Anonymity Networks with Shekyl

Currently only Tor and I2P have been integrated into Shekyl. 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.

PQC note:

• the rebooted chain's hybrid signatures will increase transaction sizes
• larger transactions can change traffic-shape characteristics and may make relay events more fingerprintable if not considered in analysis
• the PQC transaction/authentication structure is specified in docs/POST_QUANTUM_CRYPTOGRAPHY.md

Measured v3 Impact on Anonymity Networks

TransactionV3 adds ~5,385 bytes of pqc_auth material per user transaction (see docs/V3_ROLLOUT.md for exact component sizes). Practical consequences for anonymity relay:

• A typical 2-in/2-out FCMP++ transaction grows from ~2–3 KB to ~7–8 KB.
• On Tor, a single cell is 512 bytes; a v3 transaction spans ~14–16 cells vs ~4–6 cells pre-PQC. The burst pattern is more distinctive.
• On I2P, tunnel messages are 1 KB; the same transaction requires ~7–8 fragments vs ~2–3.
• Fragmented Levin messages (white noise feature) help pad smaller payloads but cannot conceal large-burst events unless dummy traffic volume is proportionally increased.

Known Leak Vectors vs Mitigations Matrix

Leak vector	Severity	Current mitigation	Residual risk
Timestamp correlation (timed sync)	Medium	System clock accuracy; future random offset	Fingerprintable if clock is skewed
ISP link timing (intermittent sync)	Medium	Keep shekyld running continuously	Users who sync-send-quit are linkable
Active bandwidth shaping	High	I2P preferred (non-circuit)	Tor circuits remain vulnerable
Stream reuse (2+ tx same circuit)	Medium	Outgoing-only selection, 5-min rotation, 20-min change lock time	Small fixed outgoing pool → non-trivial reuse probability
v3 tx size burst (new)	Medium	Fragmentation + dummy messages	Burst pattern still larger than pre-PQC; requires tuning dummy volume
Levin 8-byte signature (DPI)	Low	SSL/BIP-151/Noise proposals	Not yet implemented; clearnet traffic is identifiable

Recommended Pre-Mainnet Testing

1. Replay a representative testnet transaction mix over Tor and I2P, measuring circuit-level timing and cell/fragment counts before and after v3.
2. Measure the ratio of real-to-dummy Levin messages required to make v3 bursts statistically indistinguishable from pre-PQC traffic.
3. Validate that the 5-minute outgoing-connection rotation is sufficient given the higher per-tx relay time for v3 payloads.

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 shekyld is shutdown and restarted with only public connections enabled.

Anonymity networks can also be used with shekyl-cli and shekyl-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).

For the rebooted chain, the transaction broadcast path should be re-evaluated with PQ-authenticated transactions because:

• serialized transactions will be materially larger
• fragmentation/delay behavior may become more visible on anonymity networks
• any anti-fingerprinting strategy should be tested against post-PQC payload sizes, not legacy assumptions

Usage

Outbound Connections

Connecting to an anonymous address requires the command line option --tx-proxy which tells shekyld 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 shekyld 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 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 shekyld the inbound address, network type, and max connections:

--anonymous-inbound rveahdfho7wo4b2m.onion:28083,127.0.0.1:28083,25
--anonymous-inbound cmeua5767mz2q5jsaelk2rxhf67agrwuetaso5dzbenyzwlbkg2q.b32.i2p,127.0.0.1:30000

which tells shekyld that a max of 25 inbound Tor connections are being received at address "rveahdfho7wo4b2m.onion:28083" and forwarded to shekyld localhost port 28083, and a default max I2P connections are being received at address "cmeua5767mz2q5jsaelk2rxhf67agrwuetaso5dzbenyzwlbkg2q.b32.i2p" and forwarded to shekyld 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 shekyld 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/shekyl
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/shekyl
HiddenServicePort 28083 127.0.0.1:28083

This will store key information in /var/lib/tor/data/shekyl and will forward "Tor port" 28083 to port 28083 of ip 127.0.0.1. The file /var/lib/tor/data/shekyl/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 Shekyl 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 Shekyl syncing

If a user only runs shekyld to send a transaction then quit, this can also be used by an ISP to link a user to a transaction.

Mitigation

Run shekyld 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

shekyld 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 shekyld 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, shekyld 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.

Transport for the staker-archival path (forward consideration — NOT a genesis commitment)

Status: captured for consideration; no transport work is scheduled. This section records the analysis so the eventual transport-selection PR starts from a stated position rather than a blank page. Every maturity / capability claim below (Arti onion-service support, the Arti 2.x LTS line, Monero's over-Tor traffic split) is to be re-verified at source when the transport PR opens, per 17-dependency-discipline.mdc — recorded here as the rationale to test, not as banked fact.

The transaction-broadcast transport above is inherited from Monero and is a light, latency-tolerant, public-content path. The staker-archival subsystem (see design/V3_STAKER_ARCHIVAL.md and the sim in design/STAKER_ARCHIVAL_SIM.md) introduces a second traffic class with a different threat profile, and the transport choice turns out to be coupled to the sim's latency findings rather than independent of them.

