From: "Russell O'Connor" <roconnor@blockstream•io>
To: Peter Todd <pete@petertodd•org>
Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Fast Merkle Trees
Date: Thu, 7 Sep 2017 11:51:14 -0400 [thread overview]
Message-ID: <CAMZUoKk=00AJbtq602P=MWtGhWUY_7oCrWEUj+K7xp_+DOOTYg@mail.gmail.com> (raw)
In-Reply-To: <20170907055557.GA12638@fedora-23-dvm>
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On Thu, Sep 7, 2017 at 1:55 AM, Peter Todd <pete@petertodd•org> wrote:
> On Wed, Sep 06, 2017 at 09:59:54PM -0400, Russell O'Connor via bitcoin-dev
> wrote:
> > The fast hash for internal nodes needs to use an IV that is not the
> > standard SHA-256 IV. Instead needs to use some other fixed value, which
> > should itself be the SHA-256 hash of some fixed string (e.g. the string
> > "BIP ???" or "Fash SHA-256").
>
> Note that in general, designs should *not* create new hash functions by
> using
> custom IVs, but rather use bog-standard SHA256, and make a fixed first
> block.
> That allows unoptimised implementations to just hash a block with the
> second
> initialization value, and optimized implementations to start with the fixed
> midstate.
I 100% agree.
With SHA256 every final state is also a valid midstate. Therefore, using a
custom IV of the SHA256 hash of some fixed string results in a hash of data
that is functionally equivalent to prefixing the data with the padded
version of the fixed string and using a regular SHA256 hash of the combined
data. This is important and I should have explicitly pointed it out.
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prev parent reply other threads:[~2017-09-07 15:51 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-07 1:59 Russell O'Connor
2017-09-07 2:20 ` Mark Friedenbach
2017-09-07 15:43 ` Russell O'Connor
2017-09-07 17:42 ` Mark Friedenbach
2017-09-07 18:55 ` Russell O'Connor
2017-09-07 20:04 ` Mark Friedenbach
2017-09-12 11:44 ` Johnson Lau
2017-09-07 5:55 ` Peter Todd
2017-09-07 15:51 ` Russell O'Connor [this message]
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