# Security Review Report

**Branch**: feature-190-wave-3b-frequency-analysis-api -> main
**Date**: 2026-06-02

## Issues in Your Changes (BLOCKING)

No blocking security issues found.

## Issues in Code You Touched (Should Fix)

No should-fix security issues found.

## Pre-existing Issues (Not Blocking)

No pre-existing security issues found.

## Suggestions (Lower Confidence)

No suggestions.

## Summary

| Category | CRITICAL | HIGH | MEDIUM | LOW |
|----------|----------|------|--------|-----|
| Blocking | 0 | 0 | 0 | - |
| Should Fix | - | 0 | 0 | - |
| Pre-existing | - | - | 0 | 0 |

**Security Score**: 10
**Recommendation**: APPROVED

## Analysis Notes

This PR adds pure-computation AST n-gram newtypes (`AstBigram`, `AstTrigram`) with vocabulary helpers and IDF weight lookup. The security analysis found no issues for the following reasons:

**Attack surface**: Zero. The module performs no I/O, no network access, no file system operations, no user input processing, no deserialization of untrusted data, and no `unsafe` code. All inputs are `u16` numeric IDs and `&str` lookups into a compile-time static vocabulary array.

**Specific checks performed**:

1. **Injection (OWASP A03)**: Not applicable. No SQL, shell, or command execution. All data flows are in-memory numeric operations and static array indexing.

2. **Integer overflow/truncation**: The `u16 -> u32` and `u16 -> u64` promotions in `encode()` use `u32::from()` / `u64::from()` which are infallible widening conversions. The `as NodeKindId` casts in `decode()` are safe because the values are masked to 16-bit range first (`& 0xFFFF`). The `vocab_lookup` cast `idx as NodeKindId` is safe because `NODE_KIND_VOCABULARY` length is validated to fit in `u16` by existing tests.

3. **Array bounds**: `vocab_resolve()` uses `.get()` (bounds-checked). `vocab_lookup()` uses `binary_search()` which returns valid indices by contract. The `NODE_KIND_VOCABULARY` is a static `&[&str]` with no runtime mutation.

4. **Secrets/credentials (OWASP A02)**: No hardcoded secrets, tokens, or credentials.

5. **Denial of service**: Not applicable. All operations are O(log n) binary search or O(1) bit manipulation on fixed-size static data. No unbounded allocation, no user-controlled loop counts.

6. **Supply chain (OWASP A06)**: No new dependencies added.

7. **`unsafe` code**: None present in the diff.

8. **Feature knowledge alignment**: Consistent with documented feature knowledge -- the ast_index module is pure computation with no I/O, no user input, no unsafe code, and all array accesses use bounds-checked `.get()` or binary search.

**Decisions context**: ADR-001 (fix all noticed issues immediately) -- no issues noticed to fix. PF-002 (avoids PF-002) -- no findings classified as deferred; there are genuinely zero security concerns in this pure-computation module.
