mirror of
https://github.com/shadowsocks/shadowsocks-libev.git
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Max LvandClaude
553cdc7945
Add `ss-local --printable-salt` / `"printable_salt": true`, disabled by default. For each outgoing TCP salt it picks a prefix length uniformly from 6-12 bytes and samples those bytes as printable ASCII (0x20-0x7e) immediately before key derivation. The salt stays 32 bytes with no framing or markers, so the wire format is unchanged and any stock server interoperates. Only chacha20-ietf-poly1305 and aes-256-gcm are accepted; other ciphers fail at startup. UDP, server responses, and AEAD-2022 are untouched. Crypto unit tests cover length distribution, empty first writes, exact framing overhead, salt-tail preservation, byte-at-a-time decryption, tampering and replay rejection. CLI tests check local-only exposure and cipher rejection via flag, config key and bad config type. interop.py gains --printable-salt and --server-bin for independent stock servers. docs/printable-salt.md records the design, entropy bound, fingerprint caveats and route-experiment procedure; docs/encoding-experiments.md and tests/experiments/ hold the Base85/lowpop85/TLS outer-transport prototypes. docs/measurements/printable-salt-2026-09-23/ archives the sanitized results of the aborted field trials. Co-authored-by: Claude <noreply@anthropic.com>
150 lines
8.0 KiB
Markdown
150 lines
8.0 KiB
Markdown
# Experimental printable TCP salt
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`ss-local --printable-salt` chooses a prefix length uniformly from 6–12 bytes
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for each outgoing TCP salt, then replaces that prefix with independent, uniformly
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sampled printable ASCII bytes (`0x20`–`0x7e`).
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It is disabled by default and accepts only `chacha20-ietf-poly1305` and
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`aes-256-gcm`, including when the cipher comes from a configuration file or URL.
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Other ciphers fail at startup. Enable it with the CLI flag or the
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`printable_salt` configuration key; either one turns it on, and the key is
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ignored by the other binaries.
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```sh
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ss-local -c client.json --printable-salt
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```
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```json
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{ "server": "example.com", "server_port": 8388, "password": "...",
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"method": "chacha20-ietf-poly1305", "printable_salt": true }
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```
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The salt remains 32 bytes. The remaining 20–26 bytes retain their original secure
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random values. Length sampling uses libsodium `6 + randombytes_uniform(7)`;
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character sampling uses `randombytes_uniform(95)` immediately
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before key derivation and transmission on the first nonempty TCP encryption call.
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Empty writes do not consume the salt; later writes do not change it. For a selected length `L`, the conditional salt entropy
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is `(32 - L) * 8 + L * log2(95)`, at least **238.84 bits** (at `L = 12`).
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This is also a conservative min-entropy bound for the mixture; the random length
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is not an independent secret field whose entropy can simply be added. Salt uniqueness
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remains essential. There are no extra framing bytes, messages, or round trips.
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UDP salt generation, server responses, authenticated framing, replay rejection,
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and AEAD-2022 are unchanged. No SIP003 plugin or fixed protocol marker is added.
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This is not TLS impersonation, nor a promise of censorship resistance.
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The prefix introduces a statistical fingerprint: every enabled connection starts
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with six printable bytes, versus `(95 / 256)^6`, approximately 0.26%, for a
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uniformly random salt. The bytes vary independently, but the printable property
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is fixed. The constrained span now varies; the untouched tail may also start
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with printable bytes, so there is no explicit boundary marker. The 6–12 range
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is an experimental tradeoff, not an optimized or measured defense. Increasing
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the span can strengthen other ASCII statistics even as the boundary varies. A classifier could combine it with subsequent encrypted bytes, packet
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lengths, endpoint information, or probing. High salt entropy does not imply low
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detectability.
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## Rationale and limits
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The [USENIX Security 2023 study](https://gfw.report/publications/usenixsecurity23/en/)
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reported an exemption for six leading printable bytes in the first TCP payload.
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Its [follow-up](https://github.com/gfw-report/usenixsecurity23-artifact) reports that
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the measured dynamic blocking stopped in March 2023. This is historical evidence;
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present-day behavior must be measured on the route of interest.
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[Outline prefixing](https://developer.getoutline.org/vpn/advanced/prefixing/)
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provides related deployment precedent. The
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[AEAD specification](https://shadowsocks.org/doc/aead.html) describes salt-based
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key derivation and the requirement for unique salts.
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An application write does not determine TCP packet boundaries. Packet captures
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must establish whether the actual first transmitted payload contains all six
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bytes. This experiment does not establish protection against every active probe,
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long-term blocking, or other classifiers.
