protocol: option to pad transaction relay to the next kB

To help protect one's privacy from traffic volume analysis
for people using Tor or I2P. This will really fly once we
relay txes on a timer rather than on demand, though.

Off by default for now since it's wasteful and doesn't bring
anything until I2P's in.

2 files changed, 33 insertions, 0 deletions

diff --git a/src/cryptonote_protocol/cryptonote_protocol_defs.h b/src/cryptonote_protocol/cryptonote_protocol_defs.h
index db159f0f4..d5bb50930 100644
--- a/src/cryptonote_protocol/cryptonote_protocol_defs.h
+++ b/src/cryptonote_protocol/cryptonote_protocol_defs.h
@@ -146,9 +146,11 @@ namespace cryptonote
     struct request
     {
       std::vector<blobdata>   txs;
+      std::string _; // padding
 
       BEGIN_KV_SERIALIZE_MAP()
         KV_SERIALIZE(txs)
+        KV_SERIALIZE(_)
       END_KV_SERIALIZE_MAP()
     };
   };
diff --git a/src/cryptonote_protocol/cryptonote_protocol_handler.inl b/src/cryptonote_protocol/cryptonote_protocol_handler.inl
index c2c660e8c..6efdcc25e 100644
--- a/src/cryptonote_protocol/cryptonote_protocol_handler.inl
+++ b/src/cryptonote_protocol/cryptonote_protocol_handler.inl
@@ -1724,8 +1724,39 @@ skip:
   bool t_cryptonote_protocol_handler<t_core>::relay_transactions(NOTIFY_NEW_TRANSACTIONS::request& arg, cryptonote_connection_context& exclude_context)
   {
     // no check for success, so tell core they're relayed unconditionally
+    const bool pad_transactions = m_core.pad_transactions();
+    size_t bytes = pad_transactions ? 9 /* header */ + 4 /* 1 + 'txs' */ + tools::get_varint_data(arg.txs.size()).size() : 0;
     for(auto tx_blob_it = arg.txs.begin(); tx_blob_it!=arg.txs.end(); ++tx_blob_it)
+    {
       m_core.on_transaction_relayed(*tx_blob_it);
+      if (pad_transactions)
+        bytes += tools::get_varint_data(tx_blob_it->size()).size() + tx_blob_it->size();
+    }
+
+    if (pad_transactions)
+    {
+      // stuff some dummy bytes in to stay safe from traffic volume analysis
+      static constexpr size_t granularity = 1024;
+      size_t padding = granularity - bytes % granularity;
+      const size_t overhead = 2 /* 1 + '_' */ + tools::get_varint_data(padding).size();
+      if (overhead > padding)
+        padding = 0;
+      else
+        padding -= overhead;
+      arg._ = std::string(padding, ' ');
+
+      std::string arg_buff;
+      epee::serialization::store_t_to_binary(arg, arg_buff);
+
+      // we probably lowballed the payload size a bit, so added a but too much. Fix this now.
+      size_t remove = arg_buff.size() % granularity;
+      if (remove > arg._.size())
+        arg._.clear();
+      else
+        arg._.resize(arg._.size() - remove);
+      // if the size of _ moved enough, we might lose byte in size encoding, we don't care
+    }
+
     return relay_post_notify<NOTIFY_NEW_TRANSACTIONS>(arg, exclude_context);
   }
   //------------------------------------------------------------------------------------------------------------------------