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From: Luke Dashjr <luke@dashjr.org>
To: Rusty Russell <rusty@rustcorp.com.au>
Date: Sat, 1 Oct 2016 05:02:09 +0000
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] BIP draft: OP_CHECKBLOCKATHEIGHT
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On Saturday, October 01, 2016 4:01:04 AM Rusty Russell wrote:
> Prefer a three-arg version (gbits-to-compare, blocknum, hash):
> - If <bits> is 0 or > 256, invalid.
> - If the hash length is not (<bits> + 7) / 8, invalid.

This means zero padding on-chain, which would be undesirable.
Rather "at most" and have the consensus implementation do the padding.

> - If the hash unused bits are not 0, invalid.

Why?

> - Otherwise <bits> of hash is compared to lower <bits> of blockhash.

Lower in what endian? Why only that endian? Why only lower? I can see a 
possible use case where one wants to look at only the high bits to ensure 
their transaction is only valid in a block with at least a certain 
difficulty...

> This version also lets you play gambling games on-chain!
> 
> Or maybe I've just put another nail in CBAH's coffin?

Or maybe resurrected it...

Luke