Motivation is similar to NO_UDBL_DIVISION.
The alternative implementation of 64-bit mult is straightforward and aims at
obvious correctness. Also, visual examination of the generate assembly show
that it's quite efficient with clang, armcc5 and arm-clang. However current
GCC generates fairly inefficient code for it.
I tried to rework the code in order to make GCC generate more efficient code.
Unfortunately the only way to do that is to get rid of 64-bit add and handle
the carry manually, but this causes other compilers to generate less efficient
code with branches, which is not acceptable from a side-channel point of view.
So let's keep the obvious code that works for most compilers and hope future
versions of GCC learn to manage registers in a sensible way in that context.
See https://bugs.launchpad.net/gcc-arm-embedded/+bug/1775263
Allowing DECRYPT with crypt_and_tag is a risk as people might fail to check
the tag correctly (or at all). So force them to use auth_decrypt() instead.
See also https://github.com/ARMmbed/mbedtls/pull/1668
The cast to void was motivated by the assumption that the functions only
return non-zero when passed bad arguments, but that might not be true of
alternative implementation, for example on hardware failure.
- need HW failure codes too
- re-use relevant poly codes for chachapoly to save on limited space
Values were chosen to leave 3 free slots at the end of the NET odd range.
That's what it is. So we shouldn't set a block size != 1.
While at it, move call to chachapoly_update() closer to the one for GCM, as
they are similar (AEAD).
This reduces clutter, making the functions more readable.
Also, it makes lcov see each line as covered. This is not cheating, as the
lines that were previously seen as not covered are not supposed to be reached
anyway (failing branches of the selftests).
Thanks to this and previous test suite enhancements, lcov now sees chacha20.c
and poly1305.c at 100% line coverage, and for chachapoly.c only two lines are
not covered (error returns from lower-level module that should never happen
except perhaps if an alternative implementation returns an unexpected error).
This module used (len, pointer) while (pointer, len) is more common in the
rest of the library, in particular it's what's used in the GCM API that
very comparable to it, so switch to (pointer, len) for consistency.
Note that the crypt_and_tag() and auth_decrypt() functions were already using
the same convention as GCM, so this also increases intra-module consistency.
This module used (len, pointer) while (pointer, len) is more common in the
rest of the library, in particular it's what's used in the CMAC API that is
very comparable to Poly1305, so switch to (pointer, len) for consistency.
In addition to making the APIs of the various AEAD modules more consistent
with each other, it's useful to have an auth_decrypt() function so that we can
safely check the tag ourselves, as the user might otherwise do it in an
insecure way (or even forget to do it altogether).
While the old name is explicit and aligned with the RFC, it's also very long,
so with the mbedtls_ prefix prepended we get a 31-char prefix to each
identifier, which quickly conflicts with our 80-column policy.
The new name is shorter, it's what a lot of people use when speaking about
that construction anyway, and hopefully should not introduce confusion at
it seems unlikely that variants other than 20/1305 be standardised in the
foreseeable future.
- in .h files: only put the context declaration inside the #ifdef _ALT
(this was changed in 2.9.0, ie after the original PR)
- in .c file: only leave selftest out of _ALT: even though some function are
trivial to build from other parts, alt implementors might want to go another
way about them (for efficiency or other reasons)
I refactored some code into the function mbedtls_constant_time_memcmp
in commit 7aad291 but this function is only used by GCM and
AEAD_ChaCha20_Poly1305 to check the tags. So this function is now
only enabled if either of these two ciphers is enabled.
