commit b3ca90bc449b851e73ed5d71824dbdecc718051f
author Gilles Peskine <Gilles.Peskine@arm.com> Wed Jun 02 13:27:03 2021 +0200
committer Gilles Peskine <Gilles.Peskine@arm.com> Mon Jun 07 21:27:51 2021 +0200
tree 06f3f0c4aeafbb519680a7cec6bae4367dded9f6
parent 646b78b92775f5f8b9d0b0c099c1040eaf847b34

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 d0e61c5..1ae5125 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -3617,7 +3617,7 @@
 
 /* ECP options */
 //#define MBEDTLS_ECP_MAX_BITS             521 /**< Maximum bit size of groups */
-//#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 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)