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NethermindEth/LeanCIL

A Lean 4 framework for machine-checked proofs of correctness and memory safety against compiled .NET CIL from C#, F#, and other .NET languages.

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What is it?

LeanCIL extracts Common Intermediate Language from compiled .NET assemblies and checks Lean 4 proofs against an explicit model of those instructions. Proofs can cover every input satisfying a contract, loop termination, checked memory access, and counterexamples to incorrect implementations. Previously proved method contracts can be composed without unfolding helper bodies.

The workflow has a CLI with commands to list methods, extract a method and its reachable calls, check extraction artifacts, and verify a named theorem with kernel replay and an axiom audit. Successful verification writes a verified.json report tied to the SHA-256 of the assembly. Consumer packages let application specifications and proofs live in a separate Lean project pinned to LeanCIL.

The supported subset covers integer arithmetic, selected control flow, managed calls, reference-free structs, and arrays and spans over supplied storage. Floating-point arithmetic, general class allocation, virtual dispatch, ordinary recursion, and exception handlers are outside the model. The project does not claim complete CLR or JIT verification.

Key features

  • 01Proves functional properties for all inputs satisfying a contract, using Lean 4 proofs.
  • 02Extracts and validates CIL from compiled C#, F# and other .NET assemblies.
  • 03Checks memory access, bounds, initialization and alignment in a checked memory model.
  • 04Composes previously proved method contracts without unfolding helper bodies.
  • 05Produces a verified.json report bound to the assembly's SHA-256 hash.

Use cases

  • Proving that a .NET arithmetic or buffer routine behaves correctly for all inputs.
  • Checking that a loop over a span terminates and leaves memory outside its writable region unchanged.
  • Finding counterexamples that show an implementation violates its intended specification.

Quick start

  1. 1

    Install Lean 4.35.0 via elan and .NET SDK 10.0.403 from the checkout

  2. 2

    Build the Lean project from the checkout

    lake build
  3. 3

    Pack the Extractor as a local NuGet package

    dotnet pack Extractor -c Release -o artifacts/packages
  4. 4

    Install the leancil global tool from the local package

    dotnet tool install --global Nethermind.LeanCIL --add-source artifacts/packages --version 0.1.0

Things to know

  • Supports only a restricted subset of CIL; general class allocation, virtual dispatch, exception handlers and floating-point arithmetic are unsupported.
  • Requires both a Lean 4 toolchain and the .NET SDK, with specific pinned versions.
  • Early-stage project at version 0.1.0 with a small community.

Topics

#formal-verification#lean4#dotnet#cil#theorem-proving#memory-safety

FAQ

What is LeanCIL?
A Lean 4 framework for machine-checked proofs of correctness and memory safety against compiled .NET CIL from C#, F#, and other .NET languages. LeanCIL extracts Common Intermediate Language from compiled .NET assemblies and checks Lean 4 proofs against an explicit model of those instructions. Proofs can cover every input satisfying a contract, loop termination, checked memory access, and counterexamples to incorrect implementations. Previously proved method contracts can be composed without unfolding helper bodies.
Is LeanCIL free to use? What license does it have?
LeanCIL is open source under the Other license. Check the license terms before using it in commercial products.
Is LeanCIL still maintained?
Actively maintained. The last commit to LeanCIL was on Nov 5, 2026.
What can I use LeanCIL for?
Proving that a .NET arithmetic or buffer routine behaves correctly for all inputs. Checking that a loop over a span terminates and leaves memory outside its writable region unchanged. Finding counterexamples that show an implementation violates its intended specification.
How do I install LeanCIL?
Install Lean 4.35.0 via elan and .NET SDK 10.0.403 from the checkout Build the Lean project from the checkout: lake build Pack the Extractor as a local NuGet package: dotnet pack Extractor -c Release -o artifacts/packages Install the leancil global tool from the local package: dotnet tool install --global Nethermind.LeanCIL --add-source artifacts/packages --version 0.1.0
Where can I get LeanCIL?
The source code and installation instructions are on GitHub: https://github.com/NethermindEth/LeanCIL

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