ESP32_Serial_Swiss_Army_Knife/docs/roadmap.md

Implementation roadmap

This document tracks the implementation and hardware-validation plan for the ESP32 Serial Swiss Army Knife. It describes intended sequencing rather than a release schedule. A phase is complete only when its implementation, documentation, build validation, and relevant hardware tests have passed.

Status legend

• Complete — implemented and validated on the target hardware.
• Implemented; validation pending — code is present and builds, but the current implementation still needs the listed hardware checks.
• Planned — accepted project direction, not yet implemented.
• Under evaluation — useful candidate whose feasibility, security, or resource cost must be measured before it becomes a commitment.

Project-wide design constraints

These constraints apply across all phases:

• UART0 remains the trusted physical administration and recovery console.
• UART1 and the MAX3243 provide the single physical RS-232 data port.
• All serial transports use the central session broker: one writer, multiple observers.
• A slow or failed client must not block UART reception or unrelated clients.
• Serial data paths remain binary-transparent; transports must not reserve in-band control sequences.
• Network serial access must be authenticated and encrypted. No unauthenticated plaintext TCP serial service is planned.
• Secrets must not be exposed by routine status output, command completion, or logs.
• Resource use must remain bounded and observable on the ESP32-S3 N16R8 target.
• New work must preserve USB CDC and UART0 recovery when Wi-Fi or a network service fails.
• Project code remains GPL-3.0-only; third-party license notices and compatibility must be preserved.

Phase overview

Phase	Scope	Status
0	RS-232 hardware characterization and diagnostics	Complete
1	Persistent UART1 serial-service foundation	Complete
2	Transport-neutral session broker	Complete
3	Native USB CDC-ACM serial transport	Complete
4	Wi-Fi station/AP foundation and network diagnostics	Complete
5A	Authenticated HTTPS administration foundation	Complete
5B	Offline xterm.js WebSocket serial terminal	Complete
6	Authenticated SSH serial transport	Complete
7	Local display and button interface	Planned
8	Security and production hardening	Planned
9	Authenticated, rollback-capable OTA	Planned
10	BLE serial transport and provisioning evaluation	Planned
11	Advanced network integration	Under evaluation
12	Optional filesystem-backed features	Under evaluation

Completed foundation

Phase 0 — RS-232 hardware characterization

Implemented and hardware-validated:

• Central board pin profile for the ESP32-S3 N16R8 and MAX3243 breakout.
• MAX3243 enable, static driver, receiver, valid-voltage, and modem-signal checks.
• UART loopback across supported framing and baud-rate combinations.
• Hardware CTS transmit gating and RTS receive-backpressure tests.
• Exclusive RS-232 port ownership and fault-safe cleanup.
• RGB status indication for diagnostic state.
• Potentially disruptive tests isolated under the debug CLI submenu; top-level status retained.

Detailed procedures are in Electrical tests and Hardware wiring.

Phase 1 — Serial-service foundation

Implemented and hardware-validated:

• Persistent, versioned UART configuration with safe defaults.
• UART1 lifecycle management and exclusive ownership.
• Nonblocking RX and TX software streams.
• Baud rate, data bits, parity, stop bits, RTS/CTS, DTR policy, and RTS threshold controls.
• Runtime counters and safe restart after working-configuration changes.
• Explicit save/load/default/reset behavior; edits are not silently persisted.

Phase 2 — Session broker

Implemented and hardware-validated:

• Up to eight generation-safe broker clients.
• Exactly one writer lease with multiple observers.
• Bounded per-client output and event queues.
• Slow-observer isolation and drop accounting.
• Writer grant, release, denial, revocation, and forced administrative reassignment.
• DTR on-connect integration.
• Partial-transfer and retryable no-progress semantics for transports.
• Broker payload storage placed preferentially in PSRAM while FreeRTOS control structures remain in internal RAM.

Phase 3 — Native USB CDC-ACM

Implemented and hardware-validated:

• Native ESP32-S3 USB CDC-ACM serial transport.
• Automatic broker connection when the host opens the port with DTR asserted.
• Opportunistic writer acquisition with observer fallback.
• Binary-transparent bidirectional serial data.
• Supported host line-coding application while USB owns the writer lease.
• Disconnect cleanup, bounded buffering, counters, and UART0 lifecycle controls.

Phase 4 — Wi-Fi foundation

Implemented and hardware-validated:

• Four persistent station profiles with priority ordering.
• WPA2/WPA3 mixed mode and WPA3-SAE-required mode.
• Explicit no-echo secret entry through UART0.
• AP policies off, fallback, and always.
• Random persistent fallback-AP password and MAC-derived default SSID.
• Bounded profile attempts, DHCP deadlines, failover, and retry backoff.
• Root and wifi aliases for ping, nslookup, and traceroute.
• Wi-Fi/lwIP payload allocation configured to prefer PSRAM with capacities pinned in sdkconfig.defaults.

