blockchain: deterministic UNIX time unlock checks

Based on a patch by TheCharlatan <seb.kung@gmail.com>

src/cryptonote_config.h

@@ -180,6 +180,7 @@
 #define HF_VERSION_EFFECTIVE_SHORT_TERM_MEDIAN_IN_PENALTY 12
 #define HF_VERSION_EXACT_COINBASE               13
 #define HF_VERSION_CLSAG                        13
+#define HF_VERSION_DETERMINISTIC_UNLOCK_TIME    13
 
 #define PER_KB_FEE_QUANTIZATION_DECIMALS        8
 

src/cryptonote_core/blockchain.cpp

@@ -2267,8 +2267,9 @@ bool Blockchain::get_outs(const COMMAND_RPC_GET_OUTPUTS_BIN::request& req, COMMA
       MERROR("Unexpected output data size: expected " << req.outputs.size() << ", got " << data.size());
       return false;
     }
+    const uint8_t hf_version = m_hardfork->get_current_version();
     for (const auto &t: data)
-      res.outs.push_back({t.pubkey, t.commitment, is_tx_spendtime_unlocked(t.unlock_time), t.height, crypto::null_hash});
+      res.outs.push_back({t.pubkey, t.commitment, is_tx_spendtime_unlocked(t.unlock_time, hf_version), t.height, crypto::null_hash});
 
     if (req.get_txid)
     {
@@ -2292,7 +2293,8 @@ void Blockchain::get_output_key_mask_unlocked(const uint64_t& amount, const uint
   key = o_data.pubkey;
   mask = o_data.commitment;
   tx_out_index toi = m_db->get_output_tx_and_index(amount, index);
-  unlocked = is_tx_spendtime_unlocked(m_db->get_tx_unlock_time(toi.first));
+  const uint8_t hf_version = m_hardfork->get_current_version();
+  unlocked = is_tx_spendtime_unlocked(m_db->get_tx_unlock_time(toi.first), hf_version);
 }
 //------------------------------------------------------------------
 bool Blockchain::get_output_distribution(uint64_t amount, uint64_t from_height, uint64_t to_height, uint64_t &start_height, std::vector<uint64_t> &distribution, uint64_t &base) const
@@ -3363,7 +3365,7 @@ bool Blockchain::check_tx_inputs(transaction& tx, tx_verification_context &tvc,
 
     // make sure that output being spent matches up correctly with the
     // signature spending it.
-    if (!check_tx_input(tx.version, in_to_key, tx_prefix_hash, tx.version == 1 ? tx.signatures[sig_index] : std::vector<crypto::signature>(), tx.rct_signatures, pubkeys[sig_index], pmax_used_block_height))
+    if (!check_tx_input(tx.version, in_to_key, tx_prefix_hash, tx.version == 1 ? tx.signatures[sig_index] : std::vector<crypto::signature>(), tx.rct_signatures, pubkeys[sig_index], pmax_used_block_height, hf_version))
     {
       MERROR_VER("Failed to check ring signature for tx " << get_transaction_hash(tx) << "  vin key with k_image: " << in_to_key.k_image << "  sig_index: " << sig_index);
       if (pmax_used_block_height) // a default value of NULL is used when called from Blockchain::handle_block_to_main_chain()
@@ -3756,7 +3758,7 @@ uint64_t Blockchain::get_dynamic_base_fee_estimate(uint64_t grace_blocks) const
 //------------------------------------------------------------------
 // This function checks to see if a tx is unlocked.  unlock_time is either
 // a block index or a unix time.
-bool Blockchain::is_tx_spendtime_unlocked(uint64_t unlock_time) const
+bool Blockchain::is_tx_spendtime_unlocked(uint64_t unlock_time, uint8_t hf_version) const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
   if(unlock_time < CRYPTONOTE_MAX_BLOCK_NUMBER)
@@ -3771,7 +3773,7 @@ bool Blockchain::is_tx_spendtime_unlocked(uint64_t unlock_time) const
   else
   {
     //interpret as time
-    uint64_t current_time = static_cast<uint64_t>(time(NULL));
+    const uint64_t current_time = hf_version >= HF_VERSION_DETERMINISTIC_UNLOCK_TIME ? get_adjusted_time(m_db->height()) : static_cast<uint64_t>(time(NULL));
     if(current_time + (get_current_hard_fork_version() < 2 ? CRYPTONOTE_LOCKED_TX_ALLOWED_DELTA_SECONDS_V1 : CRYPTONOTE_LOCKED_TX_ALLOWED_DELTA_SECONDS_V2) >= unlock_time)
       return true;
     else
@@ -3783,7 +3785,7 @@ bool Blockchain::is_tx_spendtime_unlocked(uint64_t unlock_time) const
 // This function locates all outputs associated with a given input (mixins)
 // and validates that they exist and are usable.  It also checks the ring
 // signature for each input.
-bool Blockchain::check_tx_input(size_t tx_version, const txin_to_key& txin, const crypto::hash& tx_prefix_hash, const std::vector<crypto::signature>& sig, const rct::rctSig &rct_signatures, std::vector<rct::ctkey> &output_keys, uint64_t* pmax_related_block_height) const
+bool Blockchain::check_tx_input(size_t tx_version, const txin_to_key& txin, const crypto::hash& tx_prefix_hash, const std::vector<crypto::signature>& sig, const rct::rctSig &rct_signatures, std::vector<rct::ctkey> &output_keys, uint64_t* pmax_related_block_height, uint8_t hf_version) const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
 
