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Date: Tue, 21 Jul 2015 14:09:19 -0400
Message-ID: <CABsx9T2ZX3iuCN4g6Yh0k8R7Ad0yx-yfx1f2XhCEPtz-vt2xsw@mail.gmail.com>
From: Gavin Andresen <gavinandresen@gmail.com>
To: Gregory Maxwell <gmaxwell@gmail.com>
Content-Type: multipart/alternative; boundary=001a1133d13c949aa4051b668d59
Cc: bitcoin-dev@lists.linuxfoundation.org
Subject: Re: [bitcoin-dev] For discussion: limit transaction size to
	mitigate CVE-2013-2292
Precedence: list

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On Mon, Jul 20, 2015 at 4:55 PM, Gregory Maxwell <gmaxwell@gmail.com> wrote:

> On Mon, Jul 20, 2015 at 7:10 PM, Gavin Andresen via bitcoin-dev
> <bitcoin-dev@lists.linuxfoundation.org> wrote:
> > Mitigate a potential CPU exhaustion denial-of-service attack by limiting
> > the maximum size of a transaction included in a block.
>
> This seems like a fairly indirect approach. The resource being watched
> for is not the size (otherwise two transactions for 200k would be
> strictly worse than one 200k transactions) but the potential of N^2
> costs related to repeated hashing in checksig; which this ignores.
>

Yes.  The tradeoff is implementation complexity: it is trivial to check
transaction size,
not as trivial to count signature operations, because
number-of-bytes-in-transaction
doesn't require any context.

But I would REALLY hate myself if in ten years a future version of me was
struggling to
get consensus to move away from some stupid 100,000 byte transaction size
limit
I imposed to mitigate a potential DoS attack.

So I agree, a limit on sigops is the right way to go. And if that is being
changed,
might as well accurately count exactly how many sigops a transaction
actually
requires to be validated...

-- 
--
Gavin Andresen

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<div dir=3D"ltr"><div class=3D"gmail_extra"><div class=3D"gmail_quote">On M=
on, Jul 20, 2015 at 4:55 PM, Gregory Maxwell <span dir=3D"ltr">&lt;<a href=
=3D"mailto:gmaxwell@gmail.com" target=3D"_blank">gmaxwell@gmail.com</a>&gt;=
</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .=
8ex;border-left:1px #ccc solid;padding-left:1ex"><span class=3D"">On Mon, J=
ul 20, 2015 at 7:10 PM, Gavin Andresen via bitcoin-dev<br>
&lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org">bitcoin-dev@li=
sts.linuxfoundation.org</a>&gt; wrote:<br>
&gt; Mitigate a potential CPU exhaustion denial-of-service attack by limiti=
ng<br>
&gt; the maximum size of a transaction included in a block.<br>
<br>
</span>This seems like a fairly indirect approach. The resource being watch=
ed<br>
for is not the size (otherwise two transactions for 200k would be<br>
strictly worse than one 200k transactions) but the potential of N^2<br>
costs related to repeated hashing in checksig; which this ignores.<br></blo=
ckquote><div><br></div><div>Yes.=C2=A0 The tradeoff is implementation compl=
exity: it is trivial to check transaction size,</div><div>not as trivial to=
 count signature operations, because number-of-bytes-in-transaction</div><d=
iv>doesn&#39;t require any context.</div><div><br></div><div>But I would RE=
ALLY hate myself if in ten years a future version of me was struggling to</=
div><div>get consensus to move away from some stupid 100,000 byte transacti=
on size limit</div><div>I imposed to mitigate a potential DoS attack.</div>=
<div><br></div><div>So I agree, a limit on sigops is the right way to go. A=
nd if that is being changed,</div><div>might as well accurately count exact=
ly how many sigops a transaction actually</div><div>requires to be validate=
d...</div></div><div><br></div>-- <br><div class=3D"gmail_signature">--<br>=
Gavin Andresen<br></div><div class=3D"gmail_signature"><br></div>
</div></div>

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