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Reduce the default ECP window size

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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>
This commit is contained in:
Gilles Peskine 2021-06-02 13:27:03 +02:00
parent 646b78b927
commit b3ca90bc44
3 changed files with 7 additions and 3 deletions
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3
ChangeLog.d/ecp-window-size.txt Normal file
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@ -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.

2
include/mbedtls/config.h
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@ -3617,7 +3617,7 @@
/* ECP options */ /* ECP options */
//#define MBEDTLS_ECP_MAX_BITS 521 /**< Maximum bit size of groups */ //#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 */ //#define MBEDTLS_ECP_FIXED_POINT_OPTIM 1 /**< Enable fixed-point speed-up */
/* Entropy options */ /* Entropy options */

5
include/mbedtls/ecp.h
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@ -258,7 +258,8 @@ mbedtls_ecp_group;
#if !defined(MBEDTLS_ECP_WINDOW_SIZE) #if !defined(MBEDTLS_ECP_WINDOW_SIZE)
/* /*
* Maximum "window" size used for point multiplication. * 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. * Minimum value: 2. Maximum value: 7.
* *
* Result is an array of at most ( 1 << ( MBEDTLS_ECP_WINDOW_SIZE - 1 ) ) * 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 * 224 475 475 453 398 342
* 192 640 640 633 587 476 * 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 */ #endif /* MBEDTLS_ECP_WINDOW_SIZE */
#if !defined(MBEDTLS_ECP_FIXED_POINT_OPTIM) #if !defined(MBEDTLS_ECP_FIXED_POINT_OPTIM)