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