(**************************************************************************)
(*                                BELENIOS                                *)
(*                                                                        *)
(*  Copyright © 2012-2022 Inria                                           *)
(*                                                                        *)
(*  This program is free software: you can redistribute it and/or modify  *)
(*  it under the terms of the GNU Affero General Public License as        *)
(*  published by the Free Software Foundation, either version 3 of the    *)
(*  License, or (at your option) any later version, with the additional   *)
(*  exemption that compiling, linking, and/or using OpenSSL is allowed.   *)
(*                                                                        *)
(*  This program is distributed in the hope that it will be useful, but   *)
(*  WITHOUT ANY WARRANTY; without even the implied warranty of            *)
(*  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU     *)
(*  Affero General Public License for more details.                       *)
(*                                                                        *)
(*  You should have received a copy of the GNU Affero General Public      *)
(*  License along with this program.  If not, see                         *)
(*  <http://www.gnu.org/licenses/>.                                       *)
(**************************************************************************)

open Belenios_platform.Platform
open Belenios_core
open Serializable_core_t
open Serializable_j
open Signatures
open Common

exception PedersenFailure of string

module MakeVerificator (G : GROUP) = struct
  let get_context x =
    let keys = cert_keys_of_string (sread G.of_string) x.s_message in
    match keys.cert_context with
    | Some x -> x
    | None -> failwith "missing context in certificate"

  let verify vk { s_message; s_signature = { challenge; response } } =
    let commitment = G.((g **~ response) *~ (vk **~ challenge)) in
    let prefix = "sigmsg|" ^ s_message ^ "|" in
    G.Zq.(challenge =% G.hash prefix [| commitment |])

  let verify_cert context x =
    let keys = cert_keys_of_string (sread G.of_string) x.s_message in
    keys.cert_context = Some context && verify keys.cert_verification x

  let compute_verification_keys coefexps =
    let n = Array.length coefexps in
    assert (n > 0);
    let threshold = Array.length coefexps.(0) in
    assert (threshold > 0);
    Array.init n (fun j ->
        let jj = G.Zq.of_int (j + 1) in
        let rec loop_compute_vk i vk =
          if i < n then (
            let c = coefexps.(i) in
            assert (threshold = Array.length c);
            let rec loop k jk accu =
              if k < threshold then
                loop (k + 1) G.Zq.(jk * jj) G.(accu *~ (c.(k) **~ jk))
              else accu
            in
            let computed_gsij = loop 0 G.Zq.one G.one in
            loop_compute_vk (i + 1) G.(vk *~ computed_gsij))
          else vk
        in
        loop_compute_vk 0 G.one)
end

module MakeCombinator (G : GROUP) = struct
  module V = MakeVerificator (G)

  let check_single { trustee_pok; trustee_public_key = y; _ } =
    G.check y
    &&
    let { challenge; response } = trustee_pok in
    let commitment = G.((g **~ response) *~ (y **~ challenge)) in
    let zkp = "pok|" ^ G.description ^ "|" ^ G.to_string y ^ "|" in
    G.Zq.(challenge =% G.hash zkp [| commitment |])

  let check_pedersen t =
    let group = G.description in
    let size = Array.length t.t_certs in
    let threshold = t.t_threshold in
    Array.for_alli
      (fun i c -> V.verify_cert { group; size; threshold; index = i + 1 } c)
      t.t_certs
    &&
    let certs =
      Array.map
        (fun x -> cert_keys_of_string (sread G.of_string) x.s_message)
        t.t_certs
    in
    Array.for_all2
      (fun cert x -> V.verify cert.cert_verification x)
      certs t.t_coefexps
    && (match t.t_signatures with
       | None -> false
       | Some sigs ->
           let n = Array.length sigs in
           n = Array.length certs
           && n = Array.length t.t_coefexps
           && Array.for_all3
                (fun cert coefexps s_signature ->
                  let x =
                    raw_coefexps_of_string (sread G.of_string)
                      coefexps.s_message
                  in
                  let s_message =
                    "certs_sig|"
                    ^ string_of_certs (swrite G.to_string)
                        (swrite G.Zq.to_string)
                        { certs = t.t_certs; coefexps = x.coefexps }
                  in
                  V.verify cert.cert_verification { s_message; s_signature })
                certs t.t_coefexps sigs)
    &&
    let coefexps =
      Array.map
        (fun x ->
          (raw_coefexps_of_string (sread G.of_string) x.s_message).coefexps)
        t.t_coefexps
    in
    Array.for_all check_single t.t_verification_keys
    &&
    let computed_vks = V.compute_verification_keys coefexps in
    t.t_threshold = Array.length coefexps.(0)
    && Array.for_all3
         G.(
           fun cert vk computed_vk ->
             vk.trustee_public_key =~ computed_vk
             &&
             match vk.trustee_signature with
             | None -> false
             | Some s_signature ->
                 V.verify cert.cert_verification
                   { s_message = to_string computed_vk; s_signature })
         certs t.t_verification_keys computed_vks

