Re: [bitcoin-dev] Simplicity: An alternative to Script

From: Mark Friedenbach <mark@friedenbach•org>
To: Matt Corallo <lf-lists@mattcorallo•com>
Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Simplicity: An alternative to Script
Date: Mon, 30 Oct 2017 15:32:42 -0700	[thread overview]
Message-ID: <6E10759E-DCEC-4C13-AD23-7DEE1BC20311@friedenbach.org> (raw)
In-Reply-To: <1689d7c6-7c32-aa78-6626-c344f19923de@mattcorallo.com>

I was just making a factual observation/correction. This is Russell’s project and I don’t want to speak for him. Personally I don’t think the particulars of bitcoin integration design space have been thoroughly explored enough to predict the exact approach that will be used.

It is possible to support a standard library of jets that are general purpose enough to allow the validation of new crypto primitives, like reusing sha2 to make Lamport signatures. Or use curve-agnostic jets to do Weil pairing validation. Or string manipulation and serialization jets to implement covenants. So I don’t think the situation is as dire as you make it sound.

> On Oct 30, 2017, at 3:14 PM, Matt Corallo <lf-lists@mattcorallo•com> wrote:
> 
> Are you anticipating it will be reasonably possible to execute more
> complicated things in interpreted form even after "jets" are put in
> place? If not its just a soft-fork to add new script operations and
> going through the effort of making them compatible with existing code
> and using a full 32 byte hash to represent them seems wasteful - might
> as well just add a "SHA256 opcode".
> 
> Either way it sounds like you're assuming a pretty aggressive soft-fork
> cadence? I'm not sure if that's so practical right now (or are you
> thinking it would be more practical if things were
> drop-in-formally-verified-equivalent-replacements?).
> 
> Matt
> 
>> On 10/30/17 17:56, Mark Friedenbach wrote:
>> Script versions makes this no longer a hard-fork to do. The script
>> version would implicitly encode which jets are optimized, and what their
>> optimized cost is.
>> 
>>> On Oct 30, 2017, at 2:42 PM, Matt Corallo via bitcoin-dev
>>> <bitcoin-dev@lists•linuxfoundation.org
>>> <mailto:bitcoin-dev@lists•linuxfoundation.org>> 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@lists•linuxfoundation.org
>>> <mailto:bitcoin-dev@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
>>>    <http://plas2017.cse.buffalo.edu/> on Programming Languages and
>>>    Analysis for Security.  You find a copy of my Simplicity paper at
>>>    https://blockstream.com/simplicity.pdf
>>>    <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 <https://elementsproject.org/>
>>>    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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>> 