Signature proving payment to destination by only revealing key derivation, not the actual tx secret key

src/crypto/crypto.cpp

@@ -197,6 +197,13 @@ namespace crypto {
     ec_point comm;
   };
 
+  struct s_comm_2 {
+    hash msg;
+    ec_point D;
+    ec_point X;
+    ec_point Y;
+  };
+
   void crypto_ops::generate_signature(const hash &prefix_hash, const public_key &pub, const secret_key &sec, signature &sig) {
     boost::lock_guard<boost::mutex> lock(random_lock);
     ge_p3 tmp3;
@@ -242,6 +249,124 @@ namespace crypto {
     return sc_isnonzero(&c) == 0;
   }
 
+  void crypto_ops::generate_tx_proof(const hash &prefix_hash, const public_key &R, const public_key &A, const public_key &D, const secret_key &r, signature &sig) {
+    // sanity check
+    ge_p3 R_p3;
+    ge_p3 A_p3;
+    ge_p3 D_p3;
+    if (ge_frombytes_vartime(&R_p3, &R) != 0) throw std::runtime_error("tx pubkey is invalid");
+    if (ge_frombytes_vartime(&A_p3, &A) != 0) throw std::runtime_error("recipient view pubkey is invalid");
+    if (ge_frombytes_vartime(&D_p3, &D) != 0) throw std::runtime_error("key derivation is invalid");
+#if !defined(NDEBUG)
+    {
+      assert(sc_check(&r) == 0);
+      // check R == r*G
+      ge_p3 dbg_R_p3;
+      ge_scalarmult_base(&dbg_R_p3, &r);
+      public_key dbg_R;
+      ge_p3_tobytes(&dbg_R, &dbg_R_p3);
+      assert(R == dbg_R);
+      // check D == r*A
+      ge_p2 dbg_D_p2;
+      ge_scalarmult(&dbg_D_p2, &r, &A_p3);
+      public_key dbg_D;
+      ge_tobytes(&dbg_D, &dbg_D_p2);
+      assert(D == dbg_D);
+    }
+#endif
+
+    // pick random k
+    ec_scalar k;
+    random_scalar(k);
+    
+    // compute X = k*G
+    ge_p3 X_p3;
+    ge_scalarmult_base(&X_p3, &k);
+    
+    // compute Y = k*A
+    ge_p2 Y_p2;
+    ge_scalarmult(&Y_p2, &k, &A_p3);
+
+    // sig.c = Hs(Msg || D || X || Y)
+    s_comm_2 buf;
+    buf.msg = prefix_hash;
+    buf.D = D;
+    ge_p3_tobytes(&buf.X, &X_p3);
+    ge_tobytes(&buf.Y, &Y_p2);
+    hash_to_scalar(&buf, sizeof(s_comm_2), sig.c);
+
+    // sig.r = k - sig.c*r
+    sc_mulsub(&sig.r, &sig.c, &r, &k);
+  }
+
+  bool crypto_ops::check_tx_proof(const hash &prefix_hash, const public_key &R, const public_key &A, const public_key &D, const signature &sig) {
+    // sanity check
+    ge_p3 R_p3;
+    ge_p3 A_p3;
+    ge_p3 D_p3;
+    if (ge_frombytes_vartime(&R_p3, &R) != 0) return false;
+    if (ge_frombytes_vartime(&A_p3, &A) != 0) return false;
+    if (ge_frombytes_vartime(&D_p3, &D) != 0) return false;
+    if (sc_check(&sig.c) != 0 || sc_check(&sig.r) != 0) return false;
+
+    // compute sig.c*R
+    ge_p2 cR_p2;
+    ge_scalarmult(&cR_p2, &sig.c, &R_p3);
+
+    // compute sig.r*G
+    ge_p3 rG_p3;
+    ge_scalarmult_base(&rG_p3, &sig.r);
+
+    // compute sig.c*D
+    ge_p2 cD_p2;
+    ge_scalarmult(&cD_p2, &sig.c, &D_p3);
+
+    // compute sig.r*A
+    ge_p2 rA_p2;
+    ge_scalarmult(&rA_p2, &sig.r, &A_p3);
+
+    // compute X = sig.c*R + sig.r*G
+    public_key cR;
+    ge_tobytes(&cR, &cR_p2);
+    ge_p3 cR_p3;
+    if (ge_frombytes_vartime(&cR_p3, &cR) != 0) return false;
+    ge_cached rG_cached;
+    ge_p3_to_cached(&rG_cached, &rG_p3);
+    ge_p1p1 X_p1p1;
+    ge_add(&X_p1p1, &cR_p3, &rG_cached);
+    ge_p2 X_p2;
+    ge_p1p1_to_p2(&X_p2, &X_p1p1);
+
+    // compute Y = sig.c*D + sig.r*A
+    public_key cD;
+    public_key rA;
+    ge_tobytes(&cD, &cD_p2);
+    ge_tobytes(&rA, &rA_p2);
+    ge_p3 cD_p3;
+    ge_p3 rA_p3;
+    if (ge_frombytes_vartime(&cD_p3, &cD) != 0) return false;
+    if (ge_frombytes_vartime(&rA_p3, &rA) != 0) return false;
+    ge_cached rA_cached;
+    ge_p3_to_cached(&rA_cached, &rA_p3);
+    ge_p1p1 Y_p1p1;
+    ge_add(&Y_p1p1, &cD_p3, &rA_cached);
+    ge_p2 Y_p2;
+    ge_p1p1_to_p2(&Y_p2, &Y_p1p1);
+
+    // compute c2 = Hs(Msg || D || X || Y)
+    s_comm_2 buf;
+    buf.msg = prefix_hash;
+    buf.D = D;
+    ge_tobytes(&buf.X, &X_p2);
+    ge_tobytes(&buf.Y, &Y_p2);
+    ec_scalar c2;
+    hash_to_scalar(&buf, sizeof(s_comm_2), c2);
+
+    // test if c2 == sig.c
+    sc_sub(&c2, &c2, &sig.c);
+    return sc_isnonzero(&c2) == 0;
+  }
+
   static void hash_to_ec(const public_key &key, ge_p3 &res) {
     hash h;
     ge_p2 point;