  let check trustees =
    trustees
    |> List.for_all (function
         | `Single t -> check_single t
         | `Pedersen t -> check_pedersen t)

  let combine_keys trustees =
    trustees
    |> List.map (function
         | `Single t -> t.trustee_public_key
         | `Pedersen p ->
             p.t_coefexps
             |> Array.map (fun x ->
                    (raw_coefexps_of_string (sread G.of_string) x.s_message)
                      .coefexps)
             |> Array.fold_left (fun accu x -> G.(accu *~ x.(0))) G.one)
    |> List.fold_left G.( *~ ) G.one

  let lagrange indexes j =
    List.fold_left
      (fun accu k ->
        let kj = k - j in
        if kj = 0 then accu else G.Zq.(accu * of_int k * invert (of_int kj)))
      G.Zq.one indexes

  let combine_factors trustees check partial_decryptions =
    (* neutral factor *)
    let dummy =
      match partial_decryptions with
      | x :: _ -> Shape.map (fun _ -> G.one) x.owned_payload.decryption_factors
      | [] -> failwith "no partial decryptions"
    in
    (* compute synthetic factor for each trustee_kind *)
    let fold pds_with_ids =
      let indexes = List.map fst pds_with_ids in
      List.fold_left
        (fun a (j, b) ->
          let l = lagrange indexes j in
          Shape.map2 G.(fun x y -> x *~ (y **~ l)) a b.decryption_factors)
        dummy pds_with_ids
    in
    let r =
      Util.compute_synthetic_factors trustees check partial_decryptions fold
    in
    (* combine all factors into one *)
    match r with
    | Ok factors ->
        Ok (List.fold_left (fun a b -> Shape.map2 G.( *~ ) a b) dummy factors)
    | Error _ as x -> x
end

(** Distributed key generation *)

module MakeSimple (G : GROUP) (M : RANDOM) = struct
  open G

  let random () = G.Zq.random (M.get_rng ())

  (** Fiat-Shamir non-interactive zero-knowledge proofs of
      knowledge *)

  let fs_prove gs x oracle =
    let w = random () in
    let commitments = Array.map (fun g -> g **~ w) gs in
    let challenge = oracle commitments in
    let response = G.Zq.(w - (x * challenge)) in
    { challenge; response }

  let generate = random

  let prove x =
    let trustee_public_key = g **~ x in
    let zkp =
      "pok|" ^ G.description ^ "|" ^ G.to_string trustee_public_key ^ "|"
    in
    let trustee_pok = fs_prove [| g |] x (G.hash zkp) in
    {
      trustee_pok;
      trustee_public_key;
      trustee_signature = None;
      trustee_name = None;
    }
end

module MakePKI (G : GROUP) (M : RANDOM) = struct
  type private_key = G.Zq.t
  type public_key = G.t

  let random () = G.Zq.random (M.get_rng ())
  let genkey () = generate_b58_token ~rng:(M.get_rng ()) ~length:22
  let derive_sk p = G.Zq.reduce_hex (sha256_hex ("sk|" ^ p))
  let derive_dk p = G.Zq.reduce_hex (sha256_hex ("dk|" ^ p))

  let sign sk s_message =
    let w = random () in
    let commitment = G.(g **~ w) in
    let prefix = "sigmsg|" ^ s_message ^ "|" in
    let challenge = G.hash prefix [| commitment |] in
    let response = G.Zq.(w - (sk * challenge)) in
    let s_signature = { challenge; response } in
    { s_message; s_signature }

  let encrypt y plaintext =
    let r = random () in
    let key = random () in
    let key = G.(g **~ key) in
    let y_alpha = G.(g **~ r) in
    let y_beta = G.((y **~ r) *~ key) in
    let key = sha256_hex ("key|" ^ G.to_string key) in
    let iv = sha256_hex ("iv|" ^ G.to_string y_alpha) in
    let y_data = Crypto_primitives.encrypt ~key ~iv ~plaintext in
    { y_alpha; y_beta; y_data }

