To improve privacy, relay locally submitted transactions (from the wallet or via RPC) to the P2P network only via Tor or I2P short-lived connections. * Introduce a new connection type for relaying sensitive data (our own transactions) with the following properties: * started whenever there are local unbroadcast transactions to be sent * only opened to Tor or I2P peers * opened regardless of max connections limits * after handshake is completed one local transaction is pushed to the peer and the connection is closed * ignore all incoming messages after handshake is completed * Relay locally submitted transactions (from the wallet or via RPC) using this new mechanism, to a few Tor or I2P peers. * Hide those transactions from `GETDATA` and `MEMPOOL` requests, as if we don't have them. * Once we get an `INV` from somebody, request the transaction with `GETDATA`, as if we didn't have it before. * After we receive the full transaction as a `TX` message, in reply to our `GETDATA` request, only then consider the transaction has propagated through the network and remove it from the unbroadcast list. * `INV` it to others as a result of the `TX` message we get, like what we do with transactions that are not ours. The messages exchange should look like this: ``` tx-sender >--- connect -------> tx-recipient tx-sender >--- VERSION -------> tx-recipient tx-sender <--- VERSION -------< tx-recipient tx-sender <--- WTXIDRELAY ----< tx-recipient (maybe) tx-sender <--- SENDADDRV2 ----< tx-recipient (maybe) tx-sender <--- SENDTXRCNCL ---< tx-recipient (maybe) tx-sender <--- VERACK --------< tx-recipient tx-sender >--- VERACK --------> tx-recipient tx-sender >--- TX ------------> tx-recipient tx-sender >--- PING ----------> tx-recipient tx-sender <--- PONG ----------< tx-recipient tx-sender disconnects ``` Whenever a new local transaction is received (from the wallet or RPC), the node will send it to 5 (`SENSITIVE_RELAY_NUM_BROADCAST_PER_TX`) recipients right away. If after 10-15 mins we still have not heard anything about the transaction from the network, then it will be sent to 5 more peers (see `PeerManagerImpl::ReattemptInitialBroadcast()`). A few considerations: * The short-lived sensitive relay connections are very cheap and fast wrt network traffic. It is expected that some of those peers will blackhole the transaction. Just one honest/proper peer is enough for successful propagation. * The peers that receive the transaction could deduce that this is initial transaction broadcast from the transaction originator. This is ok, they can't identify the sender. TODO: * [x] Disable if `-connect` is used or Tor and I2P are not reachable. * [x] Use I2P transient connections even if listening on I2P. * [x] Do something with the user-agent string (could reveal identity if it has been personalized) and make sure no identifying information in the `VERSION` message. Done: took the [suggestion below](https://github.com/bitcoin/bitcoin/pull/27509#issuecomment-1517896418). * [x] Improve the condition when we consider the transaction seen by the network: right now a single INV from somebody suffices (this is still an improvement over `master` which is ok right after pushing the tx to a peer). We could wait for more than one INV, from peers that we have selected (outbound) and that are specifically not the ones we broadcasted to (via short-lived connection). Edit: One `INV` should be enough because after that we broadcast the transaction to everybody we are connected to (like if it is not ours). * [ ] Idea: start with one connection per transaction (not 5 as it is now), optimistically assuming it will be sufficient. (numbers are examples) After 1 minute, if still not received from the network, try 2 connections, after a few minutes try more. If still unsuccessful after 10 minutes, then fall back to the old mechanism. This is supposed to address: https://github.com/bitcoin/bitcoin/issues/3828 https://github.com/bitcoin/bitcoin/issues/14692 https://github.com/bitcoin/bitcoin/issues/19042 https://github.com/bitcoin/bitcoin/issues/24557 https://github.com/bitcoin/bitcoin/issues/25450

Relay own transactions only via short-lived Tor or I2P connections