From Coq Require Extraction.
Require Import Strings.String.

(*
Abstract cryptographic primitives.

For proof-checking, this will resolve to a set of axioms, but when
extracting to ocaml, this should be replaced with the real implementation
of the cryptographic primitives.
*)
Module Type AbsCryptoPrims.
  Definition principal : Type := string.

  Inductive label : Set :=
    Public : label
  .
End AbsCryptoPrims.

Module NullCryptoPrims : AbsCryptoPrims.
  Definition principal : Type := string.

  Inductive label : Set :=
    Public : label
  .
End NullCryptoPrims.

Extraction Language OCaml.
Set Extraction Optimize.
Set Extraction AccessOpaque.
Extraction "CryptoPrims" NullCryptoPrims.