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feat(rs): surface rotation drain signal via decrypt_indexed (LAB-1678) - #74

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Closes LAB-1678. Consumes the cachekit-core 0.6.0 decrypt_indexed surface (cachekit-io/cachekit-core#73) that #63 deferred: a previous-key hit is now observable, so an operator can watch a rotation drain reach zero before dropping the retired master key.

What changed

  • crates/cachekit/Cargo.toml: cachekit-core pin 0.50.6 (features unchanged; 0.6.0 is additive).
  • EncryptionLayer::decrypt reads via Keyring::decrypt_indexed. Plaintext, attempt sequencing (core-owned, LAB-683) and the error-class mapping (KeyDerivation/KeyringIndexOutOfRangeConfig, else Encryption) are unchanged; existing rotation tests pass unmodified.
  • Drain signal: one AtomicU64 per previous key on the layer. Index 0 (current key) is not counted; index i ≥ 1 increments slot i - 1. EncryptionLayer::previous_key_hits() -> Vec<u64> (hits[i]previous_keys[i]) plus a SecureCache::previous_key_hits() passthrough so builder-configured clients can reach it. Pull-style, matching the crate's MetricsProvider pattern; no new dependency, no user code on the decrypt hot path. The payload is positions and counts only, no key material, by type.
  • Tests: unit tests for the index-0 silent case, the index-N counted-at-position case (current=k3, previous=[k2, k1]), and a failed decrypt counting nothing; an integration test reading the signal off SecureCache; a doc-test on previous_key_hits asserting the drain signal.
  • Docs: README ### Key Rotation gains the operator workflow paragraph; EncryptionLayer rustdoc gains a "Rotation drain signal" section mirroring core's decrypt_indexed language.

Verification

  • cargo clippy --all-targets --features "cachekitio,redis,encryption,l1,macros,memcached,file" -- -D warnings: clean.
  • cargo test with the same feature set: all suites green (92 lib tests, doc-tests included).
  • cargo +1.85 check --all-targets --features "cachekitio,redis,encryption,l1,macros": passes (MSRV).
  • cargo check --target wasm32-unknown-unknown --no-default-features --features workers,cachekitio,encryption: builds; the 33 unused-import warnings are pre-existing in backend modules.
  • prek run --all-files: every hook passes except detect-secrets, which flags tests/vectors/interop-mode.json on a clean main too (pre-existing, not touched here).

Dependency bump evidence (cachekit-core 0.5 → 0.6)

  • Security workflow (cargo deny check advisories/bans/licenses/sources with --all-features + cargo audit on Cargo.lock) passed on this branch: run 33617757729.
  • OSV query for cachekit-core@0.6.0 (crates.io) returns no advisories (2026-09-03).
  • Cargo.lock: sole change against main is cachekit-core 0.5.0 → 0.6.0, checksum 93adc5646956ba8da140f4179a02e60a2cc7a401b83ebb1270cc4d03748e1fac (matches the published .crate); no new transitive packages. (main already carries the async-trait 0.1.92 bump that clears the beta double_must_use canary, so this PR no longer carries it — the branch is merged up to main @ 92637cc.)
  • The 0.5→0.6 source diff is additive (verified independently by the panel's security-specialist and by the adversarial reviewer's packaged-crate comparison).

Adversarial review (Helly R) — applied at 067ad65

The "safe to drop" guidance omitted the runbook's write-fence and longest-TTL preconditions. README ### Key Rotation and the previous_key_hits rustdoc now state: complete the two-phase promotion first, start the clock only when that deploy completes fleet-wide (a lagging instance still writes under the retiring key and reads it silently as index 0), wait at least the longest TTL in use including explicit set_with_ttl values, aggregate across instances, and only then read a flat counter as drained. Positions are comparable across instances only once they share one keyring configuration.

Out of scope, deliberately: switching the layer to core's new TenantKeyring (LAB-1638 follow-up), py/ts exposure (neither has multi-key decrypt), the docs.cachekit.io runbook page (LAB-687).

Summary by CodeRabbit

  • New Features

    • Added visibility into reads still using previous encryption keys, helping teams monitor key-rotation progress.
    • Previous-key usage is tracked separately for each rotation key and excludes reads using the current key.
    • Failed authentication attempts do not increase previous-key usage counts.
  • Documentation

    • Expanded key-rotation guidance to explain how to interpret usage counts, account for multiple running instances, wait through a complete TTL period, and safely remove old keys.

Summary

Despite the PR title referencing a rotation drain signal via decrypt_indexed (LAB-1678), the actual code changes in this PR are limited to documentation updates in README.md.

