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Reduce the default MBEDTLS_ECP_WINDOW_SIZE value to 2

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As tested in https://github.com/Mbed-TLS/mbedtls/issues/6790,
after introducing side-channel counter-measures to bignum,
the performance of RSA decryption in correlation to the
MBEDTLS_ECP_WINDOW_SIZE has changed.
The default value of 2 has been chosen as it provides best
or close-to-best results for tests on Cortex-M4 and Intel i7.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
This commit is contained in:
Andrzej Kurek 2023-02-24 07:44:57 -05:00
parent a3b625b0a1
commit 86f30ff626
3 changed files with 10 additions and 3 deletions
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7
ChangeLog.d/mpi-window-perf Normal file
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@ -0,0 +1,7 @@
Changes
* Changed the default MBEDTLS_ECP_WINDOW_SIZE from 6 to 2.
As tested in issue 6790, the correlation between this define and
RSA decryption performance has changed lately due to security fixes.
To fix the performance degradation when using default values the
window was reduced from 6 to 2, a value that gives the best or close
to best results when tested on Cortex-M4 and Intel i7.

4
include/mbedtls/bignum.h
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@ -63,7 +63,7 @@
#if !defined(MBEDTLS_MPI_WINDOW_SIZE)
/*
* Maximum window size used for modular exponentiation. Default: 6
* Maximum window size used for modular exponentiation. Default: 2
* Minimum value: 1. Maximum value: 6.
*
* Result is an array of ( 2 ** MBEDTLS_MPI_WINDOW_SIZE ) MPIs used
@ -71,7 +71,7 @@
*
* Reduction in size, reduces speed.
*/
#define MBEDTLS_MPI_WINDOW_SIZE 6 /**< Maximum window size used. */
#define MBEDTLS_MPI_WINDOW_SIZE 2 /**< Maximum window size used. */
#endif /* !MBEDTLS_MPI_WINDOW_SIZE */
#if !defined(MBEDTLS_MPI_MAX_SIZE)

2
include/mbedtls/mbedtls_config.h
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@ -3559,7 +3559,7 @@
* comment in the specific module. */
/* MPI / BIGNUM options */
//#define MBEDTLS_MPI_WINDOW_SIZE 6 /**< Maximum window size used. */
//#define MBEDTLS_MPI_WINDOW_SIZE 2 /**< Maximum window size used. */
//#define MBEDTLS_MPI_MAX_SIZE 1024 /**< Maximum number of bytes for usable MPIs. */
/* CTR_DRBG options */