Re: [Hash2] Result extension

From: Vinnie Falco (vinnie.falco_at_[hidden])
Date: 2024-12-09 17:21:01

On Mon, Dec 9, 2024 at 8:43 AM Peter Dimov via Boost <boost_at_[hidden]>
wrote:

> What's important here is that it's not possible to provide
> an extended result of better quality from the outside; the
> hash algorithm is in the best place to provide it because it
> has access to more bits of internal state than it lets out.
>
> This requirement effectively mandates that all _hash
> algorithms_ be _extendable-output hash functions_:
>
> https://en.wikipedia.org/wiki/Extendable-output_function

It seems to me that Hash2 is primarily aimed at users who wish to opt-in to
a superior framework for use in unordered style containers. For this
use-case, hash algorithms only need the digest finalized once. All of the
work in the paper is focused on this use-case. What you have done, in your
own words, is innovative. From the documentation:

> If we’re using one of these algorithms to produce file or content
checksums, do not tolerate collisions, and operate on a large number of
files or items (many millions), it might be better to use a 128 bit digest
instead.

This obviously is out of scope for unordered containers. I think it would
be better to change HashAlgorithm to only allow one call to finalize() (and
rename it from "result"), and no calls to update() after finalize, using
the language or similar language to what I have already posted. And there
is nothing inherently wrong with innovating, therefore I would propose that
you simply add a new concept:

    ExtendableOutputHashAlgorithm

And in this new concept impose your additional requirements. Most users
won't care about extendable output and can just stick with the elements of
the library needed for working with unordered containers. Those users will
find it easy to adapt external implementations of hash algorithms, as they
do not need to also have the expertise for ensuring that these external
implementations meet the more stringent requirements of
ExtendableOutputHashAlgorithm.

There is an added benefit of relaxing the requirements for HashAlgorithm:
users can call into existing algorithms without modifying them, often a
requirement for security certifications.

Thanks