From b3ca90bc449b851e73ed5d71824dbdecc718051f Mon Sep 17 00:00:00 2001 From: Gilles Peskine Date: Wed, 2 Jun 2021 13:27:03 +0200 Subject: [PATCH] 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 --- ChangeLog.d/ecp-window-size.txt | 3 +++ include/mbedtls/config.h | 2 +- include/mbedtls/ecp.h | 5 +++-- 3 files changed, 7 insertions(+), 3 deletions(-) create mode 100644 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 000000000..909d4e8a4 --- /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. diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h index d0e61c545..1ae51252e 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 */ diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h index b974e5984..f278d105d 100644 --- a/include/mbedtls/ecp.h +++ b/include/mbedtls/ecp.h @@ -258,7 +258,8 @@ 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 ) ) @@ -275,7 +276,7 @@ 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)