  let decrypt x { y_alpha; y_beta; y_data } =
    let key =
      sha256_hex G.("key|" ^ to_string (y_beta *~ invert (y_alpha **~ x)))
    in
    let iv = sha256_hex ("iv|" ^ G.to_string y_alpha) in
    Crypto_primitives.decrypt ~key ~iv ~ciphertext:y_data

  let make_cert ~sk ~dk ~context =
    let cert_keys =
      {
        cert_verification = G.(g **~ sk);
        cert_encryption = G.(g **~ dk);
        cert_context = Some context;
      }
    in
    let cert = string_of_cert_keys (swrite G.to_string) cert_keys in
    sign sk cert

  include MakeVerificator (G)
end

module MakeChannels
    (G : GROUP)
    (M : RANDOM)
    (P : PKI with type private_key = G.Zq.t and type public_key = G.t) =
struct
  type private_key = P.private_key
  type public_key = P.public_key

  let send sk c_recipient c_message =
    let msg = { c_recipient; c_message } in
    let msg = string_of_channel_msg (swrite G.to_string) msg in
    let msg = P.sign sk msg in
    P.encrypt c_recipient (string_of_signed_msg (swrite G.Zq.to_string) msg)

  let recv dk vk msg =
    let msg = P.decrypt dk msg |> signed_msg_of_string (sread G.Zq.of_string) in
    if not (P.verify vk msg) then
      failwith "invalid signature on received message";
    let msg = channel_msg_of_string (sread G.of_string) msg.s_message in
    let { c_recipient; c_message } = msg in
    if not G.(c_recipient =~ g **~ dk) then
      failwith "invalid recipient on received message";
    c_message
end

module MakePedersen
    (G : GROUP)
    (M : RANDOM)
    (P : PKI with type private_key = G.Zq.t and type public_key = G.t)
    (C : CHANNELS with type private_key = G.Zq.t and type public_key = G.t) =
struct
  type scalar = G.Zq.t
  type element = G.t

  open G
  module K = MakeSimple (G) (M)
  module V = MakeVerificator (G)
  module L = MakeCombinator (G)

  let random () = Zq.random (M.get_rng ())

  let step1 context =
    let seed = P.genkey () in
    let sk = P.derive_sk seed in
    let dk = P.derive_dk seed in
    let cert = P.make_cert ~sk ~dk ~context in
    (seed, cert)

  let step1_check = P.verify_cert

  let step2 certs =
    assert (Array.length certs > 0);
    let group = G.description in
    let size = Array.length certs in
    let threshold = (P.get_context certs.(0)).threshold in
    Array.iteri
      (fun i cert ->
        if P.verify_cert { group; size; threshold; index = i + 1 } cert then ()
        else
          let msg = Printf.sprintf "certificate %d does not validate" (i + 1) in
          raise (PedersenFailure msg))
      certs;
    threshold

  let eval_poly polynomial x =
    let cur = ref Zq.one and res = ref Zq.zero in
    for i = 0 to Array.length polynomial - 1 do
      (res := Zq.(!res + (!cur * polynomial.(i))));
      cur := Zq.(!cur * x)
    done;
    !res

  let step3 certs seed =
    let n = Array.length certs in
    let threshold = step2 certs in
    let sk = P.derive_sk seed and dk = P.derive_dk seed in
    let mk_signature coefexps =
      let m =
        "certs_sig|"
        ^ string_of_certs (swrite G.to_string) (swrite Zq.to_string)
            { certs; coefexps }
      in
      Some (P.sign sk m).s_signature
    in
    let certs =
      Array.map
        (fun x -> cert_keys_of_string (sread G.of_string) x.s_message)
        certs
    in
    let vk = g **~ sk and ek = g **~ dk in
    let i =
      Array.findi
        (fun i cert ->
          if cert.cert_verification =~ vk && cert.cert_encryption =~ ek then
            Some (i + 1)
          else None)
        certs
    in
    let () =
      match i with
      | None -> raise (PedersenFailure "could not find my certificate")
      | Some _ -> ()
    in
    let polynomial = Array.make threshold Zq.zero in
    let rec fill_polynomial i =
      if i < threshold then (
        let a = random () in
        polynomial.(i) <- a;
        fill_polynomial (i + 1))
      else ()
    in
    let () = fill_polynomial 0 in
    let p_polynomial =
      let x =
        C.send sk ek
          (string_of_raw_polynomial (swrite Zq.to_string) { polynomial })
      in
      string_of_encrypted_msg (swrite G.to_string) x
    in
    let coefexps = Array.map (fun x -> g **~ x) polynomial in
    let p_signature = mk_signature coefexps in
    let coefexps = string_of_raw_coefexps (swrite G.to_string) { coefexps } in
    let p_coefexps = P.sign sk coefexps in
    let p_secrets = Array.make n "" in
    let rec fill_secrets j =
      if j < n then (
        let secret = eval_poly polynomial (Zq.of_int (j + 1)) in
        let secret = string_of_secret (swrite Zq.to_string) { secret } in
        let x = C.send sk certs.(j).cert_encryption secret in
        p_secrets.(j) <- string_of_encrypted_msg (swrite G.to_string) x;
        fill_secrets (j + 1))
      else ()
    in
    let () = fill_secrets 0 in
    { p_polynomial; p_secrets; p_coefexps; p_signature }

