design: Fixed size large numbers

design: Fixed size large numbers

For the longest time, we have mitigated security issues related to large
numbers (BIGNUM) and constant time in a piece-meal fashion, without really
looking at the problem from a zoomed out, holistic perspective.

An interesting aspect in this problem is that large numbers can vary in
size, and that depending on their combined sizes, the time to perform
mathematical calculations with them vary equally much, and may thereby
unintentionally leak information on those numbers.

To mitigate that sort of timing issue, we introduce fixed size numbers,
which are designed to have payload sizes that are pre-determined, usually by
the crypto system that uses them.  This means that even a very small number
(let's take 1 as a ridiculous example) would have the same size payload as a
much larger number, and calculations using them would perform across all
payload bits of all input numbers combined.

These fixed size numbers primarly differ from BIGNUMs in that once they have
been allocated to a certain size, that size will not change throughout its
lifetime.

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28522)