Adding initial support for broadcasting transactions over Tor

- Support for ".onion" in --add-exclusive-node and --add-peer
  - Add --anonymizing-proxy for outbound Tor connections
  - Add --anonymous-inbounds for inbound Tor connections
  - Support for sharing ".onion" addresses over Tor connections
  - Support for broadcasting transactions received over RPC exclusively
    over Tor (else broadcast over public IP when Tor not enabled).

ANONYMITY_NETWORKS.md

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+# Anonymity Networks with Monero
+
+Currently only Tor has been integrated into Monero. Providing support for
+Kovri/I2P should be minimal, but has not yet been attempted. The usage of
+these networks is still considered experimental - there are a few pessimistic
+cases where privacy is leaked. The design is intended to maximize privacy of
+the source of a transaction by broadcasting it over an anonymity network, while
+relying on IPv4 for the remainder of messages to make surrounding node attacks
+(via sybil) more difficult.
+
+
+## Behavior
+
+If _any_ anonymity network is enabled, transactions being broadcast that lack
+a valid "context" (i.e. the transaction did not come from a p2p connection),
+will only be sent to peers on anonymity networks. If an anonymity network is
+enabled but no peers over an anonymity network are available, an error is
+logged and the transaction is kept for future broadcasting over an anonymity
+network. The transaction will not be broadcast unless an anonymity connection
+is made or until `monerod` is shutdown and restarted with only public
+connections enabled.
+
+
+## P2P Commands
+
+Only handshakes, peer timed syncs, and transaction broadcast messages are
+supported over anonymity networks. If one `--add-exclusive-node` onion address
+is specified, then no syncing will take place and only transaction broadcasting
+can occur. It is therefore recommended that `--add-exclusive-node` be combined
+with additional exclusive IPv4 address(es).
+
+
+## Usage
+
+Anonymity networks have no seed nodes (the feature is still considered
+experimental), so a user must specify an address. If configured properly,
+additional peers can be found through typical p2p peerlist sharing.
+
+### Outbound Connections
+
+Connecting to an anonymous address requires the command line option
+`--proxy` which tells `monerod` the ip/port of a socks proxy provided by a
+separate process. On most systems the configuration will look like:
+
+> `--proxy tor,127.0.0.1:9050,10`
+> `--proxy i2p,127.0.0.1:9000`
+
+which tells `monerod` that ".onion" p2p addresses can be forwarded to a socks
+proxy at IP 127.0.0.1 port 9050 with a max of 10 outgoing connections and
+".i2p" p2p addresses can be forwarded to a socks proxy at IP 127.0.0.1 port 9000
+with the default max outgoing connections. Since there are no seed nodes for
+anonymity connections, peers must be manually specified:
+
+> `--add-exclusive-node rveahdfho7wo4b2m.onion:28083`
+> `--add-peer rveahdfho7wo4b2m.onion:28083`
+
+Either option can be listed multiple times, and can specify any mix of Tor,
+I2P, and IPv4 addresses. Using `--add-exclusive-node` will prevent the usage of
+seed nodes on ALL networks, which will typically be undesireable.
+
+### Inbound Connections
+
+Receiving anonymity connections is done through the option
+`--anonymous-inbound`. This option tells `monerod` the inbound address, network
+type, and max connections:
+
+> `--anonymous-inbound rveahdfho7wo4b2m.onion:28083,127.0.0.28083,25`
+> `--anonymous-inbound foobar.i2p:5000,127.0.0.1:30000`
+
+which tells `monerod` that a max of 25 inbound Tor connections are being
+received at address "rveahdfho7wo4b2m.onion:28083" and forwarded to `monerod`
+localhost port 28083, and a default max I2P connections are being received at
+address "foobar.i2p:5000" and forwarded to `monerod` localhost port 30000.
+These addresses will be shared with outgoing peers, over the same network type,
+otherwise the peer will not be notified of the peer address by the proxy.
+
+### Network Types
+
+#### Tor
+
+Options `--add-exclusive-node` and `--add-peer` recognize ".onion" addresses,
