On Thu, Nov 3, 2016 at 3:37 AM, Daniel Robinson <danrobinson010@gmail.com> wrote:

Really cool!

How about "poison transactions," the other covenants use case proposed by Möser, Eyal, and Sirer? (I think OP_CHECKSIGFROMSTACKVERIFY will also make it easier to check fraud proofs, the other prerequisite for poison transactions.)

Seems a little wasteful to do those two "unnecessary" signature checks, and to have to construct the entire transaction data structure, just to verify a single output in the transaction. Any plans to add more flexible introspection opcodes to Elements, such as OP_CHECKOUTPUTVERIFY?

I used to be hesitant to the idea of adding transaction introspection operations, because the script design seemed to be deliberately avoiding doing that.  One of the big takeaways from this work, for me at least, is that since the transaction data is so easily recoverable anyways, adding transaction introspection operations isn't really going to provide any more power to script; it will just save everyone a bunch of work.  There are no specific plans to put transaction introspection opcodes into Elements at this moment, but I feel that the door for that possibility is wide open now.

Really minor nit: "Notice that we have appended 0x83 to the end of the transaction data"—should this say "to the end of the signature"?

On Thu, Nov 3, 2016 at 12:28 AM Russell O'Connor via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

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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