Subject: Re: [Bitcoin-development] New opcodes and transaction version numbers (was 'relax the IsStandard rules for P2SH transactions')

From: Peter Todd <[email-masked]>
Date: Mon, 29 Sep 2014 01:35:26 -0400

On Mon, Sep 29, 2014 at 12:30:11AM -0400, Alan Reiner wrote:
> On 09/28/2014 10:35 PM, Peter Todd wrote:
> > This can be solved by upgrading the address format at
> > the same time to let senders know they must send the funds in a
> > transaction with an increased version number, but obviously needing new
> > addresses for every new opcode defeats the purpose of P2SH.
> 
> Can't this be solved with a single update to the address format,
> allowing a tx version number to be part of the address serialization? 
> Then the sending software will apply that version to the payment tx.   
> Of course, I'm not sure if allowing nodes to create transactions with
> version numbers outside of their programming is safe.  It seems like it
> should be since we're talking about soft forks anyway, but there's
> probably some subtleties I'm overlooking.

Keep in mind that when a *wallet* - not a node - creates a transaction
the scriptPubKeys in the transaction outputs have been specified by the
receiver(s) and aren't executed until they are spent. Modulo sigops(1)
there is absolutely no reason why the wallet should care what the
contents of those scriptPubKeys are at all.

This is particularly apparent when you remember that there may be
multiple recipients of a transaction. If I'm paying Alice and Bob, who
have specified that they want the transaction to have version number 2
and 3 respectively, now what? Do we take the highest of the two and
constrain ourselves for how scripts are interpreted for all eternity? It
just doesn't make very much sense.

Meanwhile the man-hours of effort that would be required to implement
that "one-time" address format change is huge - it took literally years
for everyone to update their software to just support P2SH addresses.
I'm working on a CHECKLOCKTIMEVERIFY implementation right now, and know
of exchanges who would like to use it ASAP. Why make them wait years for
everyone to upgrade?

On that basis alone I think the question ought to be why should we use
transaction version numbers to enable new opcodes rather than just
enabling them globally based on block version numbers.

1) Satoshi implemented a per-block sigop limit to prevent blocks from
causing an unreasonable number of signature checking operations, but
rather than computing that limit based on the scripts actually executed,
the limit is computed based on the contents of all scriptSigs and
scriptPubKeys in the block. This is bizzare given that the contents of
the latter are *not* executed, and the former misses the prevout
scriptPubKeys that *are* executed. In short this means you can create a
block that passes the sigop limit, yet executes millions of expensive
signature operations by being filled with scriptSigs spending txouts
with large numbers of sigops in their scriptPubKeys. P2SH improves on
this situation somewhat by counting the sigops in redeemScripts towards
the limit, but missed the opportunity to just count all sigops in all
scriptSigs directly.

-- 
'peter'[:-1]@petertodd.org
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