Reducing stack memory consumption

[pouyaashraf]

Description

When using the mbedtls library on certain embedded platforms, a stack-allocated buffer (buf in mbedtls_rsa_rsassa_pss_verify_ext) provokes a stack overflow. The platform in question is LittleKernel running on ARM, where we unfortunately don't have the option of increasing the stack alotted to the kernel. Changing the buf variable to have static storage duration resolves this, but then the function becomes thread un-safe which is not desirable.

Issue request type

[ ] Question
[x] Enhancement
[ ] Bug

[pouyaashraf]

To add to this, the amount of memory allocated by buf is 1KiB, and it's done here: https://github.com/ARMmbed/mbed-crypto/blob/development/library/rsa.c#L2165

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/IOTCRYPT-1042