mbedtls_mpi_write_string() could be reduced to use less RAM.

[RonEld]

Description

• Type: Enhancement\Feature Request
• Priority: Major
  As mentioned in Refactor mpi_write_hlp to not be recursive  #2214 (review), mbedtls_mpi_write_string() could be reduced to use less RAM.

Enhancement\Feature Request

Justification - why does the library need this feature?
It always best to reduce RAM usage, and mbedtls_mpi_write_string() is not time-critical

Suggested enhancement
From #2214 (review):

Let's safe some bytes and RAM here instead, by not using T. Suggestion: Document that mpi_write_hlp() ignores the sign and just pass X instead of the local copy T.

From #2214 (review):

I think we should call mpi_write_hlp() even for radix 16 and get rid of the radix == 16 branch - it might not be as fast as the hand-coded version, but mbedtls_mpi_write_string() is not time-critical anyway.

[daverodgman]

Closing - this is only likely to be used in debug builds, so impact is very limited.