+ */
+int ec_GFp_mont_field_inv(const EC_GROUP *group, BIGNUM *r, const BIGNUM *a,
+ BN_CTX *ctx)
+{
+ BIGNUM *e = NULL;
+ BN_CTX *new_ctx = NULL;
+ int ret = 0;
+
+ if (group->field_data1 == NULL)
+ return 0;
+
+ if (ctx == NULL && (ctx = new_ctx = BN_CTX_secure_new()) == NULL)
+ return 0;
+
+ BN_CTX_start(ctx);
+ if ((e = BN_CTX_get(ctx)) == NULL)
+ goto err;
+
+ /* Inverse in constant time with Fermats Little Theorem */
+ if (!BN_set_word(e, 2))
+ goto err;
+ if (!BN_sub(e, group->field, e))
+ goto err;
+ /*-
+ * Exponent e is public.
+ * No need for scatter-gather or BN_FLG_CONSTTIME.
+ */
+ if (!BN_mod_exp_mont(r, a, e, group->field, ctx, group->field_data1))
+ goto err;
+
+ /* throw an error on zero */
+ if (BN_is_zero(r)) {
+ ECerr(EC_F_EC_GFP_MONT_FIELD_INV, EC_R_CANNOT_INVERT);
+ goto err;
+ }
+
+ ret = 1;
+
+ err:
+ BN_CTX_end(ctx);
+ BN_CTX_free(new_ctx);
+ return ret;
+}
+