[bitcoin-dev] BIP Proposal: The Great Consensus Cleanup

[Matt Corallo <lf-lists@mattcorallo.com>]

The following is a proposed BIP to soft-fork out some oddities in the 
current Bitcoin consensus rules, resolving several vulnerabilities, in 
addition to fixing the timewarp vulnerability. I'd like to ask the BIP 
editor to assign a BIP number.

The latest version of the BIP can be found at 
https://github.com/TheBlueMatt/bips/blob/cleanup-softfork/bip-XXXX.mediawiki 
(a text copy is included below).

Some things that may be worth discussing:

  * Note that the activation times in this BIP may result in the 
activation of the new soft-fork rules on the same block as the scheduled 
block-subsidy halving. Sadly, avoiding this either requires a 
significantly compressed BIP activation time (which may result in the 
rules not activating for benign reasons) or beginning the activation 
process significantly into the future.

  * The BIP proposes allowing timestamps on the difficulty-adjustment 
block to go backwards by 600 seconds which has the nice property of 
making the difficulty-adjustment algorithm target almost exactly one 
block per 600 seconds in the worst-case (where miners are attempting to 
exploit the timewarp attack), while avoiding any potential hardware 
bricking (assuming upgrades on the part of mining pools). Alternatively, 
some have proposed allowing the time to go backwards 7200 seconds, which 
introduces some small level of inflation in the case of a miner attack 
(though much less than we've had historically simply due to the rapidly 
growing hashrate) but avoids any requirements for upgrades as the 
existing 7200-second-in-the-future check implies miners will only ever 
build on blocks for which they can set the next timestamp to their 
current time.

  * The 4th change (making non-standard signature hash types invalid) 
may be worth discussing. In order to limit the number of potential 
signature hashes which could be used per-input (allowing us to cache 
them to avoid re-calculation), we can disable non-standard sighash 
types. Alternatively, however, most of the same effect could be achieved 
by caching the just-before-the-last-byte sighash midstate and hashing 
only the last byte when a checking signatures. Still, them having been 
non-standard for many years makes me doubt there is much risk involved 
in disabling them, and I don't see much potential use-case for keeping 
them around so I'd like to just remove them.

As for why the timewarp vulnerability should (IMO rather obviously) be 
fixed, it seems rather clear that the only potential use for exploiting 
it would be either to inflate the currency supply maliciously by miners 
or to fork in what amounts to extension blocks. As for why extension 
blocks are almost certainly not the right approach to such changes, its 
likely worth reading this old post: 
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-January/013510.html


<pre>
BIP: XXXX
Layer: Consensus (soft fork)
Title: The Great Consensus Cleanup
Author: Matt Corallo
Status: Draft
Type: Standards Track
Created: 2019-01-28
License: PD
</pre>

==Abstract==

This BIP defines a set of consensus changes which reduce the complexity 
of Bitcoin implementations and improve worst-case validation times, 
fixing a number of long-standing vulnerabilities.

==Motivation==

BIP 143 significantly improved certain aspects of Bitcoin's consensus 
rules, key to this being changes to the format of the data which is 
hashed and signed in CHECKSIG operations during script execution. 
However, several improvements were left for later forks to avoid 
bloating the original activation with unrelated changes. This BIP seeks 
to make some of these changes as well as a few other simplifications. 
Specifically, this BIP proposes the following changes:

* Worst-case validation time for non-BIP 143 transactions has long been 
considered a significant vulnerability. To address this, both 
OP_CODESEPARATOR in non-BIP 143 scripts and FindAndDelete fail script 
validation, among other cleanups. This drastically reduces worst-case 
validation time for non-BIP 143 transactions by enabling Signature Hash 
caching on a per-input basis. While validation time of large, simple 
non-BIP 143 transactions can still be excessively high on their own, 
removing these multipliers goes a long way towards resolving the issue.

* By further restricting nTime fields on difficulty adjustment blocks, 
we propose fixing the long-standing "timewarp" inflation vulnerability 
in Bitcoin's difficulty adjustment without risking existing mining 
hardware becoming unusable. This limits the worst-case difficulty 
adjustment target in case of attack from the current exponential growth, 
to once every roughly 600 seconds. Note that no change in default 
behavior is proposed, keeping the existing target of one block every 
~600.6 seconds[1] in the common case (ie we limit the attack scenario to 
about a 0.1% inflation rate, much smaller than the historical inflation 
rate due to rapid hashrate growth).

* Several vulnerabilities where Bitcoin clients needed to check for 
specific cases of malleation in the merkle tree construction are 
resolved by making certain transaction sizes invalid.

