• Registered: 2019-11-01
  • Posts: 12

Topic: How many bytes stack size needed for FAST_MATH

Hello,

I wonder how many bytes of stack is needed to enable fast math?

Thanks.

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  • Registered: 2015-10-07
  • Posts: 430

Re: How many bytes stack size needed for FAST_MATH

Hi burakkirazli,

It depends on the algorithms enabled/used and the FP_MAX_BITS you have defined. The ECC and RSA algorithms use fast math.

Typically during a TLS connection you'll have several of these on the stack. I recommend starting with 10KB of stack and then looking at your stack high watermark to determine how much to reduce.

For RSA the FP_MAX_BITS is double the maximum key size. For RSA 2048-bit requires double max bits so 4096. If you have just ECC defined than FP_MAX_BITS can be ECC max key size + 32. For both RSA and ECC you can reduce ECC stack usage by defining ALT_ECC_SIZE, which will use heap for the ECC point instead of stack.

We have some good examples for configuration the math library cases here:
https://github.com/wolfSSL/wolfssl/blob … settings.h

Thanks,
David Garske, wolfSSL

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