Add ability to check whether a given onion service is online

When another peer connects to us over Tor the connecting peer can lie
about its onion address. We can challenge the peer to sign a payload to
proof that the connecting peer has the private key of the provided onion
address. This change adds the ability to check whether the onion service
is known to the Tor directory servers.

network/src/main/java/bisq/network/p2p/node/transport/ClearNetTransport.java

@@ -83,4 +83,13 @@ public CompletableFuture<Void> shutdown() {
     public Optional<Address> getServerAddress(String serverId) {
         return Optional.empty();
     }
+
+    @Override
+    public boolean isAddressAvailable(Address address) {
+        try (Socket ignored = getSocket(address)) {
+            return true;
+        } catch (IOException e) {
+            return false;
+        }
+    }
 }

network/src/main/java/bisq/network/p2p/node/transport/I2PTransport.java

@@ -175,6 +175,11 @@ public Socket getSocket(Address address) throws IOException {
         }
     }
 
+    @Override
+    public boolean isAddressAvailable(Address address) {
+        throw new UnsupportedOperationException("isAddressAvailable needs to be implemented for I2P.");
+    }
+
     @Override
     public CompletableFuture<Void> shutdown() {
         initializeCalled = false;

network/src/main/java/bisq/network/p2p/node/transport/TorTransport.java

@@ -90,6 +90,11 @@ public Socket getSocket(Address address) throws IOException {
         return socket;
     }
 
+    @Override
+    public boolean isAddressAvailable(Address address) {
+        return tor.isHiddenServiceAvailable(address.getHost());
+    }
+
     public Optional<Socks5Proxy> getSocksProxy() throws IOException {
         return Optional.of(tor.getSocks5Proxy(null));
     }

network/src/main/java/bisq/network/p2p/node/transport/Transport.java

@@ -83,5 +83,7 @@ default Optional<Socks5Proxy> getSocksProxy() throws IOException {
 
     Optional<Address> getServerAddress(String serverId);
 
+    boolean isAddressAvailable(Address address);
+
     CompletableFuture<Void> shutdown();
 }

tor/src/main/java/bisq/tor/Tor.java

@@ -282,6 +282,10 @@ public Optional<String> getHostName(String serverId) {
         return Optional.empty();
     }
 
+    public boolean isHiddenServiceAvailable(String onionUrl) {
+        return torController.isHiddenServiceAvailable(onionUrl);
+    }
+
     private void setState(State newState) {
         log.info("Set new state {}", newState);
         checkArgument(newState.ordinal() > state.get().ordinal(),

tor/src/main/java/bisq/tor/TorController.java

@@ -119,6 +119,14 @@ int getProxyPort() throws IOException {
         return Integer.parseInt(port);
     }
 
+    boolean isHiddenServiceAvailable(String onionUrl) {
+        try {
+            return torControlConnection().isHSAvailable(onionUrl);
+        } catch (IOException e) {
+            return false;
+        }
+    }
+
     TorControlConnection.CreateHiddenServiceResult createHiddenService(int hiddenServicePort,
                                                                        int localPort) throws IOException {
         assertState();