# Bc Check > [object Object] - Skill: `trustwallet/bc-check` (Agent Skill) - Install (CLI): `npx skillmds@latest add trustwallet/bc-check` - Raw SKILL.md: https://api.skillmd.com/api/skills/trustwallet/bc-check/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: trustwallet (https://skillmd.com/u/trustwallet) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/trustwallet/bc-check --- You are auditing the current branch's diff against the PR's base branch for **backward-compatibility risk against persisted user data and wire formats**. Find cases where this PR will reject, mis-parse, or mishandle inputs that older versions of our own software already wrote to disk, to backups, or onto the network. **Do not trust the PR description's framing.** "Just a security fix" / "stricter validation" is precisely the framing that hides this class of bug. Before starting: 1. Read `docs/bc-footguns.md` if it exists. Every entry is a known constraint. 2. Determine the base branch and get the full diff: ```bash BASE=$(gh pr view --json baseRefName -q .baseRefName 2>/dev/null || echo "master") git diff origin/$BASE...HEAD ``` If `gh pr view` fails (e.g. no open PR yet), fall back to `origin/master`. 3. Identify which files are on persistence paths (see WalletCore context below). --- ### WalletCore persistence context | Path | What lives there | Backup / off-device | |---------------------------------|---------------------------------------------------------------------|------------------------------------------------------| | `src/Keystore/` | JSON keystore files — user's encrypted private keys and mnemonics | iCloud, Google Drive, manual export | | `src/proto/*.proto` | Protobuf `SigningInput` / `SigningOutput` wire formats | Potentially stored by app layer | | `include/TrustWalletCore/TW*.h` | Public C ABI — function signatures, enum values, exception types | SDK consumers, compiled language bindings | | `registry.json` | Coin metadata: SLIP44 ID, derivation paths, address encoding, curve | Drives every derived address for all wallets | | `src/Keystore/StoredKey.cpp` | Keystore load / save / migration logic | Same as Keystore above | | `src/PrivateKey*` | Private key encoding, validation, and signing entry points | Keys serialized into keystore files | | `src/PublicKey*` | Public key parsing and validation | Derived and stored in account records | | `src/HDWallet*` | HD wallet derivation — BIP32/39/44 logic | Drives all derived addresses; mnemonic import/export | | `swift/Sources/KeyStore*` | Swift-layer keystore read/write/migration | Same backup destinations as C++ keystore | | `swift/Sources/Wallet.swift` | Swift wallet account type | Serialized in Swift keystore JSON | | `swift/Sources/Watch.swift` | Swift watch-only account type | Serialized in Swift keystore JSON | | `wasm/src/keystore/` | WASM/TypeScript keystore bindings | Browser-side keystore files (localStorage, export) | **Blast-radius framing:** a bug in any of these paths means a user cannot decrypt their wallet or derives wrong addresses → cannot access funds. Even a 0.01 % blast radius is unacceptable. Treat BLOCKER verdicts as such. --- Walk the 5 steps below. Cite file:line and commit SHAs everywhere. ### Step 1 — Classify (a) Tighten input validation, (b) change parsing of a persisted format, (c) change which exception type a public API throws, (d) remove an `if missing use default` fallback, or (e) move validation earlier? If none → "No persistence risk", stop. ### Step 2 — Historical baseline For each tightened rule: - Could a prior version have *produced* data that violates the new rule? `git log -p` / `git blame` going back ≥2 years. Cite SHAs. - Was there a partial migration ("regenerate on next user action")? Assume a non-trivial fraction of users **never triggered it**. - Does the format ever leave the device? Backup / export / sync. ### Step 3 — Concrete failure scenarios For every "yes": old client version → action that produced the data → where it lives → exact upgrade code path (file:line) → user-visible symptom → blast radius. No hand-waving. ### Step 4 — Red-flag checklist Answer yes/no with file:line evidence: **Generic:** - New `throw` on read/load/decode/import path? - Removed `if (field missing) use default`? - New length/range/enum check on >1-year-old field? - Constructor: lenient parse → parse + validate? - Changed which exception type a public API throws? - Format ever in backup/export/sync? - Prior PR shipped "regenerate on next user action" partial migration? (cite SHA) **WalletCore-specific:** - Proto: field number reused or removed from an existing message? - Proto: field type changed (e.g. `bytes` → `string`) or `required` added to existing field? - Proto: enum value renumbered or removed? - Keystore: JSON key renamed, removed, or made required without a default fallback? - Keystore: `validate()` now called from the JSON-load constructor for the first time? - Registry: `slip44`, `curve`, or address-encoding field changed? (changes derived addresses) - Registry: `TWCoinType` enum value reordered or removed? - C ABI: `TW*` function removed, renamed, or its return / exception behaviour changed? ### Step 5 — Required mitigations - Accept legacy at read time, normalize on write *(preferred)*. - Gate strict check behind "newly created" flag. - One-time migration with explicit failure UX. - Apply tightening to write paths only. ### Token rules for "Suggested PR comment" | Verdict | Valid token | Invalid tokens | |---------------------|------------------------------------------------------|-----------------| | SAFE | `[bc-check: Pass]` | — | | RISK | `[bc-check: Mitigated]` (after fix) | `Pass`, `N/A` | | RISK | `[bc-check: Risk-Accepted]` (investigated, no fix) | `Pass`, `N/A` | | BLOCKER | `[bc-check: Mitigated]` (after fix) | `Pass`, `N/A` | | BLOCKER | `[bc-check: Risk-Accepted]` (investigated, no fix) | `Pass`, `N/A` | | No persistence risk | `[bc-check: N/A]` | — | **`N/A` means the scanner fired on a file that has zero BC relevance** (a renamed variable, a comment edit, a test fixture that happens to match the path filter). It does NOT mean "I found a real BC issue but accept the risk." Using `N/A` when the audit found a RISK or BLOCKER is incorrect and bypasses the gate dishonestly. **`Risk-Accepted` is for cases where the finding is real but investigation confirms the blast radius is effectively zero** — e.g. telemetry or team confirmation that no user data in the wild can trigger the new restriction. It requires the same bar as `Mitigated`: full audit output pasted, plus explicit evidence for why the risk is acceptable (who confirmed it, what data or reasoning supports it). The audit trail must be visible to reviewers. ### Output format Emit all sections below as formatted markdown (bold, inline code, headers). Do not wrap the output in a code fence. --- # BC-risk audit — \ → \ @ \ — \ ## Verdict \ — one sentence. ## Step 1 — Classification ... ## Step 2 — Historical baseline ... ## Step 3 — Concrete failure scenarios ... ## Step 4 — Red-flag checklist ... ## Step 5 — Required mitigations ... ## Suggested PR comment Emit this block as a GitHub-ready comment. Use markdown: **bold** for the verdict and key findings, `inline code` for file paths, function names, constants, commit SHAs, and token strings. Use a bullet list for multiple findings or mitigations if needed. The comment body must follow this structure exactly so the workflow gate accepts it. Wrap it in a fenced code block labelled `markdown` so the author can copy the raw text. The author must paste **only the content inside the fences** into GitHub — not the ` ```markdown ` / ` ``` ` lines themselves. ```` ```markdown [bc-check: ] # BC-risk audit — → @ — Verdict: ``` ```` ## Suggested addition to docs/bc-footguns.md \ --- The skill never auto-posts to GitHub. Author copies the verdict + suggested comment into the PR; the human signs off.