Improve capture performance: shorter RX lock, PMKID-first save, channel sweep

- Reduce time spent in capture spinlock by snapshotting slot BSSIDs and
  only locking for shared-state updates
- Add PMKID KDE detection to save on EAPOL M1 when present
- Sweep channels with remaining WPA targets instead of camping one channel

Made-with: Cursor

handshake-capture-c6/IMPROVEMENTS.md

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+# Firmware Performance Improvements
+
+Review of handshake-capture-c6 for ESP32-C6 1.47" LCD. Ordered by impact and effort.
+
+---
+
+## 1. Promiscuous Callback — Reduce Critical Section Scope
+
+**File:** `HandshakeCapture.cpp` ~lines 352–385
+
+**Issue:** The entire slot loop runs inside `portENTER_CRITICAL` / `portEXIT_CRITICAL`. For every frame we hold the spinlock across up to 3 slot checks, each with several `memcmp` calls. This blocks all other cores/tasks and can cause missed packets at high traffic.
+
+**Change:** Shrink the critical section to only the minimal shared writes:
+- Copy `slots`, `pending_save_slots`, `is_capturing`, `capturePhase` into locals at callback entry.
+- Do all reads and logic outside the lock.
+- Enter critical only for: `pcapAppendSlot`, `addClientToSlot`, `pending_save_slots[i] = i`, `slot->eapol_count++`, `slot->client_count` updates, `latest_eapol_count`.
+
+**Effort:** Medium | **Impact:** High (fewer dropped frames)
+
+---
+
+## 2. Early-Exit in Callback for Non-Target Frames
+
+**File:** `HandshakeCapture.cpp` ~lines 320–385
+
+**Issue:** For every data frame we iterate all 3 slots and do BSSID comparisons even when the frame has nothing to do with our targets. On busy channels that’s a lot of wasted `memcmp`.
+
+**Change:** Build a quick-reject set: hash or simple bloom of our target BSSIDs. If neither Addr1 nor Addr2 matches any target, `continue` before the slot loop. Alternatively, compare against slot BSSIDs only when frame type is beacon or data, and bail immediately when no match.
+
+**Effort:** Low | **Impact:** Medium (less CPU per frame)
+
+---
+
+## 3. Staggered Deauth Bursts
+
+**File:** `HandshakeCapture.cpp` ~`sendDeauthBurstAll`, `sendDeauthToClient`
+
+**Issue:** All slots are deauthed back-to-back. Radio TX blinds RX; a burst of many deauths increases the chance we miss the following EAPOL.
+
+**Change:** Interleave deauth with short RX gaps:
+- 5 frames to client A → 50–100ms delay → 5 frames to client B → delay → etc.
+- Or: 1 frame per client, round-robin, 2–3 rounds with short gaps.
+
+**Effort:** Low | **Impact:** Medium–High (better EAPOL capture window)
+
+---
+
+## 4. PMKID-First Immediate Save
+
+**File:** `HandshakeCapture.cpp` promiscuous callback, EAPOL branch
+
+**Issue:** We only set `pending_save_slots[i] = i` when `eapol_count >= 2`. PMKID is in EAPOL Message 1; hashcat can use it alone. If we never see M2, we discard usable data.
+
+**Change:** On first EAPOL (M1), parse key info byte; if PMKID flag is set, set `pending_save_slots[i] = i` immediately. Keep current logic for M2 as well. Ensures we don’t drop PMKID-only handshakes.
+
+**Effort:** Low | **Impact:** High (more crackable captures)
+
+---
+
+## 5. Replace `String` with Fixed Buffers in Hot Paths
+
+**File:** `HandshakeCapture.h` `WifiNetwork`, `HandshakeCapture.cpp` `handshakeCaptureProcessPending` ~line 89
+
+**Issue:** `String` uses dynamic allocation. In `handshakeCaptureProcessPending`, `filename += s->ssid` and similar can fragment heap. `WifiNetwork::ssid` and `encryption` as `String` add overhead in scan/capture paths.
+
+**Change:** Use `char ssid[33]` and `char encryption[16]` in `WifiNetwork`. In `handshakeCaptureProcessPending`, use `snprintf` into a stack buffer for the filename. Reduces heap use and allocation during capture.
+
+**Effort:** Medium | **Impact:** Medium (stability, less heap pressure)
+
+---
+
+## 6. SD Write Off Main Loop
+
+**File:** `HandshakeCapture.cpp` `handshakeCaptureProcessPending`
+
+**Issue:** SD writes block the main loop. A large PCAP (e.g. 4KB) can take 50–200ms; during that time `handshakeCaptureLoop` doesn’t run, LVGL stutters, and we might miss phase transitions.
+
+**Change:** Defer SD write to a lower-priority task or timer callback. `handshakeCaptureProcessPending` only marks a slot “ready to write” and copies its buffer to a secondary buffer. A separate task or `loop()`-called `processSdQueue()` does the actual `SD.open` / `write` / `close`. Alternatively, use non-blocking SPI if available.
+
+**Effort:** Medium–High | **Impact:** Medium (smoother UI, fewer timing glitches)
+
+---
+
+## 7. Channel Hop Strategy — Full Sweep
+
+**File:** `HandshakeCapture.cpp` `getBestChannel`, `handshake-capture-c6.ino` channel logic
+
+**Issue:** We stay on the single “best” channel (most targets) for the full timeout. APs on ch 2–5, 7–10, 12–14 are never visited.