src/crypto/crypto.h

@@ -123,6 +123,10 @@ namespace crypto {
     friend void generate_signature(const hash &, const public_key &, const secret_key &, signature &);
     static bool check_signature(const hash &, const public_key &, const signature &);
     friend bool check_signature(const hash &, const public_key &, const signature &);
+    static void generate_tx_proof(const hash &, const public_key &, const public_key &, const public_key &, const secret_key &, signature &);
+    friend void generate_tx_proof(const hash &, const public_key &, const public_key &, const public_key &, const secret_key &, signature &);
+    static bool check_tx_proof(const hash &, const public_key &, const public_key &, const public_key &, const signature &);
+    friend bool check_tx_proof(const hash &, const public_key &, const public_key &, const public_key &, const signature &);
     static void generate_key_image(const public_key &, const secret_key &, key_image &);
     friend void generate_key_image(const public_key &, const secret_key &, key_image &);
     static void generate_ring_signature(const hash &, const key_image &,
@@ -200,6 +204,16 @@ namespace crypto {
     return crypto_ops::check_signature(prefix_hash, pub, sig);
   }
 
+  /* Generation and checking of a tx proof; given a tx pubkey R, the recipient's view pubkey A, and the key 
+   * derivation D, the signature proves the knowledge of the tx secret key r such that R=r*G and D=r*A
+   */
+  inline void generate_tx_proof(const hash &prefix_hash, const public_key &R, const public_key &A, const public_key &D, const secret_key &r, signature &sig) {
+    crypto_ops::generate_tx_proof(prefix_hash, R, A, D, r, sig);
+  }
+  inline bool check_tx_proof(const hash &prefix_hash, const public_key &R, const public_key &A, const public_key &D, const signature &sig) {
+    return crypto_ops::check_tx_proof(prefix_hash, R, A, D, sig);
+  }
+
   /* To send money to a key:
    * * The sender generates an ephemeral key and includes it in transaction output.
    * * To spend the money, the receiver generates a key image from it.

