225 lines
10 KiB
Markdown
225 lines
10 KiB
Markdown
# Anonymity Networks with Monero
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Currently only Tor and I2P have been integrated into Monero. The usage of
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these networks is still considered experimental - there are a few pessimistic
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cases where privacy is leaked. The design is intended to maximize privacy of
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the source of a transaction by broadcasting it over an anonymity network, while
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relying on IPv4 for the remainder of messages to make surrounding node attacks
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(via sybil) more difficult.
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## Behavior
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If _any_ anonymity network is enabled, transactions being broadcast that lack
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a valid "context" (i.e. the transaction did not come from a p2p connection),
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will only be sent to peers on anonymity networks. If an anonymity network is
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enabled but no peers over an anonymity network are available, an error is
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logged and the transaction is kept for future broadcasting over an anonymity
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network. The transaction will not be broadcast unless an anonymity connection
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is made or until `monerod` is shutdown and restarted with only public
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connections enabled.
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Anonymity networks can also be used with `monero-wallet-cli` and
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`monero-wallet-rpc` - the wallets will connect to a daemon through a proxy. The
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daemon must provide a hidden service for the RPC itself, which is separate from
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the hidden service for P2P connections.
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## P2P Commands
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Only handshakes, peer timed syncs and transaction broadcast messages are
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supported over anonymity networks. If one `--add-exclusive-node` p2p address
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is specified, then no syncing will take place and only transaction broadcasting
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can occur. It is therefore recommended that `--add-exclusive-node` be combined
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with additional exclusive IPv4 address(es).
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## Usage
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Anonymity networks have no seed nodes (the feature is still considered
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experimental), so a user must specify an address. If configured properly,
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additional peers can be found through typical p2p peerlist sharing.
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### Outbound Connections
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Connecting to an anonymous address requires the command line option
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`--tx-proxy` which tells `monerod` the ip/port of a socks proxy provided by a
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separate process. On most systems the configuration will look like:
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> `--tx-proxy tor,127.0.0.1:9050,10`
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> `--tx-proxy i2p,127.0.0.1:9000`
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which tells `monerod` that ".onion" p2p addresses can be forwarded to a socks
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proxy at IP 127.0.0.1 port 9050 with a max of 10 outgoing connections and
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".b32.i2p" p2p addresses can be forwarded to a socks proxy at IP 127.0.0.1 port
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9000 with the default max outgoing connections. Since there are no seed nodes
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for anonymity connections, peers must be manually specified:
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> `--add-exclusive-node rveahdfho7wo4b2m.onion:28083`
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> `--add-peer rveahdfho7wo4b2m.onion:28083`
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Either option can be listed multiple times, and can specify any mix of Tor,
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I2P, and IPv4 addresses. Using `--add-exclusive-node` will prevent the usage of
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seed nodes on ALL networks, which will typically be undesireable.
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### Inbound Connections
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Receiving anonymity connections is done through the option
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`--anonymous-inbound`. This option tells `monerod` the inbound address, network
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type, and max connections:
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> `--anonymous-inbound rveahdfho7wo4b2m.onion:28083,127.0.0.1:28083,25`
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> `--anonymous-inbound cmeua5767mz2q5jsaelk2rxhf67agrwuetaso5dzbenyzwlbkg2q.b32.i2p:5000,127.0.0.1:30000`
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which tells `monerod` that a max of 25 inbound Tor connections are being
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received at address "rveahdfho7wo4b2m.onion:28083" and forwarded to `monerod`
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localhost port 28083, and a default max I2P connections are being received at
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address "cmeua5767mz2q5jsaelk2rxhf67agrwuetaso5dzbenyzwlbkg2q.b32.i2p:5000" and
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forwarded to `monerod` localhost port 30000.
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These addresses will be shared with outgoing peers, over the same network type,
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otherwise the peer will not be notified of the peer address by the proxy.
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### Wallet RPC
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An anonymity network can be configured to forward incoming connections to a
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`monerod` RPC port - which is independent from the configuration for incoming
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P2P anonymity connections. The anonymity network (Tor/i2p) is
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[configured in the same manner](#configuration), except the localhost port
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must be the RPC port (typically 18081 for mainnet) instead of the p2p port:
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> HiddenServiceDir /var/lib/tor/data/monero
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> HiddenServicePort 18081 127.0.0.1:18081
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Then the wallet will be configured to use a Tor/i2p address:
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> `--proxy 127.0.0.1:9050`
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> `--daemon-address rveahdfho7wo4b2m.onion`
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The proxy must match the address type - a Tor proxy will not work properly with
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i2p addresses, etc.
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i2p and onion addresses provide the information necessary to authenticate and
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encrypt the connection from end-to-end. If desired, SSL can also be applied to
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the connection with `--daemon-address https://rveahdfho7wo4b2m.onion` which
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requires a server certificate that is signed by a "root" certificate on the
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machine running the wallet. Alternatively, `--daemon-cert-file` can be used to
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specify a certificate to authenticate the server.
