ESP32 Serial Swiss Army Knife

ESP32-S3 firmware for a secure, multi-transport RS-232 adapter. It operates one MAX3243-backed UART1 serial port and safely shares it between native USB CDC-ACM, an HTTPS/WebSocket browser terminal, and SSH: one connected client can write while others observe. The firmware also provides persistent serial and Wi-Fi configuration, a UART0 recovery console, and hardware diagnostics; it is not a general-purpose router, captive portal, or unauthenticated TCP serial server.

Initial hardware target

• ESP32-S3-DevKitC-1-compatible development board
• ESP32-S3-WROOM-1-N16R8 module
• 16 MB flash
• 8 MB octal PSRAM
• Adafruit MAX3243 full-pinout RS-232 breakout; the male connector version is preferred (see Hardware wiring for supported connector variants)

Development status

Hardware characterization, the serial core, USB CDC-ACM, Wi-Fi, HTTPS/WebSocket, SSH serial transport, and the local display/control interface are implemented and Phase 7 target-hardware validated. Phase 8A's bounded role-based user database and UART0 administration, Phase 8B's role-aware HTTPS/SSH authentication and revocation, and Phase 8C's shared UART0/admin-SSH command shell are target-hardware validated. Phase 8D.3 browser login/logout is implemented, host-tested and build-verified; M1 is validated by user sign-off after both-role login, mixed-client operation and post-soak evidence. Numeric memory reserve gates remain open. Browser admin-shell mode, typed settings, and contextual administration remain planned, including preserving serial writer ownership across future terminal-mode changes. Configurable STA-only mDNS naming as sak-<suffix>.local is implemented with independent NVS persistence; target-hardware validation is pending. See the Roadmap for phase status and validation details.

Documentation

• Hardware wiring: hardware profile, GPIO assignments, connector guidance, and safety notes.
• Electrical tests: OLED/buttons, MAX3243, UART loopback, and session-broker verification procedures.
• Role-based user database and UART0 administration: user provisioning and administration, HTTPS/SSH authentication, session revocation, and the planned integrated web-administration acceptance matrix.
• Command reference: UART0/admin-SSH administration, serial, broker, USB, Wi-Fi, mDNS, web, SSH, and diagnostic commands.

Flash partition layout

The N16R8 target has 16 MiB flash and 8 MiB octal PSRAM. PlatformIO uses the custom partitions.csv layout:

Partition	Offset	Size	Purpose
nvs	0x009000	512 KiB	Serial, Wi-Fi, mDNS hostname, local-display, role-based user, HTTPS identity, and SSH host-key data
otadata	0x089000	8 KiB	Active OTA-slot selection metadata
phy_init	0x08B000	4 KiB	Optional PHY initialization data
nvs_key	0x08C000	4 KiB	Reserved for future encrypted-NVS keys
coredump	0x08D000	128 KiB	Reserved for flash core dumps
ota_0	0x0B0000	4 MiB	Primary application/OTA slot
ota_1	0x4B0000	4 MiB	Alternate application/OTA slot
storage	0x8B0000	7488 KiB	Reserved for future LittleFS web assets, logs, and files

Application offsets are aligned to the ESP32-S3's required 64 KiB boundary. The final storage partition ends at 0x1000000, exactly the end of the 16 MiB flash chip.

The table reserves OTA and storage space; it does not implement OTA downloads, rollback confirmation, core-dump handling, NVS encryption, or filesystem mounting.

One-time migration from the default partition table

The previous 1 MiB factory application began at 0x10000, which now lies inside the enlarged NVS range. A normal upload does not erase stale data in that range. When first switching to this layout, erase the flash completely:

pio run --target erase
pio run --target upload
pio device monitor -b 115200

This removes saved serial configuration and all other flash contents. The firmware recreates NVS with safe defaults. Subsequent ordinary uploads do not need a full erase.

Build

pio run

Upload and monitor

Connect the board's USB-to-UART port for firmware upload and the UART0 development console, then run:

pio run --target upload
pio device monitor -b 115200

The firmware provides an interactive UART0 console at serial-tool>. Run help for available commands. The USB-to-UART bridge normally appears as /dev/ttyUSB*; it is separate from the native USB CDC serial transport, which normally appears as /dev/ttyACM*.

The console supports session history, line editing, cursor movement, and hierarchical Tab completion. After an unattended boot, attach an ANSI-capable terminal and press Enter once to enable enhanced editing; this avoids blocking while no terminal is attached.

Serial, Wi-Fi, and mDNS hostname edits remain in RAM until explicitly saved with serial save, wifi save, or mdns save. Authenticated admin SSH sessions expose the shared operational administration registry, including interactive secrets, TLS/SSH identity management, network diagnostics, and deferred reboot/SSH lifecycle commands. Create the first administrator on UART0 with user add <username> admin (optionally --generate). Explicit recovery of an unavailable user database remains UART0-only and rebuilds it empty; it refuses a healthy database. An administrator also cannot generate a replacement password for its own account over SSH, preventing the one-time value from being lost when that mutation revokes the session. Legacy web credential commands and user bootstrap are removed.

Security notes

The HTTPS interface uses a device-specific self-signed certificate and a same-origin login page with bounded server-side cookie sessions; HTTP Basic is no longer accepted. Open / or /login, sign in with a user-database password, and use Sign out before switching accounts. Four sessions have a one-hour absolute lifetime, including active serial connections; logout closes only that session's serial access. Login is globally limited to five credential verifications per 60 seconds, with explicit capacity/backoff errors. Direct-IP and mDNS access use separate host-only Secure/HttpOnly/SameSite=Strict cookies. Non-browser clients also require cookies, strict Origin and CSRF for mutations rather than Basic credentials. There is no plaintext HTTP or TCP serial listener. SSH accepts role-based passwords and authorized Ed25519/ECDSA P-256 public keys. User passwords are stored as salted PBKDF2-HMAC-SHA256 verifiers, but the HTTPS private key, SSH private key, and Wi-Fi credentials remain recoverable from unencrypted application-owned NVS blobs. Offline password guessing and stale append-oriented flash copies also remain possible. The reserved nvs_key partition does not enable encryption. Do not treat this firmware as resistant to physical flash or RAM extraction until the planned hardening work is complete.

License

This project is licensed under the GNU General Public License version 3 only (GPL-3.0-only). Third-party components remain subject to their respective licenses. The integration baseline uses Espressif registry components espressif/mdns 1.12.0, wolfssl/wolfssl 5.8.2~1, and wolfssl/wolfssh 1.4.20; review upstream security releases before production use.

Legacy credential removal

Missing user storage is persisted as an empty database; no shared credential is imported or synchronized. Existing valid v1 user records retain their accounts, roles, IDs, verifiers and keys without a schema change. HTTPS web_sec/material upgrades valid 1,392-byte v1 storage to 1,340-byte TLS-only v2, retaining exact certificate/key DER, fingerprint and generation, and committing before publication. Invalid records or migration failures fail closed rather than triggering fallback replacement. web certificate rotate --force remains available; web reset --force replaces TLS identity only, not users.

Downgrade warning: older v1-only firmware cannot read v2 HTTPS material. Logical NVS replacement is not a secure flash wipe; historical plaintext credentials can remain in flash. This cleanup requires no factory/partition erase. See implementation and evidence limits; final integration build evidence is pending.