+
+**Change:** Maintain a channel list sorted by target count. After exhausting retries on channel N, move to channel N+1 instead of rescanning. Rotate through all channels that have uncaptured WPA targets. Reduces time spent on empty channels and improves coverage.
+
+**Effort:** Medium | **Impact:** High (more handshakes per run)
+
+---
+
+## 8. RSSI-Based Slot Ordering
+
+**File:** `HandshakeCapture.cpp` `startMultiCapture`, `scanNetworksSortedByRSSI`
+
+**Issue:** Slots are filled in scan order. Stronger-signal APs often have faster reconnects and easier captures.
+
+**Change:** When filling slots, sort `networks` by RSSI for the current channel, then assign slots. Prioritize high-RSSI targets. Optional: during observation, weight client mapping by RSSI.
+
+**Effort:** Low | **Impact:** Low–Medium (better success on marginal APs)
+
+---
+
+## 9. LVGL Update Throttling
+
+**File:** `handshake-capture-c6.ino` `updateCaptureVisuals`
+
+**Issue:** We iterate all networks and update LVGL every 400ms. When there are many networks, this does a lot of `lv_label_set_text` and `lv_obj_set_style_text_color` per cycle.
+
+**Change:** Track `last_updated_index` and only refresh labels that actually changed (compare new text/color to previous). Batch LVGL updates or skip when `captureState != STATE_CAPTURING`.
+
+**Effort:** Low | **Impact:** Low (smoother UI, less CPU)
+
+---
+
+## 10. `wasBssidCaptured` Linear Scan
+
+**File:** `HandshakeCapture.cpp` `wasBssidCaptured`, `scanNetworksSortedByRSSI` ~line 219
+
+**Issue:** `wasBssidCaptured` is O(n) per network during scan. With 20 networks and 32 cached BSSIDs, that’s up to 640 comparisons per scan.
+
+**Change:** Use a small hash set or sorted array + binary search for the BSSID cache. Or cache the last few lookups. For 32 entries, linear is acceptable; optimization is only if scan becomes a bottleneck.
+
+**Effort:** Low | **Impact:** Low
+
+---
+
+## 11. Progress Bar Animation
+
+**File:** `LVGL_Driver.cpp` `Ui_SetProgress` ~line 64
+
+**Issue:** `lv_bar_set_value(..., LV_ANIM_ON)` triggers an animation every 400ms when `updateCaptureVisuals` runs, even when the value is unchanged.
+
+**Change:** Only call `lv_bar_set_value` when `pct` or `current`/`total` actually changed. Use `LV_ANIM_OFF` for progress bar to avoid unnecessary redraws.
+
+**Effort:** Trivial | **Impact:** Low (less LVGL work)
+
+---
+
+## 12. Disable Wi-Fi AP Mode When Not Needed
+
+**File:** `HandshakeCapture.cpp` `handshakeCaptureInit`
+
+**Issue:** `WiFi.mode(WIFI_AP_STA)` keeps the soft AP up. You’ve said no web interface; the AP may be unnecessary and draws power.
+
+**Change:** Use `WIFI_STA` only if no AP functionality is required. Verify promiscuous mode works in STA-only. If it does, drop AP to save power.
+
+**Effort:** Low | **Impact:** Low (power, simpler state)
+
+---
+
+## 13. `millis()` in Critical Section
+
+**File:** `HandshakeCapture.cpp` `pcapAppendSlot` ~line 249
+
+**Issue:** `millis()` is called inside the spinlock-protected `pcapAppendSlot`. `millis()` can have non-trivial cost on some platforms.
+
+**Change:** Call `millis()` once at callback entry, store in a local, pass it into `pcapAppendSlot` as a parameter. Keeps the critical section shorter.
+
+**Effort:** Trivial | **Impact:** Low
+
+---
+
+## 14. `WiFiScanner_Init` Unused
+
+**File:** `WiFi_Scanner.cpp` defines `WiFiScanner_Init`; it’s never called from the main sketch.
+
+**Change:** Remove dead code or call it from `setup()` if it’s meant to configure WiFi before scanning.
+
+**Effort:** Trivial | **Impact:** None (cleanup)
+
+---
+
+## 15. Scan: Hidden Network Support
+
+**File:** `HandshakeCapture.cpp` `scanNetworksSortedByRSSI` ~line 205
+
+**Issue:** `if (ssid.isEmpty()) continue` skips hidden networks entirely.
+
+**Change:** Use `WiFi.scanNetworks(true, true)` to include hidden, and use a placeholder like `"<hidden>"` for empty SSID. Store BSSID and channel; we can still target by BSSID. Requires handling `<hidden>` in filenames and UI.
+
+**Effort:** Low | **Impact:** Medium (more targets)
+
+---
+
+## Quick Wins (minimal code, clear benefit)
+
+| # | Change | File |
+|---|--------|------|
+| 3 | Stagger deauth with RX gaps | HandshakeCapture.cpp |
+| 4 | PMKID-first save on M1 | HandshakeCapture.cpp |
+| 8 | RSSI-sort slots in `startMultiCapture` | HandshakeCapture.cpp |
+| 11 | Only update progress bar when value changes | LVGL_Driver.cpp |
+| 13 | Pass `millis()` into `pcapAppendSlot` | HandshakeCapture.cpp |
+
+---
+
+## Higher Effort, Higher Impact
+
+| # | Change |
+|---|--------|
+| 1 | Shrink callback critical section |
+| 5 | Replace String with fixed buffers |
+| 6 | Offload SD write from main loop |
+| 7 | Full channel sweep instead of single best |