tree 56a28ac677c264d70da0bd0393ab7a3a3ff941d2
parent d03949e2a48ca7b9b6a1e5995daaf923b3bb2048
author Hanno Becker <hanno.becker@arm.com> 1565707406 +0100
committer Hanno Becker <hanno.becker@arm.com> 1568018757 +0100

Add config.h option to hardcode choice of single MD algorithm

This commit introduces the configuration option

  MBEDTLS_MD_SINGLE_HASH

which can be used to hardcode support for a single digest algorithm
at compile-time, at the benefit of reduced code-size.

To use, it needs to be defined to evaluate to a macro of the form
MBEDTLS_MD_INFO_{DIGEST}, and macros MBEDTLS_MD_INFO_{DIGEST}_FIELD
must be defined, giving rise to the various aspects (name, type,
size, ...) of the chosen digest algorithm. MBEDTLS_MD_INFO_SHA256
provides an example, but other algorithms can be added if needed.

At the moment, the effect of using MBEDTLS_MD_SINGLE_HASH is that
the implementation of the MD API (e.g. mbedtls_md_update()) need no
longer to through the abstraction of the mbedtls_md_info structures
by calling their corresponding function pointers fields (akin to
virtual functions in C++), but the directly call the corresponding
core digest function (such as mbedtls_sha256_update()).

Therefore, MBEDTLS_MD_SINGLE_HASH so far removes the second layer
of indirection in the chain

  User calls MD API -> MD API calls underlying digest impl'n
                    -> Core digest impl'n does the actual work,

but the first indirection remains, as the MD API remains untouched
and cannot yet be inlined. Studying to what extend inlining the
shortened MD API implementations would lead to further code-savings
is left for a later commit.