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SigFox

SigFox

  • Battery conscious of up to 10 years on a AA battery
  • Limits devices to 140, 12 byte data grams per day
  • Ultra narrow Band
    • Makes the signal to noise ratio better
  • Uses Phase Modulation in the form of D-BPSK
  • Uses shift registers to encode the data using (5,7) Constitutional code
    • This sends the same packet three times with no encoding, constitutional 5 and constitutional 7 encoding.
    • This is used provide error correction
  • Has a Message Authentication Code (MAC)
    • Digital Signature with symmetric key
    • May be on the flash chip in plaintext
    • Uses CBC-MAC with the Input being
      • Flags
      • Sequence ID
      • Unique Device ID
      • Payload

Example Hex Packets:

aaaa35f0a23db334d00dedbeef9a54ad20
aaaa35f0a24db334d00dedbeef85845cad
aaaa35f0a25db334d00dedbeef92387857
aaaa35f0a26db334d00