src/simplewallet/simplewallet.cpp

@@ -47,6 +47,7 @@
 #include "common/command_line.h"
 #include "common/util.h"
 #include "common/dns_utils.h"
+#include "common/base58.h"
 #include "p2p/net_node.h"
 #include "cryptonote_protocol/cryptonote_protocol_handler.h"
 #include "simplewallet.h"
@@ -714,6 +715,8 @@ simple_wallet::simple_wallet()
   m_cmd_binder.set_handler("rescan_spent", boost::bind(&simple_wallet::rescan_spent, this, _1), tr("Rescan blockchain for spent outputs"));
   m_cmd_binder.set_handler("get_tx_key", boost::bind(&simple_wallet::get_tx_key, this, _1), tr("Get transaction key (r) for a given <txid>"));
   m_cmd_binder.set_handler("check_tx_key", boost::bind(&simple_wallet::check_tx_key, this, _1), tr("Check amount going to <address> in <txid>"));
+  m_cmd_binder.set_handler("get_tx_proof", boost::bind(&simple_wallet::get_tx_proof, this, _1), tr("Generate a signature to prove payment to <address> in <txid> using the transaction secret key (r) without revealing it"));
+  m_cmd_binder.set_handler("check_tx_proof", boost::bind(&simple_wallet::check_tx_proof, this, _1), tr("Check tx proof for payment going to <address> in <txid>"));
   m_cmd_binder.set_handler("show_transfers", boost::bind(&simple_wallet::show_transfers, this, _1), tr("show_transfers [in|out|pending|failed|pool] [<min_height> [<max_height>]] - Show incoming/outgoing transfers within an optional height range"));
   m_cmd_binder.set_handler("unspent_outputs", boost::bind(&simple_wallet::unspent_outputs, this, _1), tr("unspent_outputs [<min_amount> <max_amount>] - Show unspent outputs within an optional amount range)"));
   m_cmd_binder.set_handler("rescan_bc", boost::bind(&simple_wallet::rescan_blockchain, this, _1), tr("Rescan blockchain from scratch"));
@@ -3232,6 +3235,85 @@ bool simple_wallet::get_tx_key(const std::vector<std::string> &args_)
   }
 }
 //----------------------------------------------------------------------------------------------------
+bool simple_wallet::get_tx_proof(const std::vector<std::string> &args)
+{
+  if(args.size() != 2 && args.size() != 3) {
+    fail_msg_writer() << tr("usage: get_tx_proof <txid> <dest_address> [<tx_key>]");
+    return true;
+  }
+  if (m_wallet->ask_password() && !get_and_verify_password()) { return true; }
+
+  cryptonote::blobdata txid_data;
+  if(!epee::string_tools::parse_hexstr_to_binbuff(args[0], txid_data) || txid_data.size() != sizeof(crypto::hash))
+  {
+    fail_msg_writer() << tr("failed to parse txid");
+    return true;
+  }
+  crypto::hash txid = *reinterpret_cast<const crypto::hash*>(txid_data.data());
+
+  cryptonote::account_public_address address;
+  bool has_payment_id;
+  crypto::hash8 payment_id;
+  if(!cryptonote::get_account_address_from_str_or_url(address, has_payment_id, payment_id, m_wallet->testnet(), args[1]))
+  {
+    fail_msg_writer() << tr("failed to parse address");
+    return true;
+  }
+
+  LOCK_IDLE_SCOPE();
+
+  crypto::secret_key tx_key, tx_key2;
+  bool r = m_wallet->get_tx_key(txid, tx_key);
+  cryptonote::blobdata tx_key_data;
+  if (args.size() == 3)
+  {
+    if(!epee::string_tools::parse_hexstr_to_binbuff(args[2], tx_key_data) || tx_key_data.size() != sizeof(crypto::secret_key))
+    {
+      fail_msg_writer() << tr("failed to parse tx_key");
+      return true;
+    }
+    tx_key2 = *reinterpret_cast<const crypto::secret_key*>(tx_key_data.data());
+  }
+  if (r)
+  {
+    if (args.size() == 3 && tx_key != rct::sk2rct(tx_key2))
+    {
+      fail_msg_writer() << tr("Tx secret key was found for the given txid, but you've also provided another tx secret key which doesn't match the found one.");
+      return true;
+    }
+  }
+  else
+  {
+    if (tx_key_data.empty())
+    {
+      fail_msg_writer() << tr("Tx secret key wasn't found in the wallet file. Provide it as the optional third parameter if you have it elsewhere.");
+      return true;
+    }
+    tx_key = tx_key2;
+  }
+
+  crypto::public_key R;
+  crypto::secret_key_to_public_key(tx_key, R);
+  crypto::public_key rA = rct::rct2pk(rct::scalarmultKey(rct::pk2rct(address.m_view_public_key), rct::sk2rct(tx_key)));
+  crypto::signature sig;
+  try
+  {
+    crypto::generate_tx_proof(txid, R, address.m_view_public_key, rA, tx_key, sig);
+  }
+  catch (const std::runtime_error &e)
+  {
+    fail_msg_writer() << e.what();
+    return true;
+  }
+
+  std::string sig_str = std::string("ProofV1") +
+    tools::base58::encode(std::string((const char *)&rA, sizeof(crypto::public_key))) +
+    tools::base58::encode(std::string((const char *)&sig, sizeof(crypto::signature)));
+
+  success_msg_writer() << tr("Signature: ") << sig_str;
+  return true;
+}
+//----------------------------------------------------------------------------------------------------
 bool simple_wallet::check_tx_key(const std::vector<std::string> &args_)
 {
   std::vector<std::string> local_args = args_;
@@ -3278,6 +3360,18 @@ bool simple_wallet::check_tx_key(const std::vector<std::string> &args_)
     return true;
   }
 
