Re: [bitcoin-dev] MIN_STANDARD_TX_NONWITNESS_SIZE and OP_RETURN

From: "Russell O'Connor" <roconnor@blockstream•com>
To: ZmnSCPxj <ZmnSCPxj@protonmail•com>,
	 Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>
Cc: Greg Sanders <gsanders87@gmail•com>
Subject: Re: [bitcoin-dev] MIN_STANDARD_TX_NONWITNESS_SIZE and OP_RETURN
Date: Wed, 27 May 2020 11:15:47 -0400	[thread overview]
Message-ID: <CAMZUoKkA+=-r-OkDWP_GpN2xCsPpTfJnetzSmXtEzD8ZjH8OxA@mail.gmail.com> (raw)
In-Reply-To: <Ptq11JJF3B5h2X94dQdis8lFf7PSm_Hg9F2uITk4MhGcXULr3eiuF3GF71fEVZpcsNZ_s_nrRCXcUmxthQQq4vPQERQpUbCNYErVA9yuNNc=@protonmail.com>

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I don't believe that 60 bytes is a problem here.  SHA256 padding includes a
length value of the original message data. Thus a padded non-64 byte
transaction can never be the same as any padded 64-byte value, and
therefore after applying the SHA256 compression function the resulting
hashes cannot be identical (unless SHA256 itself is broken).

P.S. SHA256 also includes 10* padding, which also suffices to ensure
messages of different lengths have different padding.

On Sat, May 23, 2020 at 8:52 PM ZmnSCPxj via bitcoin-dev <
bitcoin-dev@lists•linuxfoundation.org> wrote:

> Good morning Thomas,
>
> > So I think the question to ask would be "why can't we just make sure
> it's not 64?"
>
> If we accept a 60-byte tx, then SHA-256 will pad it to 64 bytes, and it
> may still be possible to mount CVE-2017-12842 attack with 32-bits of work.
> Of course some other details will be changed from the standard SHA-256 in
> mounting this attack, but from my poor understanding it seems safer to just
> avoid the area around length 64.
>
> It *might* be safe to accept 65-byte or larger (but do not believe me, I
> only play a cryptographer on the Internet), but that does not help your
> specific application, which uses 60 byte tx.
>
> Regards,
> ZmnSCPxj
>
> >
> > On Sat, May 23, 2020 at 11:24 AM Greg Sanders <gsanders87@gmail•com>
> wrote:
> >
> > > AFAIU the number was picked to protect against CVE-2017-12842
> covertly. See: https://github.com/bitcoin/bitcoin/pull/16885 which
> updated the text to explicitly mention this fact.
> > >
> > > On Sat, May 23, 2020 at 11:20 AM Thomas Voegtlin via bitcoin-dev <
> bitcoin-dev@lists•linuxfoundation.org> wrote:
> > >
> > > > Hello list,
> > > >
> > > > I have been trying to CPFP a transaction using OP_RETURN, because the
> > > > remaining output value would have been lower than the dust threshold.
> > > >
> > > > The scriptPubkey of the output was OP_RETURN + OP_0, and there was a
> > > > single p2wsh input.
> > > >
> > > > The result is a 60 bytes transaction (without witness), that gets
> > > > rejected because it is lower than MIN_STANDARD_TX_NONWITNESS_SIZE,
> which
> > > > is equal to 82 bytes.
> > > >
> > > > Why is that value so high? Would it make sense to lower it to 60?
> > > >
> > > > Thomas
> > > > _______________________________________________
> > > > bitcoin-dev mailing list
> > > > bitcoin-dev@lists•linuxfoundation.org
> > > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
> _______________________________________________
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