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eddy d6f1062a44 Replace HMAC-SHA256 with AES-256-GCM + ECDH key exchange for BLE relay commands (issue #2 )

- New: src/ecdh_helper.h — ECDH keypair gen, shared secret, pubkey HMAC sign/verify
- Server (src/main.cpp): ECDH handshake callback, AES-GCM decrypt, PSK from NVS
- Client (src/main.cpp→client.cpp): ECDH after connect, AES-GCM encrypt, PSK from NVS
- Android: ECDH (KeyAgreement) + AES-GCM (Cipher) in BleRelayManager.kt
- PSK provisioned via USB CDC SET_KEY (SettingsActivity.kt)
- PrefsHelper: hmac_key → psk
- Bumped versions: server=18, client=4
- NVS namespace: hmac→syl, key:hmac_key→psk
- BLE ECDH characteristic: beb5483e-...-f26dc (WRITE_NR|NOTIFY)

2026-08-03 04:34:26 +02:00

2.3 KiB

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Plan: Issue #2 — AES-256-GCM + ECDH + PSK voor BLE relay commands

Beveiligingsmodel

Provisioning (eenmalig, USB CDC)

Android genereert 32B PSK (pre-shared key) en provisioned beide ESP's:

SET_KEY → Server ESP (NVS syl/psk)
SET_KEY → Client ESP (NVS syl/psk)

BLE ECDH handshake (elke sessie)

Client                                         Server
  |── pubkey(65B) || HMAC(PSK,pubkey)(32B) ─────>|  verify HMAC met PSK
  |                                               |  genereer eigen keypair
  |<─ pubkey(65B) || HMAC(PSK,pubkey)(32B) ──────|  verify HMAC met PSK
  |                                               |
AES_KEY = SHA-256(ECDH_shared_secret)         AES_KEY = SHA-256(ECDH_shared_secret)

PSK verlaat nooit het apparaat over BLE — alleen HMAC ervan
Zonder PSK kan MITM geen geldige HMAC voor fake pubkey produceren
Per-sessie AES keys → perfect forward secrecy
PSK gelekt? → her-provisionen via USB CDC

Packet structuren

ECDH handshake packet (97 bytes)

pubkey[65] || HMAC-SHA256(PSK, pubkey)[32]

BLE characteristic: beb5483e-...-f26dc, properties WRITE_NR | NOTIFY

Relay command (29 bytes, AES-256-GCM)

seq(4) || nonce(8) || ciphertext_cmd(1) || tag(16)

Nonce voor GCM = seq(4) || random(8) = 12 bytes. Sequence > lastSeq voor replay protection per sessie.

NVS / SharedPreferences

Opslag	Sleutel	Waarde
ESP NVS `syl`	`psk`	32B PSK
ESP NVS `syl`	`uuid`	16B device UUID
Android Prefs	`psk`	64-char hex PSK
Android Prefs	`device_uuid`	32-char hex UUID

Bestandswijzigingen

Nieuw

src/ecdh_helper.h — ECDH keypair generatie + shared secret (mbedtls/ecdh.h)

Gewijzigd

src/main.cpp — HMAC → ECDH handshake + AES-GCM decrypt; PSK uit NVS
src/client.cpp — HMAC → ECDH handshake + AES-GCM encrypt; PSK uit NVS
android/.../BleRelayManager.kt — HMAC → ECDH (KeyAgreement) + AES-GCM (Cipher)
android/.../SettingsActivity.kt — SET_KEY schrijft voortaan PSK
android/.../PrefsHelper.kt — hmac_key → psk
version.txt — bump server + client versies

Werkt NIET met oude firmware/app

Geen backward compatibility: schone break. HMAC packets worden niet meer geaccepteerd.

2.3 KiB Raw Blame History