@@ -3795,14 +3797,15 @@ bool Blockchain::check_tx_input(size_t tx_version, const txin_to_key& txin, cons
   {
     std::vector<rct::ctkey >& m_output_keys;
     const Blockchain& m_bch;
-    outputs_visitor(std::vector<rct::ctkey>& output_keys, const Blockchain& bch) :
+    const uint8_t hf_version;
-      m_output_keys(output_keys), m_bch(bch)
+    outputs_visitor(std::vector<rct::ctkey>& output_keys, const Blockchain& bch, uint8_t hf_version) :
+      m_output_keys(output_keys), m_bch(bch), hf_version(hf_version)
     {
     }
     bool handle_output(uint64_t unlock_time, const crypto::public_key &pubkey, const rct::key &commitment)
     {
       //check tx unlock time
-      if (!m_bch.is_tx_spendtime_unlocked(unlock_time))
+      if (!m_bch.is_tx_spendtime_unlocked(unlock_time, hf_version))
       {
         MERROR_VER("One of outputs for one of inputs has wrong tx.unlock_time = " << unlock_time);
         return false;
@@ -3821,7 +3824,7 @@ bool Blockchain::check_tx_input(size_t tx_version, const txin_to_key& txin, cons
   output_keys.clear();
 
   // collect output keys
-  outputs_visitor vi(output_keys, *this);
+  outputs_visitor vi(output_keys, *this, hf_version);
   if (!scan_outputkeys_for_indexes(tx_version, txin, vi, tx_prefix_hash, pmax_related_block_height))
   {
     MERROR_VER("Failed to get output keys for tx with amount = " << print_money(txin.amount) << " and count indexes " << txin.key_offsets.size());
@@ -3840,12 +3843,38 @@ bool Blockchain::check_tx_input(size_t tx_version, const txin_to_key& txin, cons
   return true;
 }
 //------------------------------------------------------------------
-//TODO: Is this intended to do something else?  Need to look into the todo there.
+// only works on the main chain
-uint64_t Blockchain::get_adjusted_time() const
+uint64_t Blockchain::get_adjusted_time(uint64_t height) const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
-  //TODO: add collecting median time
+
-  return time(NULL);
+  // if not enough blocks, no proper median yet, return current time
+  if(height < BLOCKCHAIN_TIMESTAMP_CHECK_WINDOW)
+  {
+      return static_cast<uint64_t>(time(NULL));
+  }
+  std::vector<uint64_t> timestamps;
+
+  // need most recent 60 blocks, get index of first of those
+  size_t offset = height - BLOCKCHAIN_TIMESTAMP_CHECK_WINDOW;
+  timestamps.reserve(height - offset);
+  for(;offset < height; ++offset)
+  {
+    timestamps.push_back(m_db->get_block_timestamp(offset));
+  }
+  uint64_t median_ts = epee::misc_utils::median(timestamps);
+
+  // project the median to match approximately when the block being validated will appear
+  // the median is calculated from a chunk of past blocks, so we use +1 to offset onto the current block
+  median_ts += (BLOCKCHAIN_TIMESTAMP_CHECK_WINDOW + 1) * DIFFICULTY_TARGET_V2 / 2;
+
+  // project the current block's time based on the previous block's time
+  // we don't use the current block's time directly to mitigate timestamp manipulation
+  uint64_t adjusted_current_block_ts = timestamps.back() + DIFFICULTY_TARGET_V2;
+
+  // return minimum of ~current block time and adjusted median time
+  // we do this since it's better to report a time in the past than a time in the future
+  return (adjusted_current_block_ts < median_ts ? adjusted_current_block_ts : median_ts);
 }
 //------------------------------------------------------------------
 //TODO: revisit, has changed a bit on upstream
@@ -3873,9 +3902,9 @@ bool Blockchain::check_block_timestamp(std::vector<uint64_t>& timestamps, const
 bool Blockchain::check_block_timestamp(const block& b, uint64_t& median_ts) const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
-  if(b.timestamp > get_adjusted_time() + CRYPTONOTE_BLOCK_FUTURE_TIME_LIMIT)
+  if(b.timestamp > (uint64_t)time(NULL) + CRYPTONOTE_BLOCK_FUTURE_TIME_LIMIT)
   {
-    MERROR_VER("Timestamp of block with id: " << get_block_hash(b) << ", " << b.timestamp << ", bigger than adjusted time + 2 hours");
+    MERROR_VER("Timestamp of block with id: " << get_block_hash(b) << ", " << b.timestamp << ", bigger than local time + 2 hours");
     return false;
   }
 

