Re: [Bitcoin-development] Defeating the block withholding attack
I don't understand how your proposal will work for decentralized pools - can you explain it more concretely? What would the new block header look like? What is required for a share to to be earned? What is required for a block to be valid (added to Block Chain)? I don't think I understand what you mean by NextBlockCandidate. Perhaps a concrete example using difficulty 1.7 million would be instructive. On Mon, Jun 4, 2012 at 2:05 PM, Luke-Jr l...@dashjr.org wrote: On Monday, June 04, 2012 8:49:48 PM Mike Koss wrote: As I understand the attack, the attacker gets compensated for the shares they earn, but the pool will be denied any valid blocks found. The attacker DOES NOT have access to the Bitcoins earned in the unreported block (only the mining pool has access to the Coinbase address and transactions in the block). With decentralized pools, the attacker does have access to the block, and can potentially submit it to the Bitcoin network directly bypassing the pool if it benefits them to do so. So it's a zero-net-cost attack for the attacker (but no chance of making a profit) to hurt the pool operator. Because of the above, there is a possibility an attacker can make a profit. The only way to detect such an attack now is to look for unlucky miners; at the current difficulty, you can't detect this cheat until many millions of shares have been earned w/o a qualifying block. Since an attacker can also create many fake identities, they can avoid detection indefinitely by abandoning each account after a million earned shares. There are other modes of detection, but nobody has bothered to implement them since attackers can easily do a simple workaround in an arms race. I don't understand your proposal for fixing this. You would have to come up with a scheme where: - The miner can detect a qualifying hash to earn a share. - Not be able to tell if the hash is for a valid block. With my proposal, miners can find shares, but won't know if it's a valid block until the subsequent block is also found (that subsequent block might not end up being a real block in the big picture). The way I would do this is to have a secret part (not shared with the miners) of a block that is part of the merkle hash, which is also used in a secondary hash. Difficulty is then divide into two parts: the first, solved by the miner (earning a share - e.g., 1 in 4 Billion hashes). And a second, solved by the pool (1 in Difficulty shares). A valid block would have to exhibit a valid Share Hash AND a valid Pool Hash in order to be accepted. This only works for centralized pools, which are contrary to the health of the Bitcoin network. Decentralized pools cannot have a secret. -- Mike Koss CTO, CoinLab (425) 246-7701 (m) A Bitcoin Primer http://coinlab.com/a-bitcoin-primer.pdf - What you need to know about Bitcoins. -- Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
Re: [Bitcoin-development] Defeating the block withholding attack
On Sat, Jun 2, 2012 at 8:52 PM, Luke-Jr l...@dashjr.org wrote: Analysis, comments, constructive criticism, etc welcome for the following: ==Background== At present, an attacker can harm a pool by intentionally NOT submitting shares that are also valid blocks. All pools are vulnerable to this attack, whether centralized or decentralized and regardless of reward system used. The attack's effectiveness is proportional to ratio of the attacker's hashrate to the rest of the pool. I'm unclear on the economics of this attack; we spent a bit of time talking about it a few months ago at CoinLab and decided not to worry about it for right now. Does it have asymmetric payoff for an attacker, that is, over time does it pay them more to spend their hashes attacking than just mining? My gut is that it pays less well than mining, meaning I think this is likely a small problem in the aggregate, and certainly not something we should try and fork the blockchain for until there's real pain. Consider, for instance, whether it pays better than just mining bitcoins and spending those on 'bonuses' for getting users to switch from a pool you hate. Watson, I don't believe the attack signature you mention is a factor here, since the pool controls the merkle, only that pool will benefit from block submission. The nonce / coinbase combo is worthless otherwise, and so this attack is just in brief get lucky, but don't submit. So, can anyone enlighten me as to some actual estimates of badness for this attack? Peter -- Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
Re: [Bitcoin-development] Defeating the block withholding attack
On Monday, June 04, 2012 1:43:42 AM Peter Vessenes wrote: Does it have asymmetric payoff for an attacker, that is, over time does it pay them more to spend their hashes attacking than just mining? That depends on the pool's reward scheme. Some complicated forms are capable of getting bonus earnings out of the pool. Under all systems, the attacker at least gains the hurt the pool benefit. Given the frequency of DDoS attacks on pools, it is clear there are people who will even pay for attacks that provide no other benefit than harming pools. Under all systems, the attacker doesn't lose out in any significant way. My gut is that it pays less well than mining, meaning I think this is likely a small problem in the aggregate, and certainly not something we should try and fork the blockchain for until there's real pain. If we wait until there's real pain, it will be a painful fork. If we plan it 1-2 years out, people have time to upgrade on their own before it breaks. Consider, for instance, whether it pays better than just mining bitcoins and spending those on 'bonuses' for getting users to switch from a pool you hate. With this attack, attackers can hurt the pool's luck factor *and* spend the bitcoins they earn to bribe users away. -- Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ ___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
