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.