Changes

The PR adds documentation to the security features section of the README describing the cache-key path encoding protection (CWE-22):

  • A new row is added to the security features table explaining that cache keys are percent-encoded into the CachekitIO request path, and that any key encoding to a reserved segment (., .., health, ttl, lock) is rejected rather than sent.
  • A detailed explanatory paragraph is added covering:
    • Keys are percent-encoded via urlencoding::encode so a key can only address /v1/cache/{key}.
    • Five reserved path segments are rejected with a permanent error to prevent path traversal / route hijacking.
    • Why encoding cannot neutralize the risk (WHATWG URL parser in reqwest strips dot segments before the request leaves the process, and health/ttl/lock are live route tokens).
    • Consistency notes comparing this behavior against the cachekit-ts twin and the stricter-than cachekit-py handling, and confirming no legitimate CacheKit keys are affected.

Note

Based solely on the provided diff, this PR contains only documentation additions. No functional/source code changes (e.g., to decrypt_indexed or a rotation drain signal) are present in the changes shown.


Based on the code changes provided, here's a description for this pull request:

Description

Despite the PR title referencing rotation drain signals via decrypt_indexed, the actual code changes in this PR are documentation and packaging updates. The changes fall into three areas:

1. Version Bump in README (0.50.7)

All dependency examples throughout the README have been updated from cachekit-rs = "0.5" to cachekit-rs = "0.7", covering the default, Redis, Memcached, File, and Cloudflare Workers configurations.

2. New "Intent Presets" Documentation

A new Intent Presets (recommended) section was added to the Quick Start guide, documenting four one-call preset builders:

  • CacheKit::minimal(url) — development/public data, speed-first, no extras
  • CacheKit::production(url) — user sessions and production services with L1, reliability, and auto-reconnect
  • CacheKit::encrypted(url, key) — PII/payments/GDPR-HIPAA data with AES-256-GCM encryption
  • CacheKit::io(api_key) — serverless/edge compute via cachekit.io without running Redis

The section includes a comparison table (backend, L1, encryption, reliability, auto-reconnect, default TTL), a runnable code example, and a detailed Resilience contract explaining connection-failure behavior at construction and mid-run for each preset (auto-reconnect vs. fail-fast semantics, initial connection handling, and master-key validation ordering).

The Overview intro was also rewritten to highlight the intent-preset approach.

3. docs.rs Build Configuration (Cargo.toml)

Added a [package.metadata.docs.rs] section that enables the cachekitio, redis, encryption, l1, reliability, macros, memcached, and file features when building on docs.rs. This ensures the Redis intent presets and optional backends appear in the rendered documentation (docs.rs otherwise builds with default features only). The workers feature is intentionally excluded due to being mutually exclusive with the other features.


Note: The code changes shown do not include any modifications to decrypt_indexed or rotation drain signal logic referenced in the PR title. The provided patches only touch README.md and crates/cachekit/Cargo.toml. If the rotation drain functionality is part of this PR, those source changes were not included in the diff provided for analysis.

Bump cachekit-core 0.5 -> 0.6 and read through Keyring::decrypt_indexed so
the winning key position reaches the SDK. EncryptionLayer counts reads served
by each previous key (AtomicU64 per position, current-key reads not counted)
and exposes them via previous_key_hits(), also on SecureCache. An operator
running a rotation grace window watches the retiring key's count stop
growing before dropping it, instead of guessing at a hard cut-over.

Error-class mapping and attempt sequencing are unchanged (core owns the
loop, LAB-683). No key material in the signal: positions and counts only.
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Walkthrough

The encryption layer now counts successful decryptions made with previous rotation keys. SecureCache exposes these counts. Tests and documentation cover rotation draining and safe key removal.

Changes

Previous-key rotation telemetry

Layer / File(s) Summary
Track previous-key decryptions
crates/cachekit/src/encryption.rs
EncryptionLayer maintains per-key atomic counters, records successful previous-key decryptions by key position, excludes the current key, and ignores failed authentication. Tests cover these behaviours.
Expose and document rotation telemetry
crates/cachekit/src/client.rs, crates/cachekit/tests/encryption_tests.rs, crates/cachekit/Cargo.toml, README.md
SecureCache::previous_key_hits() returns the counters. Integration coverage verifies a previous-key read. The crate uses cachekit-core 0.6. The README documents counter aggregation and safe key removal.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant SecureCache
  participant EncryptionLayer
  participant Keyring
  SecureCache->>EncryptionLayer: decrypt payload
  EncryptionLayer->>Keyring: request indexed decryption
  Keyring-->>EncryptionLayer: return plaintext and key position
  EncryptionLayer->>EncryptionLayer: increment previous-key hit counter
  EncryptionLayer-->>SecureCache: return decrypted value
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Merge Risk: ⚪ Minimal · up to c00c1

