From: Rusty Russell <rusty@rustcorp.com.au>
To: Russell O'Connor <roconnor@blockstream.io>,
	Pieter Wuille <pieter.wuille@gmail.com>
In-Reply-To: <CAMZUoKn4noCEQR6eqf9hiZSMdk-3b8UHR1NrEFrKNoLSMzVjGQ@mail.gmail.com>
References: <87608btgyd.fsf@rustcorp.com.au>
	<DB7E57AC-5588-4BBA-9ABC-B9B4F6BAECE2@friedenbach.org>
	<CAPg+sBgRrqZryiETZYCWiqHNOmN2bCfsvr30znjN-gKiU5Vfjg@mail.gmail.com>
	<CAMZUoKn4noCEQR6eqf9hiZSMdk-3b8UHR1NrEFrKNoLSMzVjGQ@mail.gmail.com>
Date: Tue, 16 Jan 2018 11:36:14 +1030
Message-ID: <87zi5ehat5.fsf@rustcorp.com.au>
MIME-Version: 1.0
Content-Type: text/plain
Cc: Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>,
	Russell O'Connor <roconnor@blockstream.com>,
	Kalle Alm <kalle.alm@gmail.com>
Subject: Re: [bitcoin-dev] BIP 117 Feedback
Precedence: list

"Russell O'Connor" <roconnor@blockstream.io> writes:
> However, if I understand correctly, the situation for BIP 117 is entirely
> different.  As far as I understand there is currently no restrictions about
> terminating a v0 witness program with a non-empty alt-stack, and there are
> no restrictions on leaving non-canonical boolean values on the main stack.

BIP-141: "The script must not fail, and result in exactly a single TRUE
on the stack."  And it has long been non-standard for P2SH scripts to
not do the same (don't know exactly when).

> There could already be commitments to V0 witness programs that, when
> executed in some reasonable context, leave a non-empty alt-stack and/or
> leave a non-canonical true value on the main stack.  Unlike the P2SH or
> Segwit soft-forks, these existing commitments could be real outputs that
> currently confer non-trivial ownership over their associated funds.  If BIP
> 117 were activated, these commitments would be subject to a new set of
> rules that didn't exist when the commitments were made.  In particular,
> these funds could be rendered unspendable.  Because segwit commitments are
> hashes of segwit programs, there is no way to even analyze the blockchain
> to determine if these commitments currently exist (and even if we could it
> probably woudln't be adequate protection).

The rule AFAICT is "standard transactions must still work".  This was
violated with low-S, but the transformation was arguably trivial.  

OTOH, use of altstack is completely standard, though in practice it's
unused and so only a theoretical concern.

My concern remains unanswered: I want hard numbers on the worst-case
time taken by sigops with the limit removed.  It's about 120 usec per
sigop (from [1]), so how bad could it be?  I think Russell had an
estimate like 1 in 3 ops, so 160 seconds to validate a block?

Thanks,
Rusty.
[1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-December/015346.html