add kdf

src/dune

@@ -17,6 +17,7 @@
   caqti-driver-pgx
   bin
   mirage-crypto
+  kdf.hkdf
   digestif
   duration
   vif

src/fdh.ml

@@ -0,0 +1,9 @@
+module Kdf = struct
+  module XTR = Hkdf.Make (Digestif.SHA512)
+  module PRF = Hkdf.Make (Digestif.SHA256)
+
+  let kdf ~xts ~ikm ~ctx ~len =
+    let prk = XTR.extract ~salt:xts ikm in
+    let okm = PRF.expand ~prk ~info:ctx len in
+    okm
+end