1 files changed, 25 insertions, 7 deletions

diff --git a/modexp.c b/modexp.c
index 8fea2ea..82c79b8 100644
--- a/modexp.c
+++ b/modexp.c
@@ -49,23 +49,41 @@
 #include "hal_internal.h"
 
 /*
- * Whether we want to use the new ModExpNG core.
+ * Enable use of the ModExpNG core, if present.
  */
 
-static int use_modexpng = 0;
+static enum {
+  unknown = -1,
+  modexpa7_core = 0,
+  modexpng_core = 1
+} which_core = unknown;
 
-hal_error_t hal_modexp_use_modexpng(const int onoff)
+static inline hal_error_t init_modexp_core(void)
 {
-  if (onoff && (hal_core_find(MODEXPNG_NAME, NULL) == NULL))
+  if (which_core != unknown)
+    return HAL_OK;
+  else if (hal_core_find(MODEXPNG_NAME, NULL) != NULL)
+    return (which_core = modexpng_core), HAL_OK;
+  else if (hal_core_find(MODEXPA7_NAME, NULL) != NULL)
+    return (which_core = modexpa7_core), HAL_OK;
+  else
     return HAL_ERROR_CORE_NOT_FOUND;
+}
 
-  use_modexpng = onoff;
-  return HAL_OK;
+hal_error_t hal_modexp_use_modexpng(const int onoff)
+{
+  if (onoff && hal_core_find(MODEXPNG_NAME, NULL) != NULL)
+    return (which_core = modexpng_core), HAL_OK;
+  else if (!onoff && hal_core_find(MODEXPA7_NAME, NULL) != NULL)
+    return (which_core = modexpa7_core), HAL_OK;
+  else
+    return HAL_ERROR_CORE_NOT_FOUND;
 }
 
 int hal_modexp_using_modexpng(void)
 {
-  return use_modexpng;
+  return (init_modexp_core() == HAL_OK &&
+          which_core == modexpng_core);
 }
 
 /*