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>
Max LvandClaude