Data format 8 (Encrypted environmental)

Lifecycle: Proposal

This is a proposed encrypted data format which is not yet implemented in Ruuvi devices outside of a few proof-of-concept projects

The encryption uses nRF52-builtin AES128 encryption in Elctronic Codebook (ECB) mode. Data to be encrypted is temprature, humidity, pressure, voltage, TX power, measurement count and movement counts. The measurement sequence counter protects against replay attacks. All measurements where encrypted data and nonce are equal can be considred duplicates.

Data format has an unencrypted header, 16 bytes of AES-128 encrypted data, 1 byte nonce and 6 bytes long MAC address for iOS devices.

Offset	Allowed values	Description
0	8	Data format.
1-2	-32767 ... 32767	Temperature in 0.005 degrees.
3-4	0 ... 40 000	Humidity (16bit unsigned) in 0.0025% (0-163.83% range, though realistically 0-100%).
5-6	0 ... 65534	Pressure (16bit unsigned) in 1 Pa units, with offset of -50 000 Pa.
7-8	0 ... 2046, 0 ... 30	Power info (11+5bit unsigned), first 11 bits is the battery voltage above 1.6V, in millivolts (1.6V to 3.646V range). Last 5 bits unsigned are the TX power above -40dBm, in 2dBm steps. (-40dBm to +20dBm range).
9-10	0 ... 65534	Movement counter (16 bit unsigned), incremented by motion detection interrupts from accelerometer
11-12	0 ... 65534	Measurement sequence number (16 bit unsigned), each time a measurement is taken, this is incremented by one, used for measurement de-duplication. Depending on the transmit interval, multiple packets with the same measurements can be sent, and there may be measurements that never were sent.
13-16	Any	Reserved for future use.
17	0 ... 255	CRC8, used to check for correct decryption.
18-23	Any valid MAC	48bit MAC address.

The encryption key is formed from 64-bit tag ID, 8 bit encryption nonce and a static password with length of 16 bytes by appending nonce to ID and XORing the 9 bytes with 9 first bytes of password

Invalid values

If a value cannot be determined for any reason, a special invalid value is sent. For unsigned values the invalid value is largest presentable number, for example 0xFFFF and for signed values the invalud value is smallest presentable number, for example 0x8000. Invalid values should be treated as NULL, NAN, NONE or similar by the parser.

Example

Data	Value
Temperature	24.58 C
Humidity	40.54 RH-%
Pressure	100453 Pa
Battery	2.765 V
TX Power	+4 dBm
Movement	15
Measurement	6353
Reserved	0
Checksum	174
MAC	0xAABBCCDDEEFF

Keys	Binary
ID	0x0011223344556677
Password	0x5275757669636f6d5275757669546167 "RuuvicomRuuviTag"

Unencrypted binary:

DF	T	H	P	B+TX	C	M	R	CH	MAC
08	1334	3F58	C515	48E6	000F	18D1	000000E0	1C	AABBCCDDEEDD

Encryption key:

Component	Binary
ID	00 11 22 33 44 55 66 77
password	52 75 75 76 69 63 6f 6d 52 75 75 76 69 54 61 67
Result	52 64 57 45 2d 36 09 1a 52 75 75 76 69 54 61 67

Encrypted data: 0x43825B56324FE019C4BD4D6D3CECAC6E

Complete message:

DF	T	H	P	B+TX	C	M	R	CH	MAC
08	4382	5B56	324F	E019	C4BD	4D6D	3CECAC6E	1C	AABBCCDDEEDD

TODO: Test vectors