In this blog post, we present the formal verification effort we started to show the absence of bugs in the ⚈ Smoo.th library, a library for optimized 〰️ elliptic curve operations in Solidity. We are using our tool coq-of-solidity to make this non-trivial verification using the generic proof assistant 🐓 Coq.

The Smoo.th library is interesting as elliptic curves are at the core of many cryptographic protocols, including authentication protocols, and having a generic and fast implementation simplifies the development of dApps in environments with missing pre-compiled (like L1s) or missing circuits (like zero-knowledge layers).

From a verification point of view, it is very challenging as it combines low-level operations (hand-optimized Yul code with bit shifts, inlined functions, ...) with higher-level reasoning on elliptic curves and arithmetic 💪.

We present improvements we made to our tool coq-of-solidity to formally verify Solidity smart contracts for any advanced properties, relying on the proof assistant 🐓 Coq. The idea is to be able to prove the full absence of bugs ✨ in very complex contracts, like L1 verifiers for zero-knowledge L2s 🕵️, or contracts with very large amounts of money 💰 (in the billions).

In this blog post, we present how we developed an effect inference mechanism to translate optimized Yul code combining variable mutations and control flow with loops and nested premature returns (break, continue, and leave) to a clean 🧼 purely functional representation in the proof system Coq.

In the previous blog post, we have seen how we represent side-effects from the Rust code of the Sui's Move type-checker of bytecode in Coq. This translation represents about 3,200 lines of Coq code excluding comments. We need to trust that this translation is faithful to the original Rust code, as we generate it by hand or with GitHub Copilot.

In this blog post, we present how we test this translation to ensure it is correct by running the type-checker on each opcode of the Move bytecode and comparing the results with the Rust code, testing the success and error cases.

We are working on formally verifying the 🦀 Rust implementation of the Move type-checker for bytecode in the proof system 🐓 Coq. You can find the code of this type-checker in the crate move-bytecode-verifier.

This requires translating all the Rust code in idiomatic Coq on which we will write our specifications and proofs. We write this translation by hand relying as much as possible on generative AI tools such as GitHub Copilot, as there are many particular cases. We plan, eventually, to prove it equivalent to the translation automatically generated by coq-of-rust.

In this blog post we present how we organize our 🔮 monad to represent the side-effects used in this Rust code. We believe this organization should work for other Rust projects as well.

In this blog post, we present what we do at Formal Land 🌲, what tools and services we are developing to provide more security for our customers 🦸. We believe that for critical applications such as blockchains (L1, L2, dApps) you should always use the most advanced technologies to find bugs, otherwise bad actors will do and overtake you in the never-ending race for security 🏎️.

Formal verification is one of the best techniques to ensure that your code is correct, as it checks every possible input ✨ of your program. For a long, formal verification was reserved for specific fields, such as the space industry 🧑‍🚀. We are making this technology accessible for the blockchain industry and general programming thanks to tools and services we develop, like coq-of-solidity and coq-of-rust.

In this blog post, we present our project to formally verify the implementation of the type checker for smart contracts of the 💧 Sui blockchain. The Sui blockchain uses the Move language to express smart contracts. This language is implemented in 🦀 Rust and compiles down to the Move bytecode that is loaded in memory when executing the smart contracts.

We will formally verify the part that checks that the bytecode is well-typed, so that when a smart contract is executed it cannot encounter critical errors. The type checker itself is also written in Rust, and we will verify it using the proof assistant Coq 🐓 and our tool coq-of-rust that translates Rust programs to Coq.

In this blog post we explain how we specify and formally verify a whole ERC-20 smart contract using our tool coq-of-solidity, which translates Solidity code to the proof assistant Coq 🐓.

The proofs are still tedious for now, as there are around 1,000 lines of proofs for 100 lines of Solidity. We plan to automate this work as much as possible in the subsequent iterations of the tool. One good thing about the interactive theorem prover Coq is that we know we can never be stuck, so we can always make progress in our proof techniques and verify complex properties even if it takes time ✨.

Formal verification with an interactive proof assistant is the strongest way to verify programs since:

• it covers all possible inputs and program states,
• it checks any kind of properties.

We continue to strengthen the security of smart contracts with our tool coq-of-solidity 🛠️. It checks for vulnerabilities or bugs in Solidity code. It uses formal verification with an interactive theorem prover (Coq 🐓) to make sure that we cover:

• all possible user inputs/storage states, even if there are infinite possibilities,
• for any security properties.

This is very important as a single bug can lead to the loss of millions of dollars in smart contracts, as we have regularly seen in the past, and we can never be sure that a human review of the code did not miss anything.

Our tool coq-of-solidity is one of the only tools using an interactive theorem prover for Solidity, together with Clear from Nethermind. This might be the most powerful approach to making code without bugs, as exemplified in this PLDI paper comparing the reliability of various C compilers. They found numerous bugs in each compiler except in the formally verified one!

In this blog post we show how we functionally specify and verify the _approve function of an ERC-20 smart contract. We will see how we prove that a refined version of the function is equivalent to the original one.

We continue to work on our open source formal verification tool for Solidity named coq-of-solidity 🛠️. Formal verification is the strongest form of code audits, as we verify that the code behaves correctly in all possible execution cases 🔍. We use the interactive theorem prover Coq to express and verify any kinds of properties.

We work by translating the Yul version of a smart contract to the formal language Coq 🐓, in which we then express the code specifications/security properties and formally verify them 🔄. The Yul language is an intermediate language used by the Solidity compiler and others to generate EVM bytecode. Yul is simpler than Solidity and at a higher level than the EVM bytecode, making it a good target for formal verification.

In this blog post we present the recent developments we made to simplify the reasoning 🧠 about Yul programs once translated in Coq.

Solidity is the most widely used smart contract language on the blockchain. As smart contracts are critical software handling a lot of money, there is a huge interest in finding all possible bugs before putting them into production.

Formal verification is a technique to test a program on all possible entries, even when there are infinitely many. This contrasts with the traditional test techniques, which can only execute a finite set of scenarios. As such, it appears to be an ideal way to bring more security to smart contract audits.

In this blog post, we present the formal verification tool coq-of-solidity that we are developing for Solidity. Its specificities are that:

1. It is open-source (GPL-3 for the translation, MIT for the proofs).
2. It uses an interactive theorem prover, the system Coq, to verify arbitrarily complex properties.

Here, we present how we translate Solidity code into Coq using the intermediate language Yul. We explain the semantics we use and what remains to be done.

The code is available in our fork of the Solidity compiler at github.com/formal-land/coq-of-solidity.