monero/docs/ANONYMITY_NETWORKS.md

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

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.

If desired, peers can be manually specified:

--add-exclusive-node 5tymba6faziy36md5ffy42vatbjzlye4vyr3gyz6lcvdfximnvwpmwqd.onion:28083
--add-peer 5tymba6faziy36md5ffy42vatbjzlye4vyr3gyz6lcvdfximnvwpmwqd.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 5tymba6faziy36md5ffy42vatbjzlye4vyr3gyz6lcvdfximnvwpmwqd.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 "5tymba6faziy36md5ffy42vatbjzlye4vyr3gyz6lcvdfximnvwpmwqd.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, 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 5tymba6faziy36md5ffy42vatbjzlye4vyr3gyz6lcvdfximnvwpmwqd.onion

The proxy must match the address type - a Tor proxy will not work properly with I2P addresses, etc.

I2P hidden service (b32.i2p) and Tor Hidden service (.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://5tymba6faziy36md5ffy42vatbjzlye4vyr3gyz6lcvdfximnvwpmwqd.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.