  let step3_check certs i polynomial =
    let x =
      raw_coefexps_of_string (sread G.of_string) polynomial.p_coefexps.s_message
    in
    let s_message =
      "certs_sig|"
      ^ string_of_certs (swrite G.to_string) (swrite Zq.to_string)
          { certs; coefexps = x.coefexps }
    in
    let certs =
      Array.map
        (fun x -> cert_keys_of_string (sread G.of_string) x.s_message)
        certs
    in
    P.verify certs.(i).cert_verification polynomial.p_coefexps
    &&
    match polynomial.p_signature with
    | None -> false
    | Some s_signature ->
        P.verify certs.(i).cert_verification { s_message; s_signature }

  let step4 certs polynomials =
    let n = Array.length certs in
    let threshold = step2 certs in
    assert (n = Array.length polynomials);
    let certs =
      Array.map
        (fun x -> cert_keys_of_string (sread G.of_string) x.s_message)
        certs
    in
    let vi_signatures =
      Some
        (Array.map
           (fun x ->
             match x.p_signature with
             | None -> raise (PedersenFailure "missing signature in polynomial")
             | Some y -> y)
           polynomials)
    in
    let vi_coefexps = Array.map (fun x -> x.p_coefexps) polynomials in
    Array.iteri
      (fun i x ->
        if P.verify certs.(i).cert_verification x then
          let x = raw_coefexps_of_string (sread G.of_string) x.s_message in
          if threshold = Array.length x.coefexps then ()
          else
            let msg = Printf.sprintf "coefexps %d has wrong length" (i + 1) in
            raise (PedersenFailure msg)
        else
          let msg = Printf.sprintf "coefexps %d does not validate" (i + 1) in
          raise (PedersenFailure msg))
      vi_coefexps;
    Array.init n (fun j ->
        let vi_polynomial = polynomials.(j).p_polynomial in
        let vi_secrets =
          Array.init n (fun i -> polynomials.(i).p_secrets.(j))
        in
        { vi_polynomial; vi_secrets; vi_coefexps; vi_signatures })

  let sign_trustee_public_key ~sk x =
    let signed = P.sign sk (G.to_string x.trustee_public_key) in
    { x with trustee_signature = Some signed.s_signature }