Two traffic classes

• Light / privacy-critical (latency-tolerant). Stake claims, archival registration, the firewalled pseudonym's liveness re-proofs, and challenge–response retention proofs. Small, infrequent, delay-insensitive — exactly the class Monero already routes over the anonymity network (handshakes, timed syncs, transaction broadcast), with the bulk left on IPv4 to make surrounding-node sybil attacks harder. Tor is a direct, battle-tested fit here.

• Heavy / archival data path (bandwidth-sensitive). Deep-shard fetch (the retrieval side of coverage). This is where Shekyl cannot reuse Monero's escape hatch. Monero keeps bulk chain-sync on clearnet because that content is public and IP exposure is an accepted leak in its threat model. Shekyl's firewalled-pseudonym requirement is precisely that the pseudonym P's network location not link to the principal, so the archival serving path must be the onion service, not clearnet. Combined with the query-privacy requirement (the fetcher is also over Tor), the heavy fetch becomes onion-service-to-Tor-client — the rendezvous configuration, the slowest path Tor has (three hops each side plus the rendezvous).

The coupling to the sim (this is the load-bearing point)

The privacy model structurally forces the heavy data path onto worst-case Tor latency. That means the L2–L6 latency band in STAKER_ARCHIVAL_SIM.md (§L10, §L10 hardening) is not a stress-test corner — it is the operating regime by construction. The transport choice and the L10 latency findings are the same physical fact seen from two sides:

• The "real Tor deep-fetch latency" the sim flagged as the post-testnet empirical unknown is Monero's documented heavy-over-Tor bandwidth pain.
• The sim has already designed around it: age-scaled bond duration is win-or-harmless across L2–L6 (L10 H1/H4), and the foundation floor backstops the L6 ceiling where backfill cannot keep pace regardless of incentive.

So Tor's heavy-path bandwidth weakness is a known, bounded, already-modeled constraint, not a discovery waiting to happen. The post-testnet measurement only tells us where on the L-curve we sit; it does not reopen the transport choice.

TCP coupling

Tor carries TCP streams and has no native UDP transport (the UDP-oriented designs are the alternatives — Lokinet/LLARP, I2P datagram mode). So the inherited Levin/TCP P2P stack drops in without an impedance mismatch, and TCP-sync and Tor reinforce each other. The flip side is that the commitment is coupled: if UDP/QUIC sync ever returns to the table, it reopens the transport question, because it fights Tor's grain. The TCP commitment and the Tor commitment stand or fall together.

Candidate transports (Tor primary; rationale to verify)

• Tor — primary. Maturity + TCP + persistent-reachable-service + longevity. The Rust-native angle is the clincher to verify: Arti (Rust Tor) is claimed to support hosting onion services as of early 2026 (Arti 2.x, committed LTS branch), which would let the archival pseudonym run a .onion responder in-process without a C dependency — fitting the Rust-canonical posture (20-rust-vs-cpp-policy.mdc). The older "Arti has no onion-service support" notes are claimed stale (client-only 1.x line). Verify at source before relying on it.
• I2P — reasonable secondary. Inward-facing (good for a closed P2P overlay), DHT-based with no directory authorities, datagram-capable. But smaller network, the mature implementation (i2pd) is C++ with no Arti-equivalent Rust story, and Monero's own I2P integration carries known privacy-leak edge cases. Dual-support à la Monero is defensible; primary it is not.
• Lokinet — rejected. Monero-lineage onion router on LLARP, UDP/layer-3 (lower latency) — but UDP is wrong for TCP sync, the network is small, and it is tied to the Oxen ecosystem, which is exactly the longevity risk Tor is chosen to avoid. The thing Tor is valued for (it is not going anywhere) is Lokinet's weakest axis.
• Nym — orthogonal, not a competitor. A mixnet trades latency for flow-correlation resistance (which Tor genuinely lacks), at a delay cost that is disqualifying for heavy fetch by design. The only conceivable fit is stronger traffic-analysis resistance on the light async class (claim/broadcast) — V4+ exotica, not a sync transport.

Forks to decide at the transport PR (not now)

1. Embed Arti in-process (Rust-canonical, viable on the LTS line if the onion-service-hosting claim verifies) vs. external Tor daemon over SOCKS (Monero's model, more battle-tested, but an external dependency). Philosophy leans embed; conservatism leans external — a deliberate call, not drift.
2. Whether to keep the I2P secondary-transport door open architecturally even if it is not built at genesis (a reversion-clause-shaped decision per 21-reversion-clause-discipline.mdc).
3. The integration's privacy edge cases are the real soundness work, not "does Tor exist." Monero still flags its anonymity-network support as experimental with cases where source privacy can leak (this doc, §Privacy Limitations), and the rendezvous-forced heavy path is a configuration Monero does not lean on — so it needs its own threat pass, not inheritance-by-assumption.

The one question that feeds the sim

When the transport pass runs, the single transport parameter that actually moves the model is: does the heavy path stay pure-rendezvous (worst-case L-regime, maximal firewall), or is there an acceptable relaxation that buys bandwidth without linking P? That is the lever that moves where Shekyl lands on the L2–L6 curve — and therefore the only transport input the staker-archival sim needs before the rest of the transport design is settled.