Cast Redact

cast-redact

Remove sensitive data from an asciicast .cast recording before it is rendered, committed, or shared. Upstream asciinema has no redact command (its official answer is "edit the NDJSON yourself"), and naive sed misses the worst case: a secret typed during recording echoes one character per event, so no single line contains it. The bundled script matches patterns on the joined per-channel stream, scrubs header fields too, and verifies its own output.

Threat model (be honest about it)

• Catches: known secret shapes (GitHub/AWS/Stripe/Slack/Google tokens, JWTs, private-key blocks, KEY=value assignments), user-supplied regexes, known literal values, home-directory paths — including matches split across events, hidden in cleared/backspaced output, or sitting in the header's command/title/env fields.
• Does not catch: secrets with no recognizable shape that you don't declare, and secrets whose characters are interleaved with ANSI/OSC control sequences or cursor-movement rewrites (styled TUI output can break a token across escape codes). A clean scan means "no configured rule matches" — it is not proof the file is safe. Pattern-based best effort, not magic.
• Non-negotiable: a real credential that was recorded is compromised no matter how well it is scrubbed — rotate it.

When NOT to use

• Secrets in source files (.env, configs) — that's not a recording.
• Trimming, speeding up, or restyling a cast → cast-render.
• Preventing leaks at capture time (clean env, fake values) → cast-record.
• PDFs, images, videos — this operates on asciicast NDJSON only.

Workflow

Never load the raw cast into the model context (that re-leaks the secret to logs/telemetry). Work through the script; it prints rule names, counts, and line numbers — never payloads.

1. Scan (read-only; exit 1 = findings, 0 = clean):

   python3 "${CLAUDE_SKILL_DIR}/scripts/redact_cast.py" --scan demo.cast
   

2. Extend the rules when needed:
   • --paths — also rewrite /Users/<name> and /home/<name> to a demo user.
   • --pattern 'REGEX' or --pattern 'REGEX=>replacement' — extra shapes (hostnames, internal domains). Regexes only; never a literal secret on the command line (argv leaks into shell history and process lists).
   • --replace-file secrets.map — known literal values, one literal=>replacement per line; keep this file gitignored.
   • --drop-input — delete all input ("i") events outright.
3. Apply to a NEW file (the source is never modified; the script re-scans its own output and fails if anything survived):

   python3 "${CLAUDE_SKILL_DIR}/scripts/redact_cast.py" demo.cast demo.redacted.cast --paths
   

4. Verify + publish path. Confirm --scan demo.redacted.cast exits 0, and for known literals verify independently of the script's own scanner (locally, e.g. grep -qF <literal> demo.redacted.cast — never in shared logs). Spot-check with asciinema play demo.redacted.cast, then render via cast-render. Keep the raw cast out of git (git check-ignore to confirm); commit only the sanitized one.
5. Rotate any real credential that appeared, and say so explicitly.

Output spec

• *.redacted.cast: valid asciicast (same version, event count, and timing as the source — except --drop-input, which removes input events while carrying their v3 intervals forward so cumulative timing is preserved), zero remaining rule matches, replacements shown as [REDACTED:<rule>].
• The original file untouched; scan output contains no secret text.

Gotchas

• Split-across-events is the norm, not the edge case — every echoed keystroke is its own event. That's why sed per line is not enough and why the script joins each channel before matching.
• The header is a leak surface: -c command strings, titles, and captured env values are scrubbed too (a token passed as a CLI argument lands there).
• Replacement changes text width, so cursor-positioned TUI output may look slightly off at the redacted spot — cosmetic; timing is untouched.
• Works on asciicast v2 and v3 (v1 unsupported: convert first with asciinema convert). Unknown event codes pass through untouched.
• --self-test runs the built-in verification suite (split-event, header, path, idempotency cases) — run it after modifying the script.
• Prevention beats cure: if the redacted artifact still feels risky, re-record with cast-record hygiene (fake values, clean HOME).