Reduce the default ECP window size
MBEDTLS_ECP_WINDOW_SIZE is a compromise between memory usage (growing based
on the value) and performance (faster with larger values). There are
disminishing returns as the value grows larger. Based on Manuel's benchmarks
recorded in https://github.com/ARMmbed/mbedtls/issues/4127, 4 is a good
compromise point, with larger values bringing little advantage. So reduce
the default from 6 to 4.
Document the default value as in optimized for performance mostly, but don't
document the specific value, so we may change it later or make it
platform-dependent.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index b974e59..f278d10 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -258,7 +258,8 @@
#if !defined(MBEDTLS_ECP_WINDOW_SIZE)
/*
* Maximum "window" size used for point multiplication.
- * Default: 6.
+ * Default: a point where higher memory usage yields disminishing performance
+ * returns.
* Minimum value: 2. Maximum value: 7.
*
* Result is an array of at most ( 1 << ( MBEDTLS_ECP_WINDOW_SIZE - 1 ) )
@@ -275,7 +276,7 @@
* 224 475 475 453 398 342
* 192 640 640 633 587 476
*/
-#define MBEDTLS_ECP_WINDOW_SIZE 6 /**< The maximum window size used. */
+#define MBEDTLS_ECP_WINDOW_SIZE 4 /**< The maximum window size used. */
#endif /* MBEDTLS_ECP_WINDOW_SIZE */
#if !defined(MBEDTLS_ECP_FIXED_POINT_OPTIM)