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Proxies can also be used to connect to "clearnet" (ipv4 addresses or ICANN
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domains), but `--daemon-cert-file` _must_ be used for authentication and
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encryption.
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### Network Types
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#### Tor & I2P
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Options `--add-exclusive-node` and `--add-peer` recognize ".onion" and
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".b32.i2p" addresses, and will properly forward those addresses to the proxy
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provided with `--tx-proxy tor,...` or `--tx-proxy i2p,...`.
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Option `--anonymous-inbound` also recognizes ".onion" and ".b32.i2p" addresses,
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and will automatically be sent out to outgoing Tor/I2P connections so the peer
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can distribute the address to its other peers.
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##### Configuration
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Tor must be configured for hidden services. An example configuration ("torrc")
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might look like:
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> HiddenServiceDir /var/lib/tor/data/monero
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> HiddenServicePort 28083 127.0.0.1:28083
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This will store key information in `/var/lib/tor/data/monero` and will forward
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"Tor port" 28083 to port 28083 of ip 127.0.0.1. The file
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`/usr/lib/tor/data/monero/hostname` will contain the ".onion" address for use
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with `--anonymous-inbound`.
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I2P must be configured with a standard server tunnel. Configuration differs by
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I2P implementation.
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## Privacy Limitations
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There are currently some techniques that could be used to _possibly_ identify
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the machine that broadcast a transaction over an anonymity network.
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### Timestamps
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The peer timed sync command sends the current time in the message. This value
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can be used to link an onion address to an IPv4/IPv6 address. If a peer first
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sees a transaction over Tor, it could _assume_ (possibly incorrectly) that the
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transaction originated from the peer. If both the Tor connection and an
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IPv4/IPv6 connection have timestamps that are approximately close in value they
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could be used to link the two connections. This is less likely to happen if the
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system clock is fairly accurate - many peers on the Monero network should have
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similar timestamps.
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#### Mitigation
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Keep the system clock accurate so that fingerprinting is more difficult. In
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the future a random offset might be applied to anonymity networks so that if
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the system clock is noticeably off (and therefore more fingerprintable),
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linking the public IPv4/IPv6 connections with the anonymity networks will be
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more difficult.
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### Intermittent Monero Syncing
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If a user only runs `monerod` to send a transaction then quit, this can also
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be used by an ISP to link a user to a transaction.
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#### Mitigation
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Run `monerod` as often as possible to conceal when transactions are being sent.
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Future versions will also have peers that first receive a transaction over an
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anonymity network delay the broadcast to public peers by a randomized amount.
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This will not completetely mitigate a user who syncs up sends then quits, in
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part because this rule is not enforceable, so this mitigation strategy is
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simply a best effort attempt.
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### Active Bandwidth Shaping
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An attacker could attempt to bandwidth shape traffic in an attempt to determine
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the source of a Tor/I2P connection. There isn't great mitigation against
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this, but I2P should provide better protection against this attack since
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the connections are not circuit based.
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#### Mitigation
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The best mitigiation is to use I2P instead of Tor. However, I2P
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has a smaller set of users (less cover traffic) and academic reviews, so there
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is a tradeoff in potential isses. Also, anyone attempting this strategy really
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wants to uncover a user, it seems unlikely that this would be performed against
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every Tor/I2P user.
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### I2P/Tor Stream Used Twice
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If a single I2P/Tor stream is used 2+ times for transmitting a transaction, the
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operator of the hidden service can conclude that both transactions came from the
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same source. If the subsequent transactions spend a change output from the
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earlier transactions, this will also reveal the "real" spend in the ring
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signature. This issue was (primarily) raised by @secparam on Twitter.
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#### Mitigation
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`monerod` currently selects two outgoing connections every 5 minutes for
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transmitting transactions over I2P/Tor. Using outgoing connections prevents an
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adversary from making many incoming connections to obtain information (this
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technique was taken from Dandelion). Outgoing connections also do not have a
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persistent public key identity - the creation of a new circuit will generate
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a new public key identity. The lock time on a change address is ~20 minutes, so
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`monerod` will have rotated its selected outgoing connections several times in
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most cases. However, the number of outgoing connections is typically a small
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fixed number, so there is a decent probability of re-use with the same public
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key identity.
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@secparam (twitter) recommended changing circuits (Tor) as an additional
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precaution. This is likely not a good idea - forcibly requesting Tor to change
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circuits is observable by the ISP. Instead, `monerod` should likely disconnect
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from peers ocassionally. Tor will rotate circuits every ~10 minutes, so
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establishing new connections will use a new public key identity and make it
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more difficult for the hidden service to link information. This process will
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have to be done carefully because closing/reconnecting connections can also
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leak information to hidden services if done improperly.
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At the current time, if users need to frequently make transactions, I2P/Tor
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will improve privacy from ISPs and other common adversaries, but still have
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some metadata leakages to unknown hidden service operators.
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