Right.  There are minor trade-offs to be made with regards to that design point of OP_CHECKSIGFROMSTACKVERIFY.  Fortunately this covenant construction isn't sensitive to that choice and can be made to work with either implementation of OP_CHECKSIGFROMSTACKVERIFY.

On Wed, Nov 2, 2016 at 11:35 PM, Johnson Lau <jl2012@xbt.hk> wrote:

Interesting. I have implemented OP_CHECKSIGFROMSTACKVERIFY in a different way from the Elements. Instead of hashing the data on stack, I directly put the 32 byte hash to the stack. This should be more flexible as not every system are using double-SHA256

https://github.com/jl2012/bitcoin/commits/mast_v3_master

On 3 Nov 2016, at 01:30, Russell O'Connor via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

Hi all,

It is possible to implement covenants using two script extensions: OP_CAT and OP_CHECKSIGFROMSTACKVERIFY.  Both of these op codes are already available in the Elements Alpha sidechain, so it is possible to construct covenants in Elements Alpha today.  I have detailed how the construction works in a blog post at <https://blockstream.com/2016/11/02/covenants-in-elements-alpha.html>.  As an example, I've constructed scripts for the Moeser-Eyal-Sirer vault.

I'm interested in collecting and implementing other useful covenants, so if people have ideas, please post them.

If there are any questions, I'd be happy to answer. 

--
Russell O'Connor

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