Set a new bytes used for elliptic curves computations to '\000'

[dinosaure]

Cstruct.create does this. If we don't initialize bytes with '\000', Field_element.zero can be something else than '\000'. It's a fix for mirleft/ocaml-x509#167.

[palainp]

To avoid setting to \000 the tmp Bytes in mul, add and sub (if I understand correctly, it write the entire Bytes), wouldn't it better to leave create() as is, and set to \000 in zero?

[dinosaure]

To avoid setting to \000 the tmp Bytes in mul, add and sub (if I understand correctly, it write the entire Bytes), wouldn't it better to leave create() as is, and set to \000 in zero?

Yes, I've thought about this minor optimisation. I leave it to @hannesm to choose whether to be paranoid or optimal.

[reynir]

This fixes a critical bug. We can always optimize in a separate PR in my opinion.

[hannesm]

first of all, thanks for your investigations and the remarks. :)

the question for me is about symmetry -- there's a create_octets function in the same functor, should it use as well Bytes.make? And there's a functor Make_Fn -- here should the create and create_octets as well zero out the data?

on a related note, is there any measurable performance impact?

and should we stay in sync with OCaml stdlib and have "create" for "allocate", and "make" for "allocate and initialize" (i.e. rename to make / make_octets)?

[hannesm]

Thanks, I looked a bit deeper, and indeed only the zero requires a cleared buffer -- so I amended your PR with a commit that ensures that the zero will be a buffer of zeroes.

Let me know if you think this is too little? Or if you can test x509 in your 32 bit container with the added commit?

Measurements on my laptop are rather noisy in this area, but across 3 runs the performance difference between the main branch (47751da) and the latest commit (81c492e) is not present, while with 3333de3 there's a small decrease.

What do you think? (/cc @reynir @dinosaure @palainp)

[dinosaure]

LGTM

[palainp]

It's ok for me too :)

[hannesm]

thanks, merged. the OCaml-CI in ocaml-x509 also reported success :)