diff --git a/twofish/primitives.mli b/twofish/primitives.mli
new file mode 100644
index 0000000..8ad5ba4
--- /dev/null
+++ b/twofish/primitives.mli
@@ -0,0 +1,4 @@
+type ctx
+val init : string -> ctx
+val encrypt : ctx -> string -> string
+val decrypt : ctx -> string -> string