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## Local reproduction
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Build an independent stock checkout from the base revision before applying this
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change, then configure/build this branch with the same toolchain:
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```sh
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cmake -S . -B build-printable -G Ninja -DSS_DEPENDENCY_MODE=system \
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-DWITH_STATIC=OFF -DWITH_DOC_MAN=OFF -DWITH_DOC_HTML=OFF
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cmake --build build-printable --parallel
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ctest --test-dir build-printable --output-on-failure
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python3 tests/interop.py --self --bin build-printable/bin \
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--server-bin build-printable-stock/bin/ss-server --printable-salt
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python3 tests/interop.py --bin build-printable/bin --printable-salt
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python3 tests/interop.py --self --bin build-printable/bin
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python3 tests/stress_test.py --bin build-printable/bin --size 10
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uvx ruff==0.15.6 check --select E9,F63,F7,F82 tests scripts
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python3 scripts/check_cli_docs.py
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python3 -m unittest discover -s tests -p test_cli_docs.py
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```
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The crypto tests exercise disabled/enabled behavior, empty first writes, unchanged
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salt tails, exact framing length, subsequent writes, byte-at-a-time decryption,
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tampering, and replay rejection. CLI tests check cipher rejection and local-only
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exposure. Interoperability tests verify concurrent bidirectional TCP and UDP;
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the default full suite covers all supported AEAD-2022 variants too.
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## Route experiment
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`tests/printable_salt_experiment.py` runs two 30-minute phases against isolated
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instances of a stock remote `ssserver`. It requires Linux with Python, tcpdump,
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`/usr/local/bin/ssserver` and `/usr/local/bin/sslocal`, verified SSH host trust,
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two unused reachable high ports, and local passwordless access to tcpdump through
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sudo. Inspect listeners and firewall rules first. The runner does not modify
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firewalls or existing services. Its remote echo origin binds to loopback only.
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```sh
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python3 tests/printable_salt_experiment.py \
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--host root@SERVER --host-key-alias VERIFIED_KNOWN_HOST \
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--address SERVER_IPV4 --interface en0 \
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--stock-bin build-printable-stock/bin/ss-local \
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--modified-bin build-printable/bin/ss-local \
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--ports 32181 32182 --output build-artifacts/printable-salt/route
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python3 tests/check_printable_pcap.py \
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build-artifacts/printable-salt/route/client.pcap --ports 32181 32182 \
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--events build-artifacts/printable-salt/route/events.jsonl
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```
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Fresh credentials are generated for each run. Proxy environment variables are
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removed from child processes. Clients swap server ports between phases. Each
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mode attempts a verified 1 KiB echo transfer every five seconds and ten 10 MiB
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echo transfers per phase. Upload and download run concurrently. Every 30 seconds,
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an SSH round trip verifies management health, process health, local listeners,
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and a verified transfer through each server using server-local stock clients.
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The run stops on a failed health check. Remote processes have a 65-minute watchdog
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and are stopped when their controlling SSH input closes or no control command
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arrives for 60 seconds. Normal cleanup removes
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experimental processes and temporary configs and retrieves packet captures.
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JSONL results record transfer success, error/timeout/reset messages, integrity
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failures, elapsed time, and effective payload throughput. `socks_handshake_ms` is
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local SOCKS setup time, **not** remote connection latency. Use small-transfer
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completion time for an end-to-end latency measure. Throughput is one-way payload
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bytes divided by bidirectional echo completion time; it is not link capacity.
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Large tests share the route with each other and the scheduled small tests.
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Captures use a 160-byte snapshot to retain TCP headers and salt prefixes while
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limiting size. Both packet captures and the raw results remain in a private local
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output directory; do not publish raw captures without reviewing address metadata.
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The packet checker accepts a first payload only after observing its SYN, checks
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its sequence number, and evaluates bytes in the packet itself.
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Call a result promising only when modified traffic repeatedly succeeds while
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stock traffic fails, including after crossover, with healthy server-local
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controls. Persistent port blocking makes crossover inconclusive. If both work,
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report compatibility and measured performance only. If both fail, investigate
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captured failures before choosing another modification. A one-hour run cannot
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establish long-term GFW resistance.
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## Additional experiments
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See [outer-transport experiments](encoding-experiments.md) for Base85,
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low-popcount encoding, and verified TLS prototypes. These require matching relays
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at both endpoints and are separate from the production CLI flag.
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Recorded results: [2026-09-23 measurements and limitations](measurements/printable-salt-2026-09-23/README.md).
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The archived 2026-09-23 measurements used the original fixed six-byte prefix.
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They do not evaluate this variable-length revision.
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