A WPA3-required profile correctly rejects an AP that does not advertise the required Protected Management Frame capability; mixed mode is the compatible choice for such an AP.

Phase 5A — Authenticated HTTPS foundation

Implemented and hardware-validated:

• HTTPS-only service on TCP port 443 using ESP-IDF mbedTLS.
• Persistent random shared admin credential.
• Persistent device-specific ECDSA P-256 certificate and private key.
• Physical-console credential retrieval, rotation, certificate inspection, rotation, and recovery.
• Explicit failure behavior that preserves UART0, USB, serial, and Wi-Fi recovery paths.
• Credential rotation revocation hooks shared with WebSocket and SSH sessions.

Phase 5B — Offline browser terminal

Implemented and hardware-validated:

• Vendored xterm.js and FitAddon assets; no Internet or CDN dependency.
• CSP-compatible page structure with scripts served as separate resources.
• Authenticated status and one-time WebSocket-ticket APIs.
• Binary WebSocket serial frames and broker writer/observer integration.
• Two bounded browser-terminal sessions.
• Writer request/release controls and role indication.
• Bounded TLS/socket behavior and slow-observer isolation.
• Viewport-constrained terminal fitting without recursive page growth.
• Validation with multiple clients, ANSI colors, advanced escape sequences, and full-screen terminal software.

Latest completed phase

Phase 6 — Authenticated SSH serial transport

The SSH implementation is complete and hardware-validated. It provides:

• wolfSSH on TCP port 22 while HTTPS remains on mbedTLS.
• Password authentication using the shared admin credential.
• A separate persistent ECDSA P-256 SSH host key and OpenSSH-compatible fingerprint.
• Interactive shell/PTY channels only.
• No exec, subsystem, SFTP, SCP, agent forwarding, or TCP forwarding.
• Two fixed session/handshake slots.
• Three password attempts and a 15-second handshake deadline.
• No broker client before successful authentication and shell negotiation.
• Binary-transparent serial forwarding with no in-band writer-control protocol.
• Opportunistic writer acquisition and observer behavior through the common broker.
• Credential-rotation revocation and UART0 session/host-key administration.
• A single bounded wolfSSH owner task pinned to CPU 1.
• wolfSSL/wolfSSH allocations that prefer PSRAM with internal-memory fallback.
• A per-call wolfSSH receive budget to prevent pipelined input from monopolizing the owner task.
• Bounded broker and WebSocket work bursts that guarantee scheduler/idle progress during sustained serial traffic.
• Software AES for PSRAM-backed mbedTLS records plus software wolfCrypt AES/SHA, preventing mbedTLS and wolfSSL from independently reconfiguring shared ESP32 crypto hardware.
• Internal task stacks retained for safety during flash/NVS cache-disable windows.
• Heap and SSH stack telemetry through memory and ssh status.

SSH login, host-key verification, serial traffic, Web Serial coexistence, and USB CDC coexistence have been validated. During concurrency testing, the previously failing three-client arrangement used USB CDC and SSH as writer in turn while the remaining transports observed.

Early runs showed healthy heap values but CPU 0 idle-task watchdog timeouts, disproving the initial memory-pressure hypothesis. One trace exposed an unbounded broker scheduling path. A later, decisive trace repeatedly stopped inside mbedTLS's external-RAM hardware-AES DMA wait while HTTPD encrypted a WebSocket record. The broker and WebSocket producer now use bounded active bursts, mbedTLS software AES avoids the hanging PSRAM DMA path, and wolfSSL AES/SHA hardware acceleration is disabled because its private locks do not coordinate with ESP-IDF's crypto lock.

Phase 6 hardware validation

The final target-hardware retest covered:

1. USB CDC, one WebSocket terminal, and one SSH terminal connected concurrently.
2. SSH as writer with USB CDC and WebSocket observing.
3. USB CDC as writer with SSH and WebSocket observing.
4. Serial traffic with active readers and writer-token release/reassignment.
5. Continued HTTPS/WebSocket, SSH, USB CDC, serial-service, and UART0 responsiveness.

The software-crypto build no longer reproduces the HTTPD watchdog stall. This validates that the failure was a shared hardware-crypto/PSRAM DMA problem rather than heap exhaustion. Phase 6 is complete; these concurrent arrangements remain regression tests for future transport, TLS, memory-placement, and ESP-IDF changes.

Planned phases

The order below is the current plan. Detailed requirements should be finalized at the start of each phase, and optional features must not weaken the completed serial and recovery paths.

