commit 5752e599b32dc0322fa3352f6c86281c01dc9ee1
author Gilles Peskine <Gilles.Peskine@arm.com> Wed Jun 02 13:27:03 2021 +0200
committer Gilles Peskine <Gilles.Peskine@arm.com> Thu Jun 17 21:46:29 2021 +0200
tree 90c2f2054e0bdab53168299ca2fd51c250869d89
parent 12b5b389817ae77ded92c3f799ef09432cc834de

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>

ChangeLog.d/ecp-window-size.txt

diff --git a/ChangeLog.d/ecp-window-size.txt b/ChangeLog.d/ecp-window-size.txt
new file mode 100644
index 0000000..909d4e8
--- /dev/null
+++ b/ChangeLog.d/ecp-window-size.txt
@@ -0,0 +1,3 @@
+Changes
+   * Reduce the default value of MBEDTLS_ECP_WINDOW_SIZE. This reduces RAM usage
+     during ECC operations at a negligible performance cost.

include/mbedtls/config.h

diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index fc10f9d..0c7e4ec 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -3145,7 +3145,7 @@
 //#define MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT      384 /**< Maximum size of (re)seed buffer */
 
 /* ECP options */
-//#define MBEDTLS_ECP_WINDOW_SIZE            6 /**< Maximum window size used */
+//#define MBEDTLS_ECP_WINDOW_SIZE            4 /**< Maximum window size used */
 //#define MBEDTLS_ECP_FIXED_POINT_OPTIM      1 /**< Enable fixed-point speed-up */
 
 /* Entropy options */

include/mbedtls/ecp.h

diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index 49e85d9..1c271c9 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -255,7 +255,8 @@
 #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 ) )
@@ -272,7 +273,7 @@
  *      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)