[bitcoin-dev] Simplicity: An alternative to Script

Matt Corallo lf-lists at mattcorallo.com
Mon Oct 30 22:14:44 UTC 2017

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


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 at lists.linuxfoundation.org
>> <mailto:bitcoin-dev at 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 at lists.linuxfoundation.org
>> <mailto: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
>>     <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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