net: introduce a new connection type for private broadcast

We will open a short-lived connection to a random Tor or I2P peer,
send our transaction to that peer and close the connection.

src/bitcoin-cli.cpp

@@ -452,6 +452,7 @@ private:
         if (conn_type == "block-relay-only") return "block";
         if (conn_type == "manual" || conn_type == "feeler") return conn_type;
         if (conn_type == "addr-fetch") return "addr";
+        if (conn_type == "private-broadcast") return "priv";
         return "";
     }
     std::string FormatServices(const UniValue& services)
@@ -703,6 +704,7 @@ public:
         "           \"manual\" - peer we manually added using RPC addnode or the -addnode/-connect config options\n"
         "           \"feeler\" - short-lived connection for testing addresses\n"
         "           \"addr\"   - address fetch; short-lived connection for requesting addresses\n"
+        "           \"priv\"   - private broadcast; short-lived connection for broadcasting our transactions\n"
         "  net      Network the peer connected through (\"ipv4\", \"ipv6\", \"onion\", \"i2p\", \"cjdns\", or \"npr\" (not publicly routable))\n"
         "  serv     Services offered by the peer\n"
         "           \"n\" - NETWORK: peer can serve the full block chain\n"

src/net.cpp

@@ -1876,6 +1876,7 @@ bool CConnman::AddConnection(const std::string& address, ConnectionType conn_typ
     switch (conn_type) {
     case ConnectionType::INBOUND:
     case ConnectionType::MANUAL:
+    case ConnectionType::PRIVATE_BROADCAST:
         return false;
     case ConnectionType::OUTBOUND_FULL_RELAY:
         max_connections = m_max_outbound_full_relay;
@@ -2666,6 +2667,7 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect, std
                     // peers from addrman.
                     case ConnectionType::ADDR_FETCH:
                     case ConnectionType::FEELER:
+                    case ConnectionType::PRIVATE_BROADCAST:
                         break;
                     case ConnectionType::MANUAL:
                     case ConnectionType::OUTBOUND_FULL_RELAY:

src/net.h

@@ -775,6 +775,7 @@ public:
             case ConnectionType::MANUAL:
             case ConnectionType::ADDR_FETCH:
             case ConnectionType::FEELER:
+            case ConnectionType::PRIVATE_BROADCAST:
                 return false;
         } // no default case, so the compiler can warn about missing cases
 
@@ -796,6 +797,7 @@ public:
         case ConnectionType::FEELER:
         case ConnectionType::BLOCK_RELAY:
         case ConnectionType::ADDR_FETCH:
+        case ConnectionType::PRIVATE_BROADCAST:
                 return false;
         case ConnectionType::OUTBOUND_FULL_RELAY:
         case ConnectionType::MANUAL:
@@ -817,6 +819,11 @@ public:
         return m_conn_type == ConnectionType::ADDR_FETCH;
     }
 
+    bool IsPrivateBroadcastConn() const
+    {
+        return m_conn_type == ConnectionType::PRIVATE_BROADCAST;
+    }
+
     bool IsInboundConn() const {
         return m_conn_type == ConnectionType::INBOUND;
     }
@@ -830,6 +837,7 @@ public:
             case ConnectionType::OUTBOUND_FULL_RELAY:
             case ConnectionType::BLOCK_RELAY:
             case ConnectionType::ADDR_FETCH:
+            case ConnectionType::PRIVATE_BROADCAST:
                 return true;
         } // no default case, so the compiler can warn about missing cases
 

src/node/connection_types.cpp

@@ -20,6 +20,8 @@ std::string ConnectionTypeAsString(ConnectionType conn_type)
         return "block-relay-only";
     case ConnectionType::ADDR_FETCH:
         return "addr-fetch";
+    case ConnectionType::PRIVATE_BROADCAST:
+        return "private-broadcast";
     } // no default case, so the compiler can warn about missing cases
 
     assert(false);

src/node/connection_types.h

@@ -75,6 +75,13 @@ enum class ConnectionType {
      * AddrMan is empty.
      */
     ADDR_FETCH,
+
+    /**
+     * Private broadcast connections are short-lived and only opened to
+     * privacy networks (Tor, I2P) for relaying privacy-sensitive data (like
+     * our own transactions) and closed afterwards.
+     */
+    PRIVATE_BROADCAST,
 };
 
 /** Convert ConnectionType enum to a string value */

src/qt/guiutil.cpp

@@ -722,6 +722,8 @@ QString ConnectionTypeToQString(ConnectionType conn_type, bool prepend_direction
     case ConnectionType::FEELER: return prefix + QObject::tr("Feeler");
     //: Short-lived peer connection type that solicits known addresses from a peer.
     case ConnectionType::ADDR_FETCH: return prefix + QObject::tr("Address Fetch");
+    //: Short-lived peer connection type that is used for broadcasting privacy-sensitive data.
+    case ConnectionType::PRIVATE_BROADCAST: return prefix + QObject::tr("Private Broadcast");
     } // no default case, so the compiler can warn about missing cases
     assert(false);
 }

src/qt/rpcconsole.cpp

@@ -484,7 +484,10 @@ RPCConsole::RPCConsole(interfaces::Node& node, const PlatformStyle *_platformSty
         tr("Outbound Feeler: short-lived, for testing addresses"),
         /*: Explanatory text for a short-lived outbound peer connection that is used
             to request addresses from a peer. */
-        tr("Outbound Address Fetch: short-lived, for soliciting addresses")};
+        tr("Outbound Address Fetch: short-lived, for soliciting addresses"),
+        /*: Explanatory text for a short-lived outbound peer connection that is used
+            to broadcast privacy-sensitive data (like our transactions). */
+        tr("Private broadcast: short-lived, for broadcasting privacy-sensitive transactions")};
     const QString connection_types_list{"<ul><li>" + Join(CONNECTION_TYPE_DOC, QString("</li><li>")) + "</li></ul>"};
     ui->peerConnectionTypeLabel->setToolTip(ui->peerConnectionTypeLabel->toolTip().arg(connection_types_list));
     const std::vector<QString> TRANSPORT_TYPE_DOC{

src/rpc/net.cpp

@@ -48,7 +48,8 @@ const std::vector<std::string> CONNECTION_TYPE_DOC{
         "inbound (initiated by the peer)",
         "manual (added via addnode RPC or -addnode/-connect configuration options)",
         "addr-fetch (short-lived automatic connection for soliciting addresses)",
-        "feeler (short-lived automatic connection for testing addresses)"
+        "feeler (short-lived automatic connection for testing addresses)",
+        "private-broadcast (short-lived automatic connection for broadcasting privacy-sensitive transactions)"
 };
 
 const std::vector<std::string> TRANSPORT_TYPE_DOC{

src/test/util/net.h

@@ -143,6 +143,7 @@ constexpr ConnectionType ALL_CONNECTION_TYPES[]{
     ConnectionType::FEELER,
     ConnectionType::BLOCK_RELAY,
     ConnectionType::ADDR_FETCH,
+    ConnectionType::PRIVATE_BROADCAST,
 };
 
 constexpr auto ALL_NETWORKS = std::array{