On Mon, Nov 19, 2018 at 10:22 PM Pieter Wuille via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> So my question is whether anyone can see ways in which this introduces
> redundant flexibility, or misses obvious use cases?
>

Hopefully my comment is on-topic for this thread:

Given that we want to move away from OP_CODESEPARATOR, because each call to
this operation effectively takes O(script-size) time, we need a replacement
for the functionality it currently provides.  While perhaps the original
motivation for OP_CODESEPARTOR is surrounded in mystery, it currently can
be used (or perhaps abused) for the task of creating signature that covers,
not only which input is being signed, but which specific branch within that
input Script code is being signed for.

For example, one can place an OP_CODESEPARATOR within each branch of an IF
block, or by placing an OP_CODESEPARATOR before each OP_CHECKSIG
operation.  By doing so, signatures created for one clause cannot be used
as signatures for another clause.  Since different clauses in Bitcoin
Script may be enforcing different conditions (such as different time-locks,
hash-locks, etc), it is useful to be able to sign in such a way that your
signature is only valid when the conditions for a particular branch are
satisfied.  In complex Scripts, it may not be practical or possible to use
different public keys for every different clause. (In practice, you will be
able to get away with fewer OP_CODESEPARATORS than one in every IF block).

One suggestion I heard (I think I heard it from Pieter) to achieve the
above is to add an internal counter that increments on every control flow
operator, OP_IF, OP_NOTIF, OP_ELSE, OP_ENDIF, and have the signature cover
the value of this counter.  Equivalently we divide every Bitcoin Script
program into blocks deliminated by these control flow operator and have the
signature cover the index of the block that the OP_CHECKSIG occurs within.
More specifically, we will want a SigHash flag to enables/disable the
signature covering this counter.

There are many different ways one might go about replacing the remaining
useful behaviour of OP_CODESEPARATOR than the one I gave above. I would be
happy with any solution.