[Bitcoin-development] New attack identified and potential solution described: Dropped-transaction spam attack against the block size limit

Tue Jun 9 17:52:05 UTC 2015

That does sound good on the surface, but how do we enforce #1 and #2?  They seem to be unenforceable, as a miner can adjust the size of the memory pool in his local source.

From: Gavin Andresen 
Sent: Tuesday, June 09, 2015 6:36 AM
To: Loi Luu 
Cc: Raystonn . ; Bitcoin Dev 
Subject: Re: [Bitcoin-development] New attack identified and potential solution described: Dropped-transaction spam attack against the block size limit

How about this for mitigating this potential attack: 

1. Limit the memory pool to some reasonable number of blocks-worth of transactions (e.g. 11)
2. If evicting transactions from the memory pool, prefer to evict transactions that are part of long chains of unconfirmed transactions.
3. Allow blocks to grow in size in times of high transaction demand.

The combination of (1) and (2) means an attacker needs to prepare lots of confirmed inputs to pull off the attack. By itself that means they MUST pay transaction fees.

(3) further mitigates the attack because it allows miners to just absorb fees that the attacker is throwing at miners.

On Tue, Jun 9, 2015 at 5:33 AM, Loi Luu <loi.luuthe at gmail.com> wrote:

    The proposed fix is to add a new rule on how
    fees are handled.  Some amount of every fee should be considered as burned
    and can never be spent.  I will propose 50% of the fee here, but there may
    be better numbers that can be discovered prior to putting this into place.
    If we'd like miners to continue to collect the same fees after this change,
    we can suggest the default fee per transaction to be doubled

  I would propose another practical rule rather than burning 50% of the fee. For example, you can
  credit 50% of the transaction fee to the next block. Thus, the attacker cannot perform
  the attack with 0-fee any more, yet you don't have to double the price of the TX fee for the fix.

  Thanks,
  Loi Luu.

  On Tue, Jun 9, 2015 at 4:07 AM, Raystonn . <raystonn at hotmail.com> wrote:

    When implemented, the block size limit was put in place to prevent the
    potential for a massive block to be used as an attack to benefit the miner
    of that block.  The theory goes that such a massive block would enrich its
    miner by delaying other miners who are now busy downloading and validating
    that huge block.  The original miner of that large-block would be free to
    continue hashing the next block, giving it an advantage.

    Unfortunately, this block size limit opened a different attack.  Prior to
    the limit, any attempt to spam the network by anyone other than someone
    mining their own transactions would have been economically unfeasible.  As
    every transaction would have a fee, there would have been a real cost for
    every minute of spam.  The end result would have been a transfer of wealth
    from spammer to Bitcoin miners, which would have harmed the spammers and
    encouraged further mining of Bitcoin, a very antifragile outcome.

    But now we have the block size limit.  Things are very different with this
    feature in place.  The beginning of a spam attack on the Bitcoin network
    will incur transaction fees, just like before.  But if spam continues at a
    rate exceeding the block size limit long enough for transactions to be
    dropped from mempools, the vast majority of spam transaction fees will never
    have to be paid.  In fact, as real users gain in desperation and pay higher
    fees to get their transactions through in a timely manner, the spammers will
    adjust their fees to minimize the cost of the attack and maximize
    effectiveness.  Using this method, they keep their fees at a point that
    causes most of the spam transactions to be dropped without confirmation
    (free spam), while forcing a floor for transaction fees.  Thus, while spam
    could be used by attackers to disable the network entirely, by paying
    high-enough fees to actually fill the blocks with spam, it can also be used
    by a single entity to force a transaction fee floor.  Real users will be
    forced to pay a transaction fee higher than the majority of the spam to get
    their transactions confirmed.  So this is an effective means for a minority
    of miners to force higher fees through spam attacks, even in the face of
    benevolent miners who would not support a higher fee floor by policy.
    Miners would simply have no way to fix this, as they can only put in the
    transactions that will fit under the block size limit.

    In the face of such a spam attack, Bitcoin's credibility and usability would
    be severely undermined.  The block size limit enables this attack, and I now
    argue for its removal.  But we can't just remove it and ignore the problem
    that it was intended to address.  We need a new fix for the large-block
    problem described in the first paragraph that does not suffer from the
    dropped-transaction spam-attack problem that is enabled by the block size
    limit today.  My proposal is likely to be controversial, and I'm very much
    open to hearing other better proposals.

    Large blocks created by a miner as a means to spam other miners out of
    competition is a problem because miners do not pay fees for their own
    transactions when they mine them.  They collect the fees they pay.  This
    breaks the economic barrier keeping people from spamming the network, as the
    spamming is essentially free.  The proposed fix is to add a new rule on how
    fees are handled.  Some amount of every fee should be considered as burned
    and can never be spent.  I will propose 50% of the fee here, but there may
    be better numbers that can be discovered prior to putting this into place.
    If we'd like miners to continue to collect the same fees after this change,
    we can suggest the default fee per transaction to be doubled.  Half of every
    fee would be burned and disappear forever, effectively distributing the
    value of those bitcoins across the entire money supply.  The other half
    would be collected by the miner of the block as is done today.  This
    solution would mean large blocks would cost a significant number of bitcoin
    to create, even when all of the transactions are created by the miner of
    that block.  For this to work, we'd need to ensure a minimum fee is paid for
    most of the transactions in every block, and the new transaction fee rule is
    in place.  Then the block size limit can be removed.

    Raystonn

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Gavin Andresen
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