wip FDH-RSA

src/fdh.ml

@@ -7,3 +7,29 @@ module Kdf = struct
     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
+  *)