[bitcoin-dev] Simplicity: An alternative to Script

Mark Friedenbach mark at friedenbach.org
Tue Oct 31 20:46:49 UTC 2017

Nit, but if you go down that specific path I would suggest making just
the jet itself fail-open. That way you are not so limited in requiring
validation of the full contract -- one party can verify simply that
whatever condition they care about holds on reaching that part of the
contract. E.g. maybe their signature is needed at the top level, and
then they don't care what further restrictions are placed.

On Tue, Oct 31, 2017 at 1:38 PM, Russell O'Connor via bitcoin-dev
<bitcoin-dev at lists.linuxfoundation.org> wrote:
> (sorry, I forgot to reply-all earlier)
> The very short answer to this question is that I plan on using Luke's
> fail-success-on-unknown-operation in Simplicity.  This is something that
> isn't detailed at all in the paper.
> The plan is that discounted jets will be explicitly labeled as jets in the
> commitment.  If you can provide a Merkle path from the root to a node that
> is an explicit jet, but that jet isn't among the finite number of known
> discounted jets, then the script is automatically successful (making it
> anyone-can-spend).  When new jets are wanted they can be soft-forked into
> the protocol (for example if we get a suitable quantum-resistant digital
> signature scheme) and the list of known discounted jets grows.  Old nodes
> get a merkle path to the new jet, which they view as an unknown jet, and
> allow the transaction as a anyone-can-spend transaction.  New nodes see a
> regular Simplicity redemption.  (I haven't worked out the details of how the
> P2P protocol will negotiate with old nodes, but I don't forsee any
> problems.)
> Note that this implies that you should never participate in any Simplicity
> contract where you don't get access to the entire source code of all
> branches to check that it doesn't have an unknown jet.
> On Mon, Oct 30, 2017 at 5:42 PM, Matt Corallo <lf-lists at mattcorallo.com>
> wrote:
>> I admittedly haven't had a chance to read the paper in full details, but I
>> was curious how you propose dealing with "jets" in something like Bitcoin.
>> AFAIU, other similar systems are left doing hard-forks to reduce the
>> sigops/weight/fee-cost of transactions every time they want to add useful
>> optimized drop-ins. For obvious reasons, this seems rather impractical and a
>> potentially critical barrier to adoption of such optimized drop-ins, which I
>> imagine would be required to do any new cryptographic algorithms due to the
>> significant fee cost of interpreting such things.
>> Is there some insight I'm missing here?
>> Matt
>> On October 30, 2017 11:22:20 AM EDT, Russell O'Connor via bitcoin-dev
>> <bitcoin-dev at lists.linuxfoundation.org> wrote:
>>> I've been working on the design and implementation of an alternative to
>>> Bitcoin Script, which I call Simplicity.  Today, I am presenting my design
>>> at the PLAS 2017 Workshop on Programming Languages and Analysis for
>>> Security.  You find a copy of my Simplicity paper at
>>> https://blockstream.com/simplicity.pdf
>>> Simplicity is a low-level, typed, functional, native MAST language where
>>> programs are built from basic combinators.  Like Bitcoin Script, Simplicity
>>> is designed to operate at the consensus layer.  While one can write
>>> Simplicity by hand, it is expected to be the target of one, or multiple,
>>> front-end languages.
>>> Simplicity comes with formal denotational semantics (i.e. semantics of
>>> what programs compute) and formal operational semantics (i.e. semantics of
>>> how programs compute). These are both formalized in the Coq proof assistant
>>> and proven equivalent.
>>> Formal denotational semantics are of limited value unless one can use
>>> them in practice to reason about programs. I've used Simplicity's formal
>>> semantics to prove correct an implementation of the SHA-256 compression
>>> function written in Simplicity.  I have also implemented a variant of ECDSA
>>> signature verification in Simplicity, and plan to formally validate its
>>> correctness along with the associated elliptic curve operations.
>>> Simplicity comes with easy to compute static analyses that can compute
>>> bounds on the space and time resources needed for evaluation.  This is
>>> important for both node operators, so that the costs are knows before
>>> evaluation, and for designing Simplicity programs, so that smart-contract
>>> participants can know the costs of their contract before committing to it.
>>> As a native MAST language, unused branches of Simplicity programs are
>>> pruned at redemption time.  This enhances privacy, reduces the block weight
>>> used, and can reduce space and time resource costs needed for evaluation.
>>> To make Simplicity practical, jets replace common Simplicity expressions
>>> (identified by their MAST root) and directly implement them with C code.  I
>>> anticipate developing a broad set of useful jets covering arithmetic
>>> operations, elliptic curve operations, and cryptographic operations
>>> including hashing and digital signature validation.
>>> The paper I am presenting at PLAS describes only the foundation of the
>>> Simplicity language.  The final design includes extensions not covered in
>>> the paper, including
>>> - full convent support, allowing access to all transaction data.
>>> - support for signature aggregation.
>>> - support for delegation.
>>> Simplicity is still in a research and development phase.  I'm working to
>>> produce a bare-bones SDK that will include
>>> - the formal semantics and correctness proofs in Coq
>>> - a Haskell implementation for constructing Simplicity programs
>>> - and a C interpreter for Simplicity.
>>> After an SDK is complete the next step will be making Simplicity
>>> available in the Elements project so that anyone can start experimenting
>>> with Simplicity in sidechains. Only after extensive vetting would it be
>>> suitable to consider Simplicity for inclusion in Bitcoin.
>>> Simplicity has a long ways to go still, and this work is not intended to
>>> delay consideration of the various Merkelized Script proposals that are
>>> currently ongoing.
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