wip FDH-RSA

src/assets.ml

@@ -1,5 +1,4 @@
 (* https://docs.taler.net/design-documents/003-tos-rendering.html
-   https://docs.taler.net/design-documents/003-tos-rendering.html
 
    must support `text/plain` and `text/markdown` *)
 

src/crypto.ml

@@ -64,6 +64,7 @@ module Binary_format_rsa = struct
         pub
     | _ -> assert false
 
+  (* custom private key binary format <> than gcrypt *)
   let priv_to_octets ({ e; d; n; p; q; dp; dq; q' } : Mirage_crypto_pk.Rsa.priv)
       =
     z_array_to_octets [| e; d; n; p; q; dp; dq; q' |]
@@ -259,15 +260,10 @@ end
 module RsaSignature : sig
   type t
 
-  val sign : key:Mirage_crypto_pk.Rsa.priv -> string -> t
   val jsont : t Jsont.t
 end = struct
   type t = string
 
-  (* TODO rsa sign *)
-  let sign ~key s =
-    Mirage_crypto_pk.Rsa.decrypt ~crt_hardening:true ~mask:`Yes ~key s
-
   let jsont =
     let of_b32 s = B32.decode s in
     let to_b32 t = B32.encode t in
@@ -282,3 +278,104 @@ type rsa_priv = RsaPrivateKey.t
 type rsa_pub = RsaPublicKey.t
 type rsa_sig = RsaSignature.t
 type denomination_hash = Hash.DenominationHash.t
+
+(* WIP *)
+module FDH_RSA = struct
+  open Mirage_crypto_pk
+
+  module Kdf = struct
+    module XTR = Hkdf.Make (Digestif.SHA512)
+    module PRF = Hkdf.Make (Digestif.SHA256)
+
+    let kdf =
+     fun ~xts ~ikm ~ctx ~len ->
+      let prk = XTR.extract ~salt:xts ikm in
+      let okm = PRF.expand ~prk ~info:ctx len in
+      okm
+
+    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 =
+        (* cat ctx ctr_be *)
+        let ctx =
+          let ctx_len = String.length ctx in
+          let b = Bytes.create (ctx_len + 2) in
+          Bytes.blit_string ctx 0 b 0 ctx_len;
+          Bytes.set_uint16_be b ctx_len ctr;
+          Bytes.unsafe_to_string b
+        in
+        let okm = kdf ~xts ~ikm ~ctx ~len in
+        assert (String.length okm = len);
+        let r = Z_extra.of_octets_be okm in
+        if Z.gt r n then go (succ ctr) else r
+      in
+      go 0
+  end
+
+  let gcd_validate r n =
+    match Z.equal (Z.gcd r n) Z.one with
+    | true -> ()
+    | false -> Fmt.failwith "RSA key is malicious"
+
+  let rsa_full_domain_hash pub msg =
+    let xts = RsaPublicKey.to_octets pub in
+    let ctx = "RSA-FDA FTpsW!" in
+    let r = Kdf.kdf_mod_n ~n:pub.n ~xts ~ikm:msg ~ctx in
+    gcd_validate r pub.n; r
+
+  let rsa_blinding_key_derive (pub : RsaPublicKey.t) bks =
+    let xts = "Blinding KDF extractor HMAC key" in
+    let ctx = "Blinding KDF" in
+    let r = Kdf.kdf_mod_n ~n:pub.n ~xts ~ikm:bks ~ctx in
+    gcd_validate r pub.n; r
+
+  let rsa_blind pub ~bks ~msg =
+    let data = rsa_full_domain_hash pub msg in
+    let bkey = rsa_blinding_key_derive pub bks in
+    (* can we just use [powm] here instead? *)
+    let r_e = Z.powm_sec bkey pub.e pub.n in
+    let data_r_e = Z.rem (Z.mul data r_e) pub.n in
+    Z_extra.to_octets_be data_r_e
+
+  (* -- WIP crypto -- *)
+
+  (* TODO crypto
+     not sure about signature scheme used by taler
+     libgnunetutil crypto_rsa.c use "(flags raw)" => no padding *)
+  (* decrypt <=> sign *)
+  let rsa_sign_z priv r =
+    let data = Z_extra.to_octets_be r in
+    Rsa.decrypt ~crt_hardening:true ~key:priv data
+
+  (* TODO crypto
+     look into mirage-crypto for this
+     use Eqaf for constant time string compare *)
+  let rsa_verify_z pub r sig_ =
+    let data = Z_extra.to_octets_be r in
+    let sig_' = Rsa.encrypt ~key:pub data in
+    match String.equal sig_ sig_' with
+    | false -> Fmt.error "RSA signature verification failed"
+    | true -> Ok ()
+
+  let rsa_sign_fdh priv msg =
+    let pub = Rsa.pub_of_priv priv in
+    let r = rsa_full_domain_hash pub msg in
+    rsa_sign_z priv r
+
+  let rsa_unblind pub ~bks ~sig_ =
+    let bkey = rsa_blinding_key_derive pub bks in
+    let r_inv =
+      try Z.invert bkey pub.n
+      with Division_by_zero ->
+        (* => gcd(r,n) <> 1, should be already checked for *)
+        assert false
+    in
+    let ubsig = Z.rem (Z.mul sig_ r_inv) pub.n in
+    ubsig
+
+  let rsa_verify pub ~msg ~sig_ =
+    let r = rsa_full_domain_hash pub msg in
+    rsa_verify_z pub r sig_
+end

src/dune

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