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

From: Peter Todd <[email-masked]>
Date: Sun, 28 Sep 2014 22:35:53 -0400

On Thu, Jun 19, 2014 at 09:54:31AM -0400, Gavin Andresen wrote:
> RE: soft-forks bumping version numbers:
> 
> Yes, we have consensus that is the way we will do it.  I should probably
> turn https://gist.github.com/gavinandresen/2355445 into an informational
> BIP.

That gist is mistaken. To see the problem consider the "re-define
OP_NOP1 to be OP_Q_CHECKSIGVERIFY" example. It says "Transactions that
use the new opcode are given a new version number." The problem is in
the definition of "use"

Lets first suppose that the evaluation of a scriptPubKey was done
according to the transaction version # of the transaction creating the
scriptPubKey. This is technically feasible as the UTXO set records the
version of the transaction creating the txout. However if I create a
P2SH address whose redeemScript makes use of a new opcode - say the new
OP_Q_CHECKSIGVERIFY - non-upgraded clients sending funds to that address
will be creating scriptPubKeys whose contents can be spent by anyone.
Not exactly ideal! 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.

On the other hand suppose scriptPubKey evaluation is done according to
the version of the transaction spending the scriptPubKey. This is
insecure as now transaction outputs using the new opcode can be
trivially spent by just spending them in a transaction with the previous
version number; the OP_Q_CHECKSIGVERIFY is evaluated as OP_NOP and
checks nothing.

If txouts be spent only by transactions with nVersion >= the nVersion of
the transaction spending them, but again you're forced to upgrade the
address format for every new opcode. Interestingly this shows that the
common assertion that "P2SH should have been done by upgrading the tx
version #" is in fact wrong except for the fact that P2SH required an
address format upgrade anyway; doing that for future opcode upgrades
would be a mistake.

With the above in mind the "Relax IsStandard rules for P2SH
transactions" pull-req(1) is incomplete as it doesn't blacklist usage of
the upgradable NOPx opcodes. After a future soft-fork redefining a NOPx
opcodes' behavior non-upgraded nodes will accept and mine transactions
that may now be invalid, the latter creating invalid blocks and thus
false confirmations. I've created a pull-req to fix this issue by
specifically blacklisting the NOPx opcodes if they are executed:

    https://github.com/bitcoin/bitcoin/pull/5000

Secondly the "Blockchain Rule Update Process" gist above should be
rewritten to say that new opcodes will be enabled for all scripts by the
block nVersion upgrade mechanism; scripts must never depend on a NOPx
opcode being executed least they be rendered unspendable by a future
upgrade.

By comparison BIP62 proposes that the transaction version # be increased
to indicate that the sender wants anti-malleability rules to be applied.
This is an appropriate usage of tx version numbers as in this case the
person creating the transaction wants the anti-malleability rules
applied; the creator of the scriptPubKey's being spent does not care
whether or not they are spent in a transaction that is or is not
malleable. Equally the new owners of the txouts being created don't in
general care how they were created. (modulo certain special-purpose
protocols where they would have the transaction anyway)

1) https://github.com/bitcoin/bitcoin/pull/4365

-- 
'peter'[:-1]@petertodd.org
000000000000000005ec432fd4c506119a0123a028911fa3981bcde0568b66ab