Phase 7 — Local display and buttons

Add a standalone local status/control interface without making it a dependency of the serial core. The planning baseline uses a 128×64 dual-color monochrome I²C OLED sold with an SSD1315 controller. Its expected SSD1306-compatible command set, I²C address, orientation, column mapping, and physical color boundary must be confirmed on the actual modules before the UI layout becomes fixed.

Hardware baseline

• Power the OLED from 3.3 V so any module-mounted I²C pull-ups remain ESP32-safe.
• Use GPIO11 for SDA and GPIO12 for SCL. These pins are currently unused and sit in the available GPIO10–14 block on the DevKit header.
• Use three active-low buttons with pull-ups: GPIO10 for previous/back, GPIO13 for select/confirm, and GPIO14 for next.
• Use short left/right presses for page or item navigation, short select for entry, a long left press for back/home, and an explicit select hold for disruptive confirmation.
• Verify whether the module provides suitable SDA/SCL pull-ups; add external pull-ups to 3.3 V if needed.
• Expect yellow rows 0–15 and blue rows 16–63, but determine the exact split with a movable one-pixel row test instead of relying on seller descriptions.
• Keep assignments centralized in the board profile rather than scattering display or button GPIO assumptions through UI code.

The persistent yellow strip is reserved for serial-service state, Wi-Fi strength, active USB/Web/SSH counts, current writer, total clients, and an alert indicator. The blue area rotates through overview, RS-232, broker-client, and network/service pages. No password, Wi-Fi secret, private-key material, or routine credential data may appear on the display.

Implementation sequence

1. Phase 7A — Electrical bring-up and diagnostics
   • Add bounded low-level display and button diagnostics under the existing debug submenu.
   • Probe only the expected 0x3C and 0x3D addresses, then validate geometry, orientation, row/column addressing, contrast, inversion, and the yellow/blue row boundary.
   • Validate each active-low button, pull-up behavior, debounce interval, short press, long press, and stuck-button handling.
2. Phase 7B — Display driver
   • Place the SSD1315 behind a small local panel interface and use ESP-IDF's SSD1306-compatible support if hardware testing confirms compatibility.
   • Use a bounded 1 KiB 128×64 framebuffer, a compact 5×7 font, and a small project-owned status-icon set; do not add LVGL for this fixed monochrome UI.
   • Prefer dirty 8-pixel-page updates, bounded I²C transaction timeouts, and nonfatal recovery after a missing or unresponsive display.
3. Phase 7C — Read-only status UI
   • Build display state from existing serial, Wi-Fi, broker, USB, WebSocket, HTTPS, and SSH snapshot APIs rather than parsing CLI output or reaching into transport internals.
   • Provide overview, RS-232/modem, broker-client/writer, and network/service pages.
   • Refresh at a bounded low rate, initially about 4 Hz, from a low-priority owner task. Never hold a service lock across an I²C transaction.
4. Phase 7D — Local controls
   • Add a shallow menu for safe serial, Wi-Fi, HTTPS, SSH, writer-release, display, and reboot actions through direct service APIs.
   • Require a visible confirmation screen and a timed select hold before stopping active services, revoking a writer, rebooting, or performing another disruptive action.
   • A local display is not a serial broker client and cannot silently acquire the writer lease.
5. Phase 7E — Reliability, persistence, and documentation
   • Add contrast and optional dim/blank timeout settings to limit OLED burn-in without making the display necessary for recovery.
   • Validate display removal, I²C NACK/timeouts, stuck buttons, queue saturation, and repeated actions.
   • Re-run concurrent USB CDC, WebSocket, and SSH traffic while the UI refreshes and confirm UART0 remains responsive.
   • Update wiring, electrical-test, command-reference, and recovery documentation.

Completion requires electrical validation on the selected module, correct operation with all current transports, bounded display/button failure behavior, safe action confirmation, no unexplained serial or broker loss, and documented UART0 recovery.

Phase 8 — Security and production hardening

Harden stored credentials, boot integrity, update trust, failure diagnostics, and dependency maintenance. This phase must be staged carefully because some ESP32 eFuse operations are irreversible.

Planned work:

• Define development, test, and production security profiles.
• Enable and validate NVS encryption for Wi-Fi, HTTPS, and SSH material.
• Evaluate flash encryption and PSRAM encryption for the target hardware and threat model.
• Enable secure boot with a documented signing-key and recovery process.
• Define secret-safe core-dump handling; do not expose credentials or session material in diagnostics.
• Audit key and session-material zeroization across application, mbedTLS, wolfSSL, and wolfSSH allocations.
• Add authentication throttling, lockout/backoff, and security-event counters where appropriate.
• Review TLS/SSH algorithms, certificate policy, password policy, and browser security headers.
• Review current wolfSSL/wolfSSH and ESP-IDF security releases instead of treating integration versions as permanent pins.
• Recheck all third-party licenses and preserve source/license notices.
• Document provisioning, key rotation, factory reset, backup, recovery, and device decommissioning.

