[bitcoin-dev] BIP proposal: Inhibiting a covert attack on the Bitcoin POW function

A month ago I was explaining the attack on Bitcoin's SHA2 hashcash which is exploited by ASICBOOST and the various steps which could be used to block it in the network if it became a problem. While most discussion of ASICBOOST has focused on the overt method of implementing it, there also exists a covert method for using it. As I explained one of the approaches to inhibit covert ASICBOOST I realized that my words were pretty much also describing the SegWit commitment structure. The authors of the SegWit proposal made a specific effort to not be incompatible with any mining system and, in particular, changed the design at one point to accommodate mining chips with forced payout addresses. Had there been awareness of exploitation of this attack an effort would have been made to avoid incompatibility-- simply to separate concerns. But the best methods of implementing the covert attack are significantly incompatible with virtually any method of extending Bitcoin's transaction capabilities; with the notable exception of extension blocks (which have their own problems). An incompatibility would go a long way to explain some of the more inexplicable behavior from some parties in the mining ecosystem so I began looking for supporting evidence. Reverse engineering of a particular mining chip has demonstrated conclusively that ASICBOOST has been implemented in hardware. On that basis, I offer the following BIP draft for discussion. This proposal does not prevent the attack in general, but only inhibits covert forms of it which are incompatible with improvements to the Bitcoin protocol. I hope that even those of us who would strongly prefer that ASICBOOST be blocked completely can come together to support a protective measure that separates concerns by inhibiting the covert use of it that potentially blocks protocol improvements. The specific activation height is something I currently don't have a strong opinion, so I've left it unspecified for the moment. <pre> BIP: TBD Layer: Consensus Title: Inhibiting a covert attack on the Bitcoin POW function Author: Greg Maxwell <greg at xiph.org> Status: Draft Type: Standards Track Created: 2016-04-05 License: PD </pre> ==Abstract== This proposal inhibits the covert exploitation of a known vulnerability in Bitcoin Proof of Work function. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. ==Motivation== Due to a design oversight the Bitcoin proof of work function has a potential attack which can allow an attacking miner to save up-to 30% of their energy costs (though closer to 20% is more likely due to implementation overheads). Timo Hanke and Sergio Demian Lerner claim to hold a patent on this attack, which they have so far not licensed for free and open use by the public. They have been marketing their patent licenses under the trade-name ASICBOOST. The document takes no position on the validity or enforceability of the patent. There are two major ways of exploiting the underlying vulnerability: One obvious way which is highly detectable and is not in use on the network today and a covert way which has significant interaction and potential interference with the Bitcoin protocol. The covert mechanism is not easily detected except through its interference with the protocol. In particular, the protocol interactions of the covert method can block the implementation of virtuous improvements such as segregated witness. Exploitation of this vulnerability could result in payoff of as much as $100 million USD per year at the time this was written (Assuming at 50% hash-power miner was gaining a 30% power advantage and that mining was otherwise at profit equilibrium). This could have a phenomenal centralizing effect by pushing mining out of profitability for all other participants, and the income from secretly using this optimization could be abused to significantly distort the Bitcoin ecosystem in order to preserve the advantage. Reverse engineering of a mining ASIC from a major manufacture has revealed that it contains an undocumented, undisclosed ability to make use of this attack. (The parties claiming to hold a patent on this technique were completely unaware of this use.) On the above basis the potential for covert exploitation of this vulnerability and the resulting inequality in the mining process and interference with useful improvements presents a clear and present danger to the Bitcoin system which requires a response. ==Background== The general idea of this attack is that SHA2-256 is a merkle damgard hash function which consumes 64 bytes of data at a time. The Bitcoin mining process repeatedly hashes an 80-byte 'block header' while incriminating a 32-bit nonce which is at the end of this header data. This means that the processing of the header involves two runs of the compression function run-- one that consumes the first 64 bytes of the header and a second which processes the remaining 16 bytes and padding. The initial 'message expansion' operations in each step of the SHA2-256 function operate exclusively on that step's 64-bytes of input with no influence from prior data that entered the hash. Because of this if a miner is able to prepare a block header with multiple distinct first 64-byte chunks but identical 16-byte second chunks they can reuse the computation of the initial expansion for multiple trials. This reduces power consumption. There are two broad ways of making use of this attack. The obvious way is to try candidates with different version numbers. Beyond upsetting the soft-fork detection logic in Bitcoin nodes this has little negative effect but it is highly conspicuous and easily blocked. The other method is based on the fact that the merkle root committing to the transactions is contained in the first 64-bytes except for the last 4 bytes of it. If the miner finds multiple candidate root values which have the same final 32-bit then they can use the attack. To find multiple roots with the same trailing 32-bits the miner can use efficient collision finding mechanism which will find a match with as little as 2^16 candidate roots expected, 2^24 operations to find a 4-way hit, though low memory approaches require more computation. An obvious way to generate different candidates is to grind the coinbase extra-nonce but for non-empty blocks each attempt will require 13 or so additional sha2 runs which is very inefficient. This inefficiency can be avoided by computing a sqrt number of candidates of the left side of the hash tree (e.g. using extra nonce grinding) then an additional sqrt number of candidates of the right side of the tree using transaction permutation or substitution of a small number of transactions. All combinations of the left and right side are then combined with only a single hashing operation virtually eliminating all tree related overhead. With this final optimization finding a 4-way collision with a moderate amount of memory requires ~2^24 hashing operations instead of the >2^28 operations that would be require for extra-nonce grinding which would substantially erode the benefit of the attack. It is this final optimization which this proposal blocks. ==New consensus rule== Beginning block X and until block Y the coinbase transaction of each block MUST either contain a BIP-141 segwit commitment or a correct WTXID commitment with ID 0xaa21a9ef. (See BIP-141 "Commitment structure" for details) Existing segwit using miners are automatically compatible with this proposal. Non-segwit miners can become compatible by simply including an additional output matching a default commitment value returned as part of getblocktemplate. Miners SHOULD NOT automatically discontinue the commitment at the expiration height. ==Discussion== The commitment in the left side of the tree to all transactions in the right side completely prevents the final sqrt speedup. A stronger inhibition of the covert attack in the form of requiring the least significant bits of the block timestamp to be equal to a hash of the first 64-bytes of the header. This would increase the collision space from 32 to 40 or more bits. The root value could be required to meet a specific hash prefix requirement in order to increase the computational work required to try candidate roots. These change would be more disruptive and there is no reason to believe that it is currently necessary. The proposed rule automatically sunsets. If it is no longer needed due to the introduction of stronger rules or the acceptance of the version-grinding form then there would be no reason to continue with this requirement. If it is still useful at the expiration time the rule can simply be extended with a new softfork that sets longer date ranges. This sun-setting avoids the accumulation of technical debt due to retaining enforcement of this rule when it is no longer needed without requiring a hard fork to remove it. == Overt attack == The non-covert form can be trivially blocked by requiring that the header version match the coinbase transaction version. This proposal does not include this block because this method may become generally available without restriction in the future, does not generally interfere with improvements in the protocol, and because it is so easily detected that it could be blocked if it becomes an issue in the future. ==Backward compatibility== ==Implementation== ==Acknowledgments== ==Copyright== This document is placed in the public domain.