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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>
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Summary
Adds an opt-in, disabled-by-default client-side experiment:
ss-local --printable-salt(or"printable_salt": truein the config). For each outgoing TCP salt it picks a prefix length uniformly from 6–12 bytes and overwrites those bytes with independently sampled printable ASCII (0x20–0x7e) right before key derivation.chacha20-ietf-poly1305andaes-256-gcmare accepted; other ciphers fail at startup, including when the cipher comes from a config file. UDP salts, server responses, and AEAD-2022 are untouched. The flag is ss-local only.(32-L)*8 + L*log2(95)≥ 238.8 bits; salt uniqueness remains essential and is still enforced by the bloom filter.This is motivated by the historical six-printable-byte exemption reported in the USENIX Security '23 GFW study. It is not TLS impersonation and not a claim of censorship resistance — see the caveats in
docs/printable-salt.md. The 2026‑09‑23 field trials archived underdocs/measurements/both aborted before crossover; all recorded transfers (stock and modified) succeeded, so they show interoperability only, not an availability advantage.Changes
src/aead.c,src/crypto.h:cipher_ctx.printable_saltflag; prefix rewrite on the first non-empty TCP encrypt, beforeaead_cipher_ctx_set_key.src/local.c,src/jconf.[ch],src/common.h,src/utils.c:--printable-saltoption,printable_saltconfig key, cipher validation, Doxygen CLI snippet, help text gated onMODULE_LOCAL.completions/,doc/shadowsocks-c.md,README.md: flag documented.tests/test_crypto.c: length distribution (all seven lengths over 512 samples), empty first write, exact framing overhead, salt-tail preservation, byte-at-a-time decrypt, tampering, replay rejection — for both ciphers, enabled and disabled.tests/test_cli.py: local-only exposure; rejection via flag, config key, and non-boolean config value.tests/interop.py:--printable-saltand--server-bin(independent stock server). New CTesttest_printable_salt_interop.tests/experiments/+docs/encoding-experiments.md: Base85 / lowpop85 / verified-TLS outer-transport prototypes (research fixtures, not plugins), with codec and supervisor-cleanup tests registered in CTest.tests/printable_salt_experiment.py,tests/check_printable_pcap.py: route-experiment runner and pcap first-payload checker.Test plan
Verified locally on macOS arm64:
cmake --build build && ctest --test-dir build -LE memcheck— 39/39 passed (includes the 3 new integration tests)python3 tests/stress_test.py --bin build/bin/ --size 10— 3/3 passeduvx ruff==0.15.6 check --select E9,F63,F7,F82 tests scripts— cleanpython3 scripts/check_cli_docs.pyandpython3 -m unittest discover -s tests -p test_cli_docs.py— pass🤖 Generated with Claude Code