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

let kdf_mod_n ~n ~xts ~ikm ~ctx =
  let nbits = Z.numbits n in
  let len = ((nbits - 1) / 8) + 1 in
  assert (8 * len = nbits);
  let rec go ctr =
    let ctr_be =
      let b = Bytes.create 2 in
      Bytes.set_uint16_be b 0 ctr;
      Bytes.unsafe_to_string b
    in
    let ctx = String.cat ctx ctr_be in
    let okm = Kdf.kdf ~xts ~ikm ~ctx ~len in
    assert (String.length okm = len);
    let r = Mirage_crypto_pk.Z_extra.of_octets_be okm in
    if Z.gt r n then go (succ ctr) else r
  in
  go 0

(*
let rsa_sign_fdh priv msg =
  let pub = Mirage_crypto_pk.Rsa.pub_of_priv priv in
  let v = rsa_full_domain_hash pub msg in
  let v = Z.to_bits v in
  Mirage_crypto_pk.Rsa.PKCS1.sig_encode ~crt_hardening:true ~key:priv v
  *)