+  crypto::key_derivation derivation;
+  if (!crypto::generate_key_derivation(address.m_view_public_key, tx_key, derivation))
+  {
+    fail_msg_writer() << tr("failed to generate key derivation from supplied parameters");
+    return true;
+  }
+
+  return check_tx_key_helper(txid, address, derivation);
+}
+//----------------------------------------------------------------------------------------------------
+bool simple_wallet::check_tx_key_helper(const crypto::hash &txid, const cryptonote::account_public_address &address, const crypto::key_derivation &derivation)
+{
   COMMAND_RPC_GET_TRANSACTIONS::request req;
   COMMAND_RPC_GET_TRANSACTIONS::response res;
   req.txs_hashes.push_back(epee::string_tools::pod_to_hex(txid));
@@ -3311,13 +3405,6 @@ bool simple_wallet::check_tx_key(const std::vector<std::string> &args_)
     return true;
   }
 
-  crypto::key_derivation derivation;
-  if (!crypto::generate_key_derivation(address.m_view_public_key, tx_key, derivation))
-  {
-    fail_msg_writer() << tr("failed to generate key derivation from supplied parameters");
-    return true;
-  }
-
   uint64_t received = 0;
   try {
     for (size_t n = 0; n < tx.vout.size(); ++n)
@@ -3340,14 +3427,6 @@ bool simple_wallet::check_tx_key(const std::vector<std::string> &args_)
           {
             rct::key Ctmp;
             //rct::key amount_key = rct::hash_to_scalar(rct::scalarmultKey(rct::pk2rct(address.m_view_public_key), rct::sk2rct(tx_key)));
-            crypto::key_derivation derivation;
-            bool r = crypto::generate_key_derivation(address.m_view_public_key, tx_key, derivation);
-            if (!r)
-            {
-              LOG_ERROR("Failed to generate key derivation to decode rct output " << n);
-              amount = 0;
-            }
-            else
             {
               crypto::secret_key scalar1;
               crypto::derivation_to_scalar(derivation, n, scalar1);
@@ -3404,6 +3483,129 @@ bool simple_wallet::check_tx_key(const std::vector<std::string> &args_)
   return true;
 }
 //----------------------------------------------------------------------------------------------------
+bool simple_wallet::check_tx_proof(const std::vector<std::string> &args)
+{
+  if(args.size() != 3) {
+    fail_msg_writer() << tr("usage: check_tx_proof <txid> <address> <signature>");
+    return true;
+  }
+
+  if (!try_connect_to_daemon())
+    return true;
+
+  // parse txid
+  cryptonote::blobdata txid_data;
+  if(!epee::string_tools::parse_hexstr_to_binbuff(args[0], txid_data) || txid_data.size() != sizeof(crypto::hash))
+  {
+    fail_msg_writer() << tr("failed to parse txid");
+    return true;
+  }
+  crypto::hash txid = *reinterpret_cast<const crypto::hash*>(txid_data.data());
+
+  // parse address
+  cryptonote::account_public_address address;
+  bool has_payment_id;
+  crypto::hash8 payment_id;
+  if(!cryptonote::get_account_address_from_str_or_url(address, has_payment_id, payment_id, m_wallet->testnet(), args[1]))
+  {
+    fail_msg_writer() << tr("failed to parse address");
+    return true;
+  }
+
+  // parse pubkey r*A & signature
+  std::string sig_str = args[2];
+  const size_t header_len = strlen("ProofV1");
+  if (sig_str.size() < header_len || sig_str.substr(0, header_len) != "ProofV1")
+  {
+    fail_msg_writer() << tr("Signature header check error");
+    return true;
+  }
+  crypto::public_key rA;
+  crypto::signature sig;
+  const size_t rA_len = tools::base58::encode(std::string((const char *)&rA, sizeof(crypto::public_key))).size();
+  const size_t sig_len = tools::base58::encode(std::string((const char *)&sig, sizeof(crypto::signature))).size();
