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+# Anonymity Networks with Monero
+
+Currently only Tor and I2P have been integrated into Monero. 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.
+
+Anonymity networks can also be used with `monero-wallet-cli` and
+`monero-wallet-rpc` - the wallets will connect to a daemon through a proxy. The
+daemon must provide a hidden service for the RPC itself, which is separate from
+the hidden service for P2P connections.
+
+
+## P2P Commands
+
+Only handshakes, peer timed syncs and transaction broadcast messages are
+supported over anonymity networks. If one `--add-exclusive-node` p2p 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
+`--tx-proxy` which tells `monerod` the ip/port of a socks proxy provided by a
+separate process. On most systems the configuration will look like:
+
+```
+--tx-proxy tor,127.0.0.1:9050,10
+--tx-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
+".b32.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 undesirable.
+
+### 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:28083,127.0.0.1:28083,25
+--anonymous-inbound cmeua5767mz2q5jsaelk2rxhf67agrwuetaso5dzbenyzwlbkg2q.b32.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 "cmeua5767mz2q5jsaelk2rxhf67agrwuetaso5dzbenyzwlbkg2q.b32.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.
+
+### Wallet RPC
+
+An anonymity network can be configured to forward incoming connections to a
+`monerod` RPC port - which is independent from the configuration for incoming
+P2P anonymity connections. The anonymity network (Tor/i2p) is
+[configured in the same manner](#configuration), except the localhost port
+must be the RPC port (typically 18081 for mainnet) instead of the p2p port:
+
+```
+HiddenServiceDir /var/lib/tor/data/monero
+HiddenServicePort 18081 127.0.0.1:18081
+```
+
+Then the wallet will be configured to use a Tor/i2p address:
+```
+--proxy 127.0.0.1:9050
+--daemon-address rveahdfho7wo4b2m.onion
+```
+
+The proxy must match the address type - a Tor proxy will not work properly with
+i2p addresses, etc.
+
+i2p and onion addresses provide the information necessary to authenticate and
+encrypt the connection from end-to-end. If desired, SSL can also be applied to
+the connection with `--daemon-address https://rveahdfho7wo4b2m.onion` which
+requires a server certificate that is signed by a "root" certificate on the
+machine running the wallet. Alternatively, `--daemon-cert-file` can be used to
+specify a certificate to authenticate the server.
+
+Proxies can also be used to connect to "clearnet" (ipv4 addresses or ICANN
+domains), but `--daemon-cert-file` _must_ be used for authentication and
+encryption.
+
+### Network Types
+
+#### Tor & I2P
+
+Options `--add-exclusive-node` and `--add-peer` recognize ".onion" and
+".b32.i2p" addresses, and will properly forward those addresses to the proxy
+provided with `--tx-proxy tor,...` or `--tx-proxy i2p,...`.
+
+Option `--anonymous-inbound` also recognizes ".onion" and ".b32.i2p" addresses,
+and will automatically be sent out to outgoing Tor/I2P 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`.
+
+I2P must be configured with a standard server tunnel. Configuration differs by
+I2P implementation.
+
+## 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.
+
+### 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 completely 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/I2P connection. There isn't great mitigation against
+this, but I2P should provide better protection against this attack since
+the connections are not circuit based.
+
+#### Mitigation
+
+The best mitigation is to use I2P instead of Tor. However, I2P
+has a smaller set of users (less cover traffic) and academic reviews, so there
+is a trade off in potential issues. Also, anyone attempting this strategy really
+wants to uncover a user, it seems unlikely that this would be performed against
+every Tor/I2P user.
+
+### I2P/Tor Stream Used Twice
+
+If a single I2P/Tor stream is used 2+ times for transmitting a transaction, the
+operator of the hidden service can conclude that both transactions came from the
+same source. If the subsequent transactions spend a change output from the
+earlier transactions, this will also reveal the "real" spend in the ring
+signature. This issue was (primarily) raised by @secparam on Twitter.
+
+#### Mitigation
+
+`monerod` currently selects two outgoing connections every 5 minutes for
+transmitting transactions over I2P/Tor. Using outgoing connections prevents an
+adversary from making many incoming connections to obtain information (this
+technique was taken from Dandelion). Outgoing connections also do not have a
+persistent public key identity - the creation of a new circuit will generate
+a new public key identity. The lock time on a change address is ~20 minutes, so
+`monerod` will have rotated its selected outgoing connections several times in
+most cases. However, the number of outgoing connections is typically a small
+fixed number, so there is a decent probability of re-use with the same public
+key identity.
+
+@secparam (twitter) recommended changing circuits (Tor) as an additional
+precaution. This is likely not a good idea - forcibly requesting Tor to change
+circuits is observable by the ISP. Instead, `monerod` should likely disconnect
+from peers occasionally. Tor will rotate circuits every ~10 minutes, so
+establishing new connections will use a new public key identity and make it
+more difficult for the hidden service to link information. This process will
+have to be done carefully because closing/reconnecting connections can also
+leak information to hidden services if done improperly.
+
+At the current time, if users need to frequently make transactions, I2P/Tor
+will improve privacy from ISPs and other common adversaries, but still have
+some metadata leakages to unknown hidden service operators.