[bitcoin-dev] Implementing Covenants with OP_CHECKSIGFROMSTACKVERIFY

[Tim Ruffing]

Not a covenant but interesting nevertheless: _One_ of OP_CAT and
OP_CHECKSIGFROMSTACKVERIFY alone is enough to implement "opt-in miner
takes double-spend" [1]:

You can create an output, which is spendable by everybody if you ever
double-spend the output with two different transactions. Then the next
miner will probably take your money (double-spending against your two
or more contradicting transactions again).

If you spend such an output, then the recipient may be willing to
accept a zero-conf transaction, because he knows that you'll lose the
money when you attempt double-spending (unless you are the lucky
miner). See the discussion in [1] for details. 

The implementation using OP_CHECKSIGFROMSTACKVERIFY is straight-
forward. You add a case to the script which allows spending if two
valid signatures on different message under the public key of the
output are given.

What is less known I think:
The same functionality can be achieved in a simpler way just using
OP_CAT, because it's possible to turn Bitcoin's ECDSA to an "opt-in
one-time signature scheme". With OP_CAT, you can create an output that
is only spendable using a signature (r,s) with a specific already fixed
first part r=x_coord(kG). Basically, the creator of this output commits
on r (and k) already when creating the output. Now, signing two
different transaction with the same r allows everybody to extract the
secret key from the two signatures.

The drawbacks of the implementation with OP_CAT is that it's not
possible to make a distinction between legitimate or illegitimate
double-spends (yet to be defined) but just every double-spend is
penalized. Also, it's somewhat hackish and the signer must store k (or
create it deterministically but that's a good idea anyway).

[1] https://www.mail-archive.com/bitcoin-development@lists.sourceforge.net/msg07122.html

Best,
Tim

On Thu, 2016-11-03 at 07:37 +0000, Daniel Robinson via bitcoin-dev
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?
> 
> 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 <bit
> coin-dev at 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 at 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 <bitc
> > > > oin-dev at 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-e
> > > > lements-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
> > > > _______________________________________________
> > > > bitcoin-dev mailing list
> > > > bitcoin-dev at lists.linuxfoundation.org
> > > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
> > 
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