phc-discussions - Re: [PHC] Argon2

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Date: Tue, 31 Mar 2015 15:32:39 +0200
From: Dmitry Khovratovich <khovratovich@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Argon2

Thank you for advice here. I will take this into account when
preparing a new code.

Dmitry

On Tue, Mar 31, 2015 at 3:30 PM, Solar Designer <solar@...nwall.com> wrote:
> On Tue, Mar 31, 2015 at 03:02:06PM +0200, Dmitry Khovratovich wrote:
>> Yes, you're right, the code is little-endian only so far (the
>> specification implicitly suggest little-endianness in the
>> initialization phase). This must be changed, and I will do it.
>
> Sounds fine.
>
> Another concern here is C strict aliasing rules.  When you cast some
> integer type to a pointer to a char-sized type, that's fine so far
> (since char is special).  However, if the function you pass this to then
> casts that to a different-sized integer type, you have a C strict
> aliasing violation there, so UB, and it may be exposed and result in
> visibly incorrect behavior if the compiler performs optimization across
> function boundaries (such as does function inlining and link-time
> optimization).  This actually affects the official scrypt's
> crypto_scrypt-nosse.c when compiled with (extreme) function inlining,
> resulting in incorrect computation.  I have not yet seen it happen
> across object file boundary, but since link-time optimization is a thing
> I expect that it may (if not now, then later).  This affects a lot of
> existing code out there.  (In the submitted yescrypt code, I tried to be
> careful about this, but e.g. some of my older code in JtR is affected.)
>
>> >         blake2b_update(&BlakeHash, (const uint8_t*)&lanes, sizeof(lanes));
> [...]
>
> Surely blake2b_update() then uses its own choice of integer or SIMD
> types for this data.
>
> A solution here is to use a union type containing all of the integer and
> SIMD types.  The called function may cast the pointer to a pointer to
> such union type, then access the fields of its desired type.  The
> compiler will then know that they might alias those other listed types.
>
> Alexander



-- 
Best regards,
Dmitry Khovratovich