+and will properly forward those addresses to the proxy provided with
+`--proxy tor,...`.
+
+Option `--anonymous-inbound` also recognizes ".onion" addresses, and will
+automatically be sent out to outgoing Tor connections so the peer can
+distribute the address to its other peers.
+
+##### Configuration
+
+Tor must be configured for hidden services. An example configuration ("torrc")
+might look like:
+
+> HiddenServiceDir /var/lib/tor/data/monero
+> HiddenServicePort 28083 127.0.0.1:28083
+
+This will store key information in `/var/lib/tor/data/monero` and will forward
+"Tor port" 28083 to port 28083 of ip 127.0.0.1. The file
+`/usr/lib/tor/data/monero/hostname` will contain the ".onion" address for use
+with `--anonymous-inbound`.
+
+#### Kovri/I2P
+
+Support for this network has not been implemented. Using ".i2p" addresses or
+specifying "i2p" will currently generate an error.
+
+
+## Privacy Limitations
+
+There are currently some techniques that could be used to _possibly_ identify
+the machine that broadcast a transaction over an anonymity network.
+
+### Timestamps
+
+The peer timed sync command sends the current time in the message. This value
+can be used to link an onion address to an IPv4/IPv6 address. If a peer first
+sees a transaction over Tor, it could _assume_ (possibly incorrectly) that the
+transaction originated from the peer. If both the Tor connection and an
+IPv4/IPv6 connection have timestamps that are approximately close in value they
+could be used to link the two connections. This is less likely to happen if the
+system clock is fairly accurate - many peers on the Monero network should have
+similar timestamps.
+
+#### Mitigation
+
+Keep the system clock accurate so that fingerprinting is more difficult. In
+the future a random offset might be applied to anonymity networks so that if
+the system clock is noticeably off (and therefore more fingerprintable),
+linking the public IPv4/IPv6 connections with the anonymity networks will be
+more difficult.
+
+### Bandwidth Usage
+
+An ISP can passively monitor `monerod` connections from a node and observe when
+a transaction is sent over a Tor/Kovri connection via timing analysis + size of
+data sent during that timeframe. Kovri should provide better protection against
+this attack - its connections are not circuit based. However, if a node is
+only using Kovri for broadcasting Monero transactions, the total aggregate of
+Kovri/I2P data would also leak information.
+
+#### Mitigation
+
+There is no current mitigation for the user right now. This attack is fairly
+sophisticated, and likely requires support from the internet host of a Monero
+user.
+
+In the near future, "whitening" the amount of data sent over anonymity network
+connections will be performed. An attempt will be made to make a transaction
+broadcast indistinguishable from a peer timed sync command.
+
+### Intermittent Monero Syncing
+
+If a user only runs `monerod` to send a transaction then quit, this can also
+be used by an ISP to link a user to a transaction.
+
+#### Mitigation
+
+Run `monerod` as often as possible to conceal when transactions are being sent.
+Future versions will also have peers that first receive a transaction over an
+anonymity network delay the broadcast to public peers by a randomized amount.
+This will not completetely mitigate a user who syncs up sends then quits, in
+part because this rule is not enforceable, so this mitigation strategy is
+simply a best effort attempt.
+
+### Active Bandwidth Shaping
+
+An attacker could attempt to bandwidth shape traffic in an attempt to determine
+the source of a Tor/Kovri/I2P connection. There isn't great mitigation against
+this, but Kovri/I2P should provide better protection against this attack since
+the connections are not circuit based.
+
+#### Mitigation
+
+The best mitigiation is to use Kovri/I2P instead of Tor. However, Kovri/I2P
+has a smaller set of users (less cover traffic) and academic reviews, so there
+is a tradeoff in potential isses. Also, anyone attempting this strategy really
+wants to uncover a user, it seems unlikely that this would be performed against
+every Tor/Kovri/I2P user.
+