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 Mbed-TLS#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

@@ -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

@@ -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

@@ -255,7 +255,8 @@ mbedtls_ecp_group;
 #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 @@ mbedtls_ecp_group;
  *      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)