Closes the v3.1 unlinkability gap and resists volume/timing correlation:
1) Per-hop client cert (identity-unlinkable hops). [[client.circuit.hops]]
now accepts {addr, cert_path, key_path, [server_name]} per hop — each
hop sees a different CN, so a relay and an exit cannot correlate the
same client by certificate. Old flat `hops = ["ip:port"]` form still
parses (serde untagged enum) and falls back to [pki] cert/key.
`aura provision-client --circuit-hops N` mints N fresh UUIDv4 certs.
2) Cell padding. CellPaddingConn wrapper pads every outgoing packet to a
fixed size (default 1280 bytes; `cell_size = N` configurable) before
it hits the inner AEAD. Format: u16_be(real_len) || pkt || zero_pad.
On-wire sizes become constant -> defeats volume/timing fingerprints.
Opt-in via [client.circuit] cell_padding = true and the mirror
[server] cell_padding_for_circuit_clients = true.
3) 3-hop support. dial_circuit now accepts N >= 2 hops; iterative
ExtendBridge nests N-1 forwarders and N handshakes. Client owns the
full chain via CircuitConnection (forwarders abort on drop).
New integration test multihop_v3_2_three_hops_end_to_end runs three
in-process actors (A relay -> B relay -> C exit) on loopback and
verifies peer_id == C's CN.
4) CIDR whitelist. [server.relay] allow_extend_to entries accept
"10.0.0.0/24" (subnet, any port), "10.0.0.0/24:443" (subnet + port),
"[2001:db8::/32]:443" (IPv6 with port), as well as exact IP:port.
Empty list keeps the v3.1 open-relay (warn).
19 new tests; workspace 276 passed (+19), clippy -D warnings clean, fmt clean.
257 baseline tests untouched; all v2 / v3.1 / LE configs work as before.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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