==Specification==

Upon activation, the following rules will be enforced on all new blocks:

* scriptSigs which contain non-push opcodes fail the script validation. 
Push opcodes are OP_0 - OP_1NEGATE and OP_1 - OP_16. Note that this 
implies any opcodes in scriptSigs greater than 0x60 will fail script 
validation, in addition to OP_RESERVED (0x50, which already fails script 
execution in executed branches, though all branches are now guaranteed 
to execute).

* OP_CODESEPARATOR in non-BIP 143 scripts fails the script validation. 
This includes OP_CODESEPARATORs in unexecuted branches of if statements, 
similar to other disabled opcodes, but unlike OP_RETURN.

* When validating signatures in non-BIP 143 scripts, if the scriptPubKey 
being executed contains, pushed as a single element using minimal 
PUSHDATA, a signature stack element being validated, the script fails 
validation. For the avoidance of doubt, any FindAndDelete matches result 
in script execution failure.

* If the sighash type byte (ie last byte in a signature being evaluated 
during the execution of OP_CHECKSIG[VERIFY] or OP_CHECKMULTISIG[VERIFY]) 
is anything other than 1, 2, 3, 0x81, 0x82, or 0x83, the script 
execution fails. This does not apply to 0-length signature stack elements.

* Transactions smaller than 65 bytes when serialized without witness 
data are invalid.

* The nTime field of each block whose height, mod 2016, is 0 must be 
greater than or equal to the nTime field of the immediately prior block 
minus 600. For the avoidance of doubt, such blocks must still comply 
with existing Median-Time-Past nTime restrictions.

==Deployment==

This BIP will be deployed by "version bits" BIP9 with the name 
"cleanups" and using bit (0-indexed) 3.

For Bitcoin mainnet, the BIP9 starttime will be midnight August 1st, 
2019 UTC (Epoch timestamp 1564617600) and BIP9 timeout will be midnight 
August 1st, 2020 UTC (Epoch timestamp 1596240000).

For Bitcoin testnet, the BIP9 starttime will be midnight June 1st, 2019 
UTC (Epoch timestamp 1559347200) and BIP9 timeout will be midnight June 
1st, 2020 UTC (Epoch timestamp 1590969600).

==Discussion==

* There are very few known uses for OP_CODESEPARATOR and none for 
FindAndDelete. None of these uses enable new functionality, and any 
efficiency gains are better made by switching to BIP 141. Further, there 
is no known use of either on the chain today, and both have been 
non-standard in Bitcoin Core since version 0.16.1, making them much more 
difficult to have mined. Both changes, together, allow for signature 
hash caching within each input script in a non-BIP 143 transaction 
today. Note that due to their non-standardness, miners using Bitcoin 
Core version 0.16.1 or later will not mine blocks which violate these 
rules today.
* Reducing valid scriptSigs to the minimal set of operations which can 
generate any stack state removes the requirement that scriptCodes need 
to be generated for scriptSig execution, reducing the possible set of 
scriptCodes which must be cached per input by 2x. Because any stack 
state can be created using only push opcodes, this does not reduce 
spendability except for pessimal scriptPubKeys which require a 
significant number of identical stack elements (ie created using 
OP_DUP). Note that such transactions have been non-standard in Bitcoin 
Core since before git history (SVN 197) and thus miners running Bitcoin 
Core will not mine such transactions today.
* Further, disabling non-canonical sighash types allows caching of the 
sighash themselves instead of midstates (as the sighash type byte is 
included in the sighash itself). Avoiding applying this rule to 0-length 
signatures avoids breaking deliberate OP_CHECKSIG failures while still 
avoiding having to ever calculate such sighashes. Such sighashes have 
been non-standard and thus miners using Bitcoin Core version 0.8 or 
higher will not mine blocks containing such transactions today.
<br/>
* While there are no known attempts to exploit the "timewarp" 
vulnerability on Bitcoin's mainnet today, and the authors do not believe 
it is likely to occur in the immediate future, removing the possibility 
has long been on various wishlists and greatly simplifies potential 
attack analysis.
** Sadly, some deployed mining hardware relies on the ability to roll 
nTime forward by up to 600 seconds[3]. Thus, only requiring that the 
nTime field move forward during difficulty adjustment would allow a 
malicious miner to prevent some competitors from mining the next block 
by setting their timestamp to two hours in the future. Thus, we allow 
nTime to go backwards by 600 seconds, ensuring that even a block with a 
timestamp two hours in the future allows for 600 seconds of nTime 
rolling on the next block.
** Note that miners today only enforce increasing timestamps against the 
median-timestamp-of-last-11-blocks, so miners who do not upgrade may 
mine a block which violates this rule at the beginning of a difficulty 
window if the last block in a difficulty window has a timestamp in the 
future. Thus, it is strongly recommended that SPV clients enforce the 
new nTime rules to avoid following any potential forks which occur.
<br/>
* The issues involved in having leaf nodes in the transaction merkle 
tree which can be confused for inner nodes are well documented. 
[4][5][6] While there are workarounds for the pitfalls, there are many 
SPV-proof-validators which do not implement them. Further, the limited 
use-cases for very small transactions does not suffice as reason to 
force the added complexity onto clients. Note that any transactions 
smaller than 83 bytes have been considered non-standard since Bitcoin 
Core version 0.17.0, so miners will not mine blocks which validate this 
rule by default.
<br/>
* There are several early-stage proposals which may affect the execution 
of scripts, including proposals such as Schnorr signatures, Taproot, 
Graftroot, and MAST. These proposals are not expected to have any 
interaction with the changes in this BIP, as they are likely to only 
apply to SegWit scripts, which are not covered by any of the new rules 
except for the sighash type byte rule. Thus, the sighash type byte rule 
defined above only applies to *current* signature-checking opcodes, as 
any new signature-checking is likely to be implemented via the 
introduction of new opcodes.
<br/>
* In spite of some suggestion that other activation methods be used, BIP 
9 is proposed as ensuring miners have upgraded to enforce new rules is 
an important part of minimizing disruption. While previous BIP 9 
soft-forks have resulted in political contention, this 
comparatively-unimportant soft-fork provides a good opportunity to 
attempt to return to utilizing BIP 9 to ensure miner upgrade prior to 
activation, which the authors believe is a critical goal. However, if 
there is broad agreement to activate these rules when the BIP 9 expiry 
time is reached, and miners have not yet signaled sufficient level of 
readiness, a later flag-day activation may be merited. For this reason, 
implementations may wish to provide a compatibility option which allows 
flag-day enforcement of these rules without an update.

