MQTT Encryption¶

Overview¶

Device	Firmware	Encryption	Direction
Charger	v0.3.6	None (plain JSON)	Both directions
Charger	v0.4.0	AES-128-CBC	Both directions
Mower	All versions	AES-128-CBC	Both directions

The charger firmware v0.3.6 sends and receives plain JSON. Charger firmware v0.4.0 adds AES-128-CBC encryption — identical key scheme as the mower. The mower encrypts all MQTT messages with AES-128-CBC.

Mower AES-128-CBC Encryption¶

Discovered via Blutter decompilation of APK v2.4.0 (encrypt_utils.dart).

Key Derivation¶

Property	Value
Algorithm	AES-128-CBC
Key formula	`"abcdabcd1234" + SN.substring(SN.length - 4)`
Key example	`abcdabcd12340238` (for mower `LFIN2230700XXX`)
IV	`abcd1234abcd1234` (static, hardcoded)
Padding	Null-byte padding to 64-byte boundary (NOT PKCS7)
Encoding	UTF-8 for key and IV

Step-by-Step Key Construction¶

graph LR
    SN[Serial Number<br/>LFIN2230700XXX] -->|Last 4 chars| Suffix[0238]
    Prefix[abcdabcd1234] -->|Concatenate| Key[abcdabcd12340238<br/>16 bytes = AES-128]
    IV_const[abcd1234abcd1234] --> IV[Static IV<br/>16 bytes]
    Key --> AES[AES-128-CBC<br/>Encrypt/Decrypt]
    IV --> AES

Take device serial number (e.g., LFIN2230700XXX)
Extract last 4 characters: 0238
Concatenate: "abcdabcd1234" + "0238" = "abcdabcd12340238" (16 bytes)
Convert to bytes via UTF-8 → AES-128 key
IV = "abcd1234abcd1234" (always fixed)

Source Code (from Blutter decompilation)¶

// encode():
0x76cc9c: r16 = "abcdabcd1234"           // Key prefix (12 chars)
0x76ccb4: r0 = _interpolate()             // "abcdabcd1234${snSuffix}" → 16 chars
0x76ccd8: r0 = Uint8List.fromList()       // Key as bytes
0x76cd84: r0 = Key()                      // encrypt package Key object
0x76cdac: r0 = AES()                      // AES cipher setup
0x76cdb0: r0 = Encrypter()                // Encrypter wrapper
0x76cdcc: r2 = "abcd1234abcd1234"         // IV string (16 chars)
0x76cdd8: r0 = Encrypted.fromUtf8()       // IV object
0x76cde8: r0 = encryptBytes()             // Encrypt!

// In mqtt.dart, decode() call:
0x765bf8: sub x1, x4, #4                  // length - 4
0x765c0c: r0 = substring()                // SN.substring(len-4) = last 4 chars
0x765c18: r0 = decode()                   // EncryptUtils.decode(data, snSuffix)

Encrypted Message Sizes¶

Report Type	Encrypted	Blocks	Plaintext	Content
`report_state_robot`	800B	50	~750B	Status, battery, GPS, errors
`report_exception_state`	144B	9	~100B	Sensors, emergency stop, WiFi
`report_state_timer_data`	480-496B	30-31	~440B	GPS, timer tasks

MQTT overhead per message: 37 bytes.

Proof of AES-CBC Mode¶

All payloads are exactly divisible by 16 (AES block size)
Shannon entropy 7.5-7.8 bits/byte — uniform byte distribution
Block boundary divergence: two type-2 payloads (480B vs 496B) identical until byte 208, then 100% divergent — this is the CBC cascade effect
Confirmed by successful decryption

Password Encryption (Separate System)¶

The app uses a different AES setup for encrypting login passwords.

Property	Value
Class	`AesEncryption` in `flutter_novabot/common/aes.dart`
Key	`1234123412ABCDEF` (UTF-8, 16 bytes)
IV	`1234123412ABCDEF` (same as key)
Mode	AES-CBC with PKCS7 padding
Output	Base64 encoded
Usage	Login, registration, password reset — NOT for MQTT

Two different AES systems

Password AES: key/IV = 1234123412ABCDEF, PKCS7 padding, Base64 output
MQTT AES: key = abcdabcd1234 + SN suffix, IV = abcd1234abcd1234, null-byte padding

App Version Differences¶

Feature	v2.3.8	v2.4.0
`encrypt_utils.dart`	Missing	Present
Mower MQTT decryption	None → `jsonDecode()` fails	`EncryptUtils.decode()` → success
Mower status visible	No (FormatException, silently dropped)	Yes
Password AES	Present	Present

The encrypt_utils.dart module is new in v2.4.0. In v2.3.8, mower messages are passed directly to jsonDecode() which throws a FormatException on the ciphertext, and the exception is silently caught and dropped.

Charger Firmware v0.4.0 — AES Confirmed¶

Fully reverse-engineered via Ghidra decompilation

Charger firmware v0.4.0 has been decompiled and analysed. It uses the exact same AES-128-CBC scheme as the mower.

Key Details¶

Property	Value
Algorithm	AES-128-CBC
Key formula	`"abcdabcd1234" + SN[-4:]` (same as mower)
IV	`"abcd1234abcd1234"` (static)
Padding	Null-byte padding to 16-byte boundary
Direction	Both: publish (encrypt) AND subscribe (decrypt)

The firmware string abcdabcd12341234abcdabcd12341234 at offset 0x23b600 is likely a test/default key variant. The actual key is device-specific: "abcdabcd1234" + SN[-4:].

MQTT Receive Path (v0.4.0)¶

graph LR
    MQTT[MQTT PUBLISH<br/>encrypted payload] --> Valid{Length > 0<br/>Length < 1024<br/>Length % 16 == 0}
    Valid -->|Yes| Decrypt[AES-128-CBC<br/>Decrypt]
    Valid -->|No| Drop[Drop packet]
    Decrypt --> Flag[Set mqtt_rec_data_flag = 1]
    Flag --> Queue[Signal FreeRTOS queue]
    Queue --> Parse[cJSON Parse]
    Parse --> Dispatch[Command dispatch<br/>FUN_4200e8c4]

cJSON_IsNull Validation¶

v0.4.0 command values must be JSON null

Unlike v0.3.6 which accepts 0 or {}, v0.4.0 uses cJSON_IsNull() to validate certain command values. Sending {"get_lora_info": 0} will be silently ignored.

Command	Expected value	cJSON check
`get_lora_info`	`null`	`cJSON_IsNull()`
`ota_version_info`	`null`	`cJSON_IsNull()`
`ota_upgrade_cmd`	`{...}` object	`cJSON_IsString()` on nested fields

Server Implementation¶

The local server handles v0.4.0 encrypted charger messages via the raw-TCP endpoint:

POST /api/dashboard/raw-tcp/:sn
Body: {"command": {"get_lora_info": null}, "qos": 1}

This endpoint:

Takes the JSON command
Encrypts with AES-128-CBC using "abcdabcd1234" + SN[-4:]
Writes the MQTT PUBLISH packet directly to the device's TCP socket (bypassing aedes)
Returns success/failure with packet size