Security features must be validated first on expendable hardware before any irreversible production configuration is recommended.

Phase 9 — Authenticated OTA and rollback

Use the existing dual 4 MiB application slots and otadata partition to add safe firmware updates.

Planned work:

• Accept only authenticated, integrity-checked firmware images compatible with the selected secure-boot policy.
• Support an update path that does not expose a plaintext management endpoint.
• Stream downloads with bounded RAM use and explicit progress/error reporting.
• Preserve serial, Wi-Fi, HTTPS, and SSH configuration across successful updates.
• Mark new firmware pending until a post-boot health check succeeds.
• Roll back automatically after failed boot or health confirmation.
• Define power-loss behavior for every update stage.
• Keep UART0 recovery and wired re-flashing documented and functional.
• Add version/compatibility checks and reject accidental downgrade unless an explicit policy allows it.

OTA is complete only after successful update, interrupted-update, invalid-image, rollback, and configuration-persistence tests on hardware.

Phase 10 — BLE

Evaluate and, if resource limits permit, add BLE as another bounded transport or provisioning path.

Planned work:

• Decide whether BLE is primarily a serial transport, a provisioning interface, or both.
• Require authenticated pairing/bonding appropriate to the device threat model.
• Integrate serial access through the existing broker rather than bypassing writer ownership.
• Preserve binary transparency through explicit framing and MTU-aware partial transfers.
• Bound connections, queues, retries, and advertising behavior.
• Measure coexistence with 2.4 GHz Wi-Fi, HTTPS, WebSocket, SSH, USB CDC, and UART service.
• Store bond/provisioning data under the hardened secret-storage policy.
• Provide UART0 controls and a physical recovery path for clearing BLE state.

BLE remains subordinate to stable Wi-Fi and serial operation; it should be omitted if coexistence or memory costs cannot be bounded acceptably.

Phase 11 — Advanced network integration

These features are candidates, not current commitments:

• mDNS advertisement for the certificate’s device-specific .local name and selected services.
• Enterprise Wi-Fi support, subject to credential-storage and certificate-validation design.
• IPv6 behavior and diagnostics beyond the current basic support.
• WireGuard feasibility evaluation, including RAM, CPU, licensing, key storage, routing, and recovery impact.

The device is not intended to become a general-purpose router. Captive-portal interception, unauthenticated DNS redirection, NAPT, and a plaintext serial listener remain out of scope unless the project requirements are explicitly revised.

Phase 12 — Optional filesystem-backed features

The storage partition is reserved but not currently mounted. Possible uses must be justified individually:

• LittleFS-backed web assets to decouple large static files from the application image.
• Bounded diagnostic logs with explicit retention and secret-redaction rules.
• Import/export of non-secret configuration.
• Optional user files needed by later administration features.

Before enabling storage, define corruption recovery, wear limits, quotas, atomic update behavior, compatibility across OTA slots, and whether encryption is required. Serial operation and recovery must not depend on a mountable filesystem.

Cross-phase release gates

Every phase should satisfy the following before being marked complete:

1. A clean release build for the ESP32-S3 N16R8 target.
2. No new compiler or project diagnostics attributable to the change.
3. Focused automated tests where practical and documented hardware tests where hardware behavior is involved.
4. UART0, native USB CDC, and previously completed network transports still operate.
5. Broker writer/observer semantics and binary transparency remain intact.
6. Failure, disconnect, stop/start, reboot, and stale-session cleanup paths are exercised.
7. Runtime memory, stack, socket, and queue bounds are measured under maximum supported concurrency.
8. Secrets remain absent from ordinary status output and logs.
9. Command reference, Hardware wiring, electrical tests, and this roadmap are updated as applicable.
10. New dependencies receive security, maintenance, and GPL-3.0 compatibility review.

Explicitly deferred work

The following are not implemented merely because flash partitions or library support exist:

• NVS, flash, or PSRAM encryption.
• Secure boot or production eFuse provisioning.
• OTA download, image confirmation, or rollback policy.
• Core-dump collection or secret-safe core-dump processing.
• Filesystem mounting.
• Public-key SSH user authentication.
• SFTP, SCP, SSH exec, forwarding, or subsystems.
• General routing, NAPT, captive-portal interception, or unauthenticated serial access.

Deferring these features is intentional: each changes the security model, recovery behavior, or bounded-resource assumptions and therefore requires its own design and validation gate.