src/cryptonote_core/blockchain.h

@@ -1042,6 +1042,21 @@ namespace cryptonote
      */
     void flush_invalid_blocks();
 
+    /**
+     * @brief get the "adjusted time"
+     *
+     * Computes the median timestamp of the previous 60 blocks, projects it
+     * onto the current block to get an 'adjusted median time' which approximates
+     * what the current block's timestamp should be. Also projects the previous
+     * block's timestamp to estimate the current block's timestamp.
+     * 
+     * Returns the minimum of the two projections, or the current local time on
+     * the machine if less than 60 blocks are available.
+     *
+     * @return current time approximated from chain data
+     */
+    uint64_t get_adjusted_time(uint64_t height) const;
+
 #ifndef IN_UNIT_TESTS
   private:
 #endif
@@ -1182,10 +1197,11 @@ namespace cryptonote
      * @param output_keys return-by-reference the public keys of the outputs in the input set
      * @param rct_signatures the ringCT signatures, which are only valid if tx version > 1
      * @param pmax_related_block_height return-by-pointer the height of the most recent block in the input set
+     * @param hf_version the consensus rules version to use
      *
      * @return false if any output is not yet unlocked, or is missing, otherwise true
      */
-    bool check_tx_input(size_t tx_version,const txin_to_key& txin, const crypto::hash& tx_prefix_hash, const std::vector<crypto::signature>& sig, const rct::rctSig &rct_signatures, std::vector<rct::ctkey> &output_keys, uint64_t* pmax_related_block_height) const;
+    bool check_tx_input(size_t tx_version,const txin_to_key& txin, const crypto::hash& tx_prefix_hash, const std::vector<crypto::signature>& sig, const rct::rctSig &rct_signatures, std::vector<rct::ctkey> &output_keys, uint64_t* pmax_related_block_height, uint8_t hf_version) const;
 
     /**
      * @brief validate a transaction's inputs and their keys
@@ -1373,10 +1389,11 @@ namespace cryptonote
      * unlock_time is either a block index or a unix time.
      *
      * @param unlock_time the unlock parameter (height or time)
+     * @param hf_version the consensus rules version to use
      *
      * @return true if spendable, otherwise false
      */
-    bool is_tx_spendtime_unlocked(uint64_t unlock_time) const;
+    bool is_tx_spendtime_unlocked(uint64_t unlock_time, uint8_t hf_version) const;
 
     /**
      * @brief stores an invalid block in a separate container
@@ -1437,16 +1454,6 @@ namespace cryptonote
     bool check_block_timestamp(std::vector<uint64_t>& timestamps, const block& b, uint64_t& median_ts) const;
     bool check_block_timestamp(std::vector<uint64_t>& timestamps, const block& b) const { uint64_t median_ts; return check_block_timestamp(timestamps, b, median_ts); }
 
-    /**
-     * @brief get the "adjusted time"
-     *
-     * Currently this simply returns the current time according to the
-     * user's machine.
-     *
-     * @return the current time
-     */
-    uint64_t get_adjusted_time() const;
-
     /**
      * @brief finish an alternate chain's timestamp window from the main chain
      *