KreMLin is a tool for extracting low-level F* programs to readable C code
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KreMLin is a tool that extracts an F* program to readable C code. If the F* program verifies against a low-level memory model that talks about the stack and the heap; if it is first-order; if it obeys certain restrictions (e.g. non-recursive data types) then KreMLin will turn it into C.

  • has a technical overview of the different transformation passes performed by KreMLin
  • contains some tips&tricks when working with KreMLin.

This work has been formalized on paper. We state that the compilation of such F* programs to C preserves semantics. We start from Low*, a subset of F*, and relate its semantics to CompCert's Clight.

  • the ML Workshop Paper provides a good, short overview
  • the submission provides a more in-depth formalization of our compilation toolchain

We have written 20,000 lines of low-level F* code, implementing the TLS 1.3 record layer. As such, KreMLin is a key component of Project Everest.

  • HACL*, our High Assurance Crypto Library, provides numerous cryptographic primitives written in F*; these primitives enjoy memory safety, functional correctness, and some degree of side-channel resistance -- they extract to C via KreMLin.

Trying out KreMLin

Make sure you run opam update first, so that by running the opam install command below you get process-0.2.1 (process version 0.2 doesn't work on Windows). Install all of the packages below, on Windows possibly following instructions from for "difficult" packages (e.g. ppx_deriving).

$ opam install ppx_deriving_yojson zarith pprint menhir ulex process fix wasm

To build just run make from this directory.

If you have the latest version of F* and fstar.exe is in your PATH then you can run the KreMLin test suite by doing make test.

File a bug if things don't work!


The simple example from the ML Workshop Paper is available in test/ML16.fst and you can compile it with make ML16.exe.

Also check out the --help flag:

$ ./krml --help


Kremlin is released under the Apache 2.0 license; see LICENSE for more details.