Subject: [bitcoin-dev] BIP Proposal: Inhibiting a covert optimization on the Bitcoin POW function

From: Sergio Demian Lerner <[email-masked]>
Date: Fri, 7 Apr 2017 17:52:17 -0300

<pre>
  BIP: TBD
  Layer: Consensus
  Title: Inhibiting a covert optimization on the Bitcoin POW function
  Author: Sergio Demian Lerner <[email-masked]>
  Status: Draft
  Type: Standards Track
  Created: 2016-04-07
  License: PD
</pre>

==Abstract==

This proposal inhibits the covert use of a known optimization 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
optimization which can allow a rational 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 applied for a patent on this
optimization. The company "Sunrise Tech Group, Llc" has offered to license
it to any interested party in the past. Sunrise Tech Group has 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 taking advantage of this optimization, as
described
by the patent:
One 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.

The use of this optimization could result in a big payoff, but the actual
sum depends on the degree of research, investment and effort put into
designing
the improved cores.

On the above basis the potential for covert use of this optimization
in the covert form and interference with useful improvements presents a
danger to the Bitcoin system.

==Background==

The general idea of this optimization 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 optimization. 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 optimization.

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 optimization.

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 optimization 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 optimization ==

A BIP for avoiding erroneous warning messages when miners use the overt
version
of the optimization was proposed several years ago, in order to deter the
covert
use of the optimization. But that BIP was rejected.
However, in light of the current discoveries, that BIP could be
reconsidered.

The over optimization does not generally interfere with improvements in the
protocol.

==Backward compatibility==

==Implementation==

==Acknowledgments==

Greg Maxwell <[email-masked]> for the original report, which contained
several errors that were corrected in the present proposal.

==Copyright==

This document is placed in the public domain.