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

bfd at cock.lu bfd at cock.lu
Thu Apr 6 07:24:03 UTC 2017

Miners blocking SegWit due to ASICBOOST requirements also means they 
would block future deployment of committed bloom filters.

On 2017-04-06 00:37, Gregory Maxwell via bitcoin-dev wrote:
> 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",
> 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.
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