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Date: Thu, 7 Aug 2014 01:03:50 +0000
From: Peter Todd <pete@petertodd.org>
To: bitcoin-development@lists.sourceforge.net
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Subject: [Bitcoin-development] SIGHASH_ANYONECANPAY extra inputs DoS attack
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tl;dr: Transactions with SIGHASH_ANYONECANPAY-using inputs can be DoS
attacked by attackers adding extra inputs to them that make the fee/byte
paid unfavorable to miners, while still being high enough to be relayed.
While just a nuisance DoS attack, this is a serious obstacle towards
using ANYONECANPAY.
Background: What uses ANYONECANPAY?
-----------------------------------
1) Crowdfunds/assurance contracts: e.g. Hearn's upcoming Lighthouse, as
well as Armory's implementation.
2) Fee bumping: receiver or sender can add inputs w/ ANYONECANPAY to get
a tx confirmed without the (expensive) overhead of a second CPFP tx.
3) Privacy: inputs are more deniable in some cases, e.g. dust used for
fees, which anyone could have added.
4) Replace-by-fee scorched earth: best implementations(1) depend on fee
bumping.
Partial defense: replace-by-fee
-------------------------------
The attacker's modified transaction will usually, but not always, be
replaced by the intended one as the latter will have higher fees.
However replace-by-fee implementations must charge adequately for
network bandwidth consumed, so there will be edge-cases where the
replacement does not happen.
Transaction fee/byte optimization
---------------------------------
Each input that does not use SIGHASH_ALL can be evaluated in terms of
whether or not it increases the fees/byte paid by the transaction. Thus
we can optimize a transaction to pay the highest fees/byte by doing the
following:
def optimize_tx(tx):
tx2 = CTransaction(vin=[], vout=tx.vout, nLockTime=tx.nLockTime)
for txin in <tx.vin sorted by fees/byte>:
if <txin depends on other txins>:
continue
if <tx2 is valid>:
prev_fee_per_byte = tx2.fees / len(tx2.serialized())
tx2.vin.append(txin)
if tx2.fees / len(tx2.serialized()) < prev_fee_per_byte:
# adding txin decreased fees/byte
tx2.vin.pop()
return tx2
else:
tx2.vin.append(txin)
return tx
Essentially txin's that reduce the profitability of the transaction are
dropped, including the attacker's added txins. Meanwhile txins that
increase the profitability can be added by anyone.
1) "[Bitcoin-development] Replace-by-fee scorched-earth without child-pays-for-parent",
Apr 28th 2014, Peter Todd,
https://www.mail-archive.com/bitcoin-development@lists.sourceforge.net/msg05211.html
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