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data-format-8-encrypted-environmental.md

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description
Lifecycle: Proposal

Data format 8 (Encrypted environmental)

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