+  std::string rA_decoded;
+  std::string sig_decoded;
+  if (!tools::base58::decode(sig_str.substr(header_len, rA_len), rA_decoded))
+  {
+    fail_msg_writer() << tr("Signature decoding error");
+    return true;
+  }
+  if (!tools::base58::decode(sig_str.substr(header_len + rA_len, sig_len), sig_decoded))
+  {
+    fail_msg_writer() << tr("Signature decoding error");
+    return true;
+  }
+  if (sizeof(crypto::public_key) != rA_decoded.size() || sizeof(crypto::signature) != sig_decoded.size())
+  {
+    fail_msg_writer() << tr("Signature decoding error");
+    return true;
+  }
+  memcpy(&rA, rA_decoded.data(), sizeof(crypto::public_key));
+  memcpy(&sig, sig_decoded.data(), sizeof(crypto::signature));
+
+  // fetch tx pubkey from the daemon
+  COMMAND_RPC_GET_TRANSACTIONS::request req;
+  COMMAND_RPC_GET_TRANSACTIONS::response res;
+  req.txs_hashes.push_back(epee::string_tools::pod_to_hex(txid));
+  if (!net_utils::invoke_http_json("/gettransactions", req, res, m_http_client) ||
+      (res.txs.size() != 1 && res.txs_as_hex.size() != 1))
+  {
+    fail_msg_writer() << tr("failed to get transaction from daemon");
+    return true;
+  }
+  cryptonote::blobdata tx_data;
+  bool ok;
+  if (res.txs.size() == 1)
+    ok = string_tools::parse_hexstr_to_binbuff(res.txs.front().as_hex, tx_data);
+  else
+    ok = string_tools::parse_hexstr_to_binbuff(res.txs_as_hex.front(), tx_data);
+  if (!ok)
+  {
+    fail_msg_writer() << tr("failed to parse transaction from daemon");
+    return true;
+  }
+  crypto::hash tx_hash, tx_prefix_hash;
+  cryptonote::transaction tx;
+  if (!cryptonote::parse_and_validate_tx_from_blob(tx_data, tx, tx_hash, tx_prefix_hash))
+  {
+    fail_msg_writer() << tr("failed to validate transaction from daemon");
+    return true;
+  }
+  if (tx_hash != txid)
+  {
+    fail_msg_writer() << tr("failed to get the right transaction from daemon");
+    return true;
+  }
+  crypto::public_key R = get_tx_pub_key_from_extra(tx);
+  if (R == null_pkey)
+  {
+    fail_msg_writer() << tr("Tx pubkey was not found");
+    return true;
+  }
+
+  // check signature
+  if (crypto::check_tx_proof(txid, R, address.m_view_public_key, rA, sig))
+  {
+    success_msg_writer() << tr("Good signature");
+  }
+  else
+  {
+    fail_msg_writer() << tr("Bad signature");
+    return true;
+  }
+
+  // obtain key derivation by multiplying scalar 1 to the pubkey r*A included in the signature
+  crypto::key_derivation derivation;
+  if (!crypto::generate_key_derivation(rA, rct::rct2sk(rct::I), derivation))
+  {
+    fail_msg_writer() << tr("failed to generate key derivation");
+    return true;
+  }
+
+  return check_tx_key_helper(txid, address, derivation);
+}
+//----------------------------------------------------------------------------------------------------
 static std::string get_human_readable_timestamp(uint64_t ts)
 {
   char buffer[64];

src/simplewallet/simplewallet.h

@@ -154,6 +154,9 @@ namespace cryptonote
     bool set_log(const std::vector<std::string> &args);
     bool get_tx_key(const std::vector<std::string> &args);
     bool check_tx_key(const std::vector<std::string> &args);
+    bool check_tx_key_helper(const crypto::hash &txid, const cryptonote::account_public_address &address, const crypto::key_derivation &derivation);
+    bool get_tx_proof(const std::vector<std::string> &args);
+    bool check_tx_proof(const std::vector<std::string> &args);
     bool show_transfers(const std::vector<std::string> &args);
     bool unspent_outputs(const std::vector<std::string> &args);
     bool rescan_blockchain(const std::vector<std::string> &args);