From: Gavin Andresen <gavinandresen@gmail•com>
To: Gregory Maxwell <gmaxwell@gmail•com>
Cc: bitcoin-dev@lists•linuxfoundation.org
Subject: Re: [bitcoin-dev] For discussion: limit transaction size to mitigate CVE-2013-2292
Date: Thu, 23 Jul 2015 11:41:18 -0400 [thread overview]
Message-ID: <CABsx9T3mEYPMFfxFbM-Jj7nADWsVg9HbvQOr0JZeEjC8gOvLZA@mail.gmail.com> (raw)
In-Reply-To: <CAAS2fgRBa47ye-ouV2jDe16MJFCKxYh0zF0Jw4BTwzpXVKgwOg@mail.gmail.com>
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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.
>
To get a feeling for the implementation complexity / correctness tradeoff,
I implemented changes to Core to count exactly how many signature operations
are performed and how many bytes are hashed to compute sighashes:
https://github.com/gavinandresen/bitcoin-git/commit/08ecd6f67d977271faa92bc1890b8f94b15c2792
I haven't benchmarked how much keeping track of the counts affects
performance (but I expect
it to be minimal compared to ECDSA signature validation, accessing inputs
from the UTXO, etc).
I like the idea of a consensus rule that directly addresses the attack--
e.g. "validating
a transaction must not require more than X megabytes hashed to compute
signature hashes."
(or: "validating a block must not require more than X megabytes hashed..."
which is
more symmetric with the current "maximum number of sigops allowed per
block")
Thinking about this and looking at block 364,292, I think I see a simple
optimization that would
speed up validation for transactions with lots of inputs: use
SIGHASH_ANYONECANPAY
for all of the inputs instead of SIGHASH_ALL.
(which would make the transaction malleable-- if that's a concern, then
make one of the inputs
SIGHASH_ALL and the rest SIGHASH_ANYONECANPAY-- I think this is a change
that
should be made to Core and other wallets should make).
---
I'd like to hear from maintainers of other full implementations: how hard
would it be for you
to keep track of the number of bytes hashed to validate a transaction or
block, and use
it as a consensus rule?
--
--
Gavin Andresen
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next prev parent reply other threads:[~2015-07-23 15:41 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-20 19:10 Gavin Andresen
2015-07-20 19:43 ` Tier Nolan
2015-07-20 20:30 ` Gavin Andresen
2015-07-20 19:58 ` Ross Nicoll
2015-07-20 20:55 ` Gregory Maxwell
2015-07-21 18:09 ` Gavin Andresen
2015-07-21 18:18 ` Jeremy Rubin
2015-07-23 15:41 ` Gavin Andresen [this message]
2015-07-24 20:59 ` Gavin Andresen
2015-07-25 0:47 ` odinn
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