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Check name Status Explanation
Docstring Coverage ✅ Passed Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 12 functions across 3 files. (2 skipped: 2…
Linked Issues check ✅ Passed Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check ✅ Passed Check skipped because no linked issues were found for this pull request.
Description Check ✅ Passed Check skipped - CodeRabbit’s high-level summary is enabled.
Title check ✅ Passed The title clearly summarises the main change: exposing the rotation-drain signal through indexed decryption. It is specific, concise, and related to the implementation.
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Found critical issues please review the requested changes

Docs: state that counts are per process and reset on restart, so operators
aggregate across every instance holding the retiring key and watch a full TTL
window before dropping it (a single restarted replica reading [0] is not a
drained key). Operator workflow now lives in one Rust location
(previous_key_hits rustdoc); type/field/handle docs point there. README code
fence that restated the inline call removed.

Code: debug_assert on the keyring index so an out-of-range index from core is
loud under test while the release path stays panic-free. Integration test
trimmed to the handle-wiring proof; index-0 silence is owned at the layer.
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Expert panel (critical stakes) — run in-session, findings applied at 048413a

Agent Result
bug-hunter-supreme NO FINDINGS
security-specialist NO FINDINGS — verified: signal is Vec<u64> only, Debug omits it, no export; core 0.6.0 decrypt is a byte-equivalent delegate of decrypt_indexed, error-class mapping unchanged; lockfile checksum matches the crates.io .crate, 0.5→0.6 source diff is additive only
code-craftsman MAJ applied: counters are per process and reset on restart, so the docs now tell operators to aggregate across every instance holding the retiring key and watch a full TTL window. MIN applied: debug_assert! on the keyring index (loud under test, release path stays panic-free)
catchphrase-agent Cuts applied: operator workflow lives in one Rust location (previous_key_hits rustdoc; type/field/handle docs point there), README code fence that restated the inline call removed, integration test trimmed to the handle-wiring proof. Rejected: folding the drain assertions into rotation_round_trip_without_reencryption — the ticket requires the existing rotation tests to pass unmodified

Verdict after fixes: SHIP. Vec<AtomicU64> + Relaxed + one guard + one delegate was judged the shortest working shape; the SecureCache passthrough is kept because both encryption fields are private and the alternative (an EncryptionLayer accessor) exposes raw encrypt/decrypt.

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Actionable comments posted: 1

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Inline comments:
In `@crates/cachekit/src/encryption.rs`:
- Line 194: Update the counter increment around hits.fetch_add in the encryption
code to compile for wasm32-unknown-unknown without atomics, using a
WASM-compatible implementation or gating the counter while preserving hit
counting on targets that support AtomicU64.

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Comment thread crates/cachekit/src/encryption.rs
…678)

The 'safe to drop' guidance said a flat counter over one TTL window was
enough. The normative runbook (protocol decisions/key-rotation.md) is
stricter, and the counter cannot see the gap: during a rolling promotion
a lagging instance still writes under the retiring key and reads it as
its own current key, which is deliberately silent (index 0). Per-entry
TTLs via set_with_ttl can also outlive the default window.

Both surfaces (README Key Rotation, previous_key_hits rustdoc) now state:
complete the two-phase promotion first, start the clock when that deploy
finishes fleet-wide, wait the longest TTL in use, aggregate across
instances, and only then read a flat counter as drained. Adversarial
finding by Helly R on PR #74.
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kodus-27b Bot previously approved these changes Sep 3, 2026
…LAB-1678)

The beta job on this PR fails on 9 double_must_use errors in
backend/mod.rs, all emitted by the async_trait macro: async-trait
<=0.1.91 injects a bare #[must_use] onto every generated method, and
beta clippy now flags that as redundant on methods already returning a
must_use Result. Upstream fixed it in 0.1.92 (LAB-2545). That bump has
sat unmerged in #72 for a week, so this PR carries the same one-package
lockfile change rather than waiting on it; whichever lands second sees
a trivial Cargo.lock conflict. Pulls syn 3.0.5 as a new transitive
build dependency alongside syn 2 (deny.toml: multiple-versions = warn).

Verified locally: beta clippy -D warnings clean, stable clippy clean,
cargo +1.85 check passes, wasm32 check builds, 279 tests green.
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Resolves the Cargo.lock conflict: main now carries async-trait 0.1.92
with syn 3.0.4 (via #67), this branch had resolved syn 3.0.5. Taken
main's lockfile and re-applied only the cachekit-core 0.6.0 pin, so the
PR's lockfile diff against main is once again that single package.
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