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.