  let step5 certs seed vinput =
    let n = Array.length certs in
    let threshold = step2 certs in
    let certs =
      Array.map
        (fun x -> cert_keys_of_string (sread G.of_string) x.s_message)
        certs
    in
    let sk = P.derive_sk seed and dk = P.derive_dk seed in
    let vk = g **~ sk and ek = g **~ dk in
    let j =
      Array.findi
        (fun i cert ->
          if cert.cert_verification =~ vk && cert.cert_encryption =~ ek then
            Some (i + 1)
          else None)
        certs
    in
    let j =
      match j with
      | None -> raise (PedersenFailure "could not find my certificate")
      | Some i -> Zq.of_int i
    in
    let { polynomial } =
      vinput.vi_polynomial
      |> encrypted_msg_of_string (sread G.of_string)
      |> C.recv dk vk
      |> raw_polynomial_of_string (sread Zq.of_string)
    in
    assert (threshold = Array.length polynomial);
    assert (n = Array.length vinput.vi_secrets);
    let secrets =
      Array.init n (fun i ->
          vinput.vi_secrets.(i)
          |> encrypted_msg_of_string (sread G.of_string)
          |> C.recv dk certs.(i).cert_verification
          |> secret_of_string (sread Zq.of_string)
          |> fun x -> x.secret)
    in
    assert (n = Array.length vinput.vi_coefexps);
    let coefexps =
      Array.init n (fun i ->
          let x = vinput.vi_coefexps.(i) in
          if not (P.verify certs.(i).cert_verification x) then
            raise
              (PedersenFailure
                 (Printf.sprintf "coefexps %d does not validate" (i + 1)));
          let res =
            (raw_coefexps_of_string (sread G.of_string) x.s_message).coefexps
          in
          assert (Array.length res = threshold);
          res)
    in
    for i = 0 to n - 1 do
      let c = coefexps.(i) in
      let rec loop k jk accu =
        if k < threshold then loop (k + 1) Zq.(jk * j) (accu *~ (c.(k) **~ jk))
        else accu
      in
      let computed_gsij = loop 0 Zq.one one in
      if not (g **~ secrets.(i) =~ computed_gsij) then
        raise
          (PedersenFailure
             (Printf.sprintf "secret %d does not validate" (i + 1)))
    done;
    let pdk_decryption_key = Array.fold_left Zq.( + ) Zq.zero secrets in
    let pdk =
      string_of_partial_decryption_key (swrite Zq.to_string)
        { pdk_decryption_key }
    in
    let vo_public_key =
      K.prove pdk_decryption_key |> sign_trustee_public_key ~sk
    in
    let private_key = C.send sk ek pdk in
    let vo_private_key =
      string_of_encrypted_msg (swrite G.to_string) private_key
    in
    { vo_public_key; vo_private_key }

  let step5_check certs i polynomials voutput =
    let n = Array.length certs in
    let certs =
      Array.map
        (fun x -> cert_keys_of_string (sread G.of_string) x.s_message)
        certs
    in
    assert (n = Array.length polynomials);
    let coefexps =
      Array.init n (fun i ->
          let x = polynomials.(i).p_coefexps in
          if not (P.verify certs.(i).cert_verification x) then
            raise
              (PedersenFailure
                 (Printf.sprintf "coefexps %d does not validate" (i + 1)));
          (raw_coefexps_of_string (sread G.of_string) x.s_message).coefexps)
    in
    let computed_vk = (V.compute_verification_keys coefexps).(i) in
    let { vo_public_key; _ } = voutput in
    L.check [ `Single vo_public_key ]
    && vo_public_key.trustee_public_key =~ computed_vk
    &&
    match vo_public_key.trustee_signature with
    | None -> false
    | Some s_signature ->
        P.verify certs.(i).cert_verification
          { s_message = G.to_string computed_vk; s_signature }

  let step6 certs polynomials voutputs =
    let n = Array.length certs in
    let t_threshold = step2 certs in
    let t_certs = certs in
    let t_signatures =
      Some
        (Array.map
           (fun x ->
             match x.p_signature with
             | None -> raise (PedersenFailure "missing signature in polynomial")
             | Some y -> y)
           polynomials)
    in
    let certs =
      Array.map
        (fun x -> cert_keys_of_string (sread G.of_string) x.s_message)
        t_certs
    in
    assert (n = Array.length polynomials);
    assert (n = Array.length voutputs);
    let coefexps =
      Array.init n (fun i ->
          let fail () =
            raise
              (PedersenFailure
                 (Printf.sprintf "coefexps %d does not validate" (i + 1)))
          in
          let x = polynomials.(i).p_coefexps in
          if not (P.verify certs.(i).cert_verification x) then fail ();
          let r =
            (raw_coefexps_of_string (sread G.of_string) x.s_message).coefexps
          in
          if not (t_threshold = Array.length r) then fail ();
          r)
    in
    let computed_vks = V.compute_verification_keys coefexps in
    for j = 0 to n - 1 do
      let voutput = voutputs.(j) in
      if not (L.check [ `Single voutput.vo_public_key ]) then
        raise
          (PedersenFailure (Printf.sprintf "pok %d does not validate" (j + 1)));
      if not (voutput.vo_public_key.trustee_public_key =~ computed_vks.(j)) then
        raise
          (PedersenFailure
             (Printf.sprintf "verification key %d is incorrect" (j + 1)))
    done;
    {
      t_threshold;
      t_certs;
      t_coefexps = Array.map (fun x -> x.p_coefexps) polynomials;
      t_signatures;
      t_verification_keys = Array.map (fun x -> x.vo_public_key) voutputs;
    }
end