==Reference Implementation==

[https://github.com/bitcoin/bitcoin/pull/15482 Bitcoin Core Pull #15482]

==References==

[1] The difficulty adjustment algorithm in Bitcoin multiplies the 
previous difficulty by (2016 / time taken to mine the last 2015 blocks). 
Intuitively[2], this implies the actual Inter-Block-Time (IBT) target is 
2016/2015*600, or about 600.3 seconds. However, the expected value of 
the inverse of an Erlang distribution (which the above is effectively 
sampling from) is actually 1/(N-1), not 1/N. Thus, the above expression 
actually targets an IBT of 2016/2014*600, or about 600.6 seconds, ie 
E(2016*600/X) = 1 where X~ErlangDistribution(k=2015, λ=1/IBT) when IBT 
is 2016/2014*600. This is equivalent to 600*E(2016*600/X) where 
X~ErlangDistribution(k=2015, λ=1/600). In the case of a miner 
deliberately reducing timestamps by 600 seconds on the 
difficulty-retargeting block, we are effectively changing the difficulty 
multiplier to (2016 / (time taken to mine the last 2016 blocks + 600)), 
or 600*E(2016*600/(X + 600)) where X~Erlang Distribution(k=2016, 
λ=1/600), which is effectively targeting an inter-block time of 
~599.9999 seconds.

[2] See [https://twitter.com/pwuille/status/1098288749098795008] for 
most peoples' intuition. For more info see Pieter's writeup at 
[https://gist.github.com/sipa/1a70884abe6d0a7cddc340c99f741a41]

[3] While no official stratum specification exists, the btc.com pool 
server (one of the most popular pool servers today) rejects shares with 
timestamps more than 600 seconds in the future at 
[https://github.com/btccom/btcpool/blob/e7c536834fd6785af7d7d68ff29111ed81209cdf/src/bitcoin/StratumServerBitcoin.cc#L384]. 
While there are few resources describing hardware operation today, 
timestamp rolling can be observed on the chain (in some rare cases) as 
block timestamps go backwards when a miner rolled one block nTime 
forward and the next does not, but only incredibly rarely more than 600 
seconds.

[4] 
[https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-June/016091.html]

[5] 
[https://lists.linuxfoundation.org/pipermail/bitcoin-dev/attachments/20180609/9f4f5b1f/attachment-0001.pdf]

[6] 
[https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-February/016697.html]

==Acknowledgments==

Thanks (in alphabetical order) to Suhas Daftuar, James Hilliard, Johnson 
Lau, Steve Lee, Greg Maxwell, John Newberry, and Pieter Wuille for their 
helpful feedback at various stages as well as the entire Bitcoin 
Protocol Development Community.