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+Cc: Bitcoin Development <bitcoin-development@lists.sourceforge.net>,

+ Flavien Charlon <flavien.charlon@coinprism.com>

+Subject: Re: [Bitcoin-development] [BIP draft] CHECKLOCKTIMEVERIFY - Prevent

+ a txout from being spent until an expiration time

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+

+On Fri, Oct 3, 2014 at 7:28 AM, Matt Whitlock <bip@mattwhitlock.name> wrote:

+> Is there a reason why we can't have the new opcode simply replace the top stack item with the block height of the txout being redeemed?

+

+This would not be soft-forking compatible.

+

+It also would be unsafe in that it would result in transactions which

+once mined could not be restored in a reorg through no fault of the

+participants, which makes the coins less fungible and differently safe

+to accept. It risks creating weird pressures around immediate block

+admission since a one additional block delay could forever censor such

+a transaction (E.g. increases the power of single miners to censor or

+steal). Avoiding this is a conscious decision in Bitcoin and also part

+of the justification for the 100 block maturity of newly generated

+coins.

+

+It also would require violating the script/transaction/block layering

+more substantially, complicating implementations, and making the

+validity of a script no longer a deterministic pure function of the

+transaction.

+

+Avoiding these issues is a conscious design in OP_CHECKLOCKTIMEVERIFY.

+I would strenuously oppose a proposal which failed in any of these

+respects.

+

+> Then arbitrary logic could be implemented, including "output cannot be spent until a certain time" and also "output can ONLY be spent until a certain time," as well as complex logic with alternative key groups with differing time constraints.

+

+You can already achieve the not spendable after logic with a

+cancellation spend that moves the coin in the usual way. (Which

+doesn't even require the participant be online, with the help of some

+network service to queue unlocked transactions).

+

+> OP_CHECKLOCKTIMEVERIFY, as conceived, seems too limited, IMHO.

+

+It is intentionally so, and yet it covers the intended use cases;

+including ones with alternative key groups, they are just not

+exclusive.

+

+