Arm Mbed and Pelion Device Management support forum

Impact on AES_FEWER/SHA256_SMALLER Tables

Hi,

I have enabled following on config.h

MBEDTLS_AES_FEWER_TABLES
MBEDTLS_SHA256_SMALLER

During performance test, we could see 2ms delay in operation. Am working on embedded platform, and I could not analyze CPU load/usage after enabling them. Would like your recommendation. The target platform is powered by AT91SAM9 family.

On SHA, could not find any difference, with & without flag enabled, our HMAC-SHA256 operation takes only 1ms. However, the AES-GCM-256(msg size less than 1k byte) encrypt takes 4 ms without fewer_tables(commented) and 6 ms with fewer_tables(un-commented).

As it is kind of embedded platform I am sure about CPU load. Could help us by providing any approximate figure CPU usage impact or Is there any benchmark available for arm91 processor for this operation, that would be useful as supportive document for recommending this change to development.

Hi @gopi219,
Many times, ROM \ RAM size reduction comes in the expense of performance.
Have you considered porting the AES hardware acceleration of your platform? According to this link, AT91SAM9M11 has a hardware based AES. Have you checked that your platform has a hardware AES?
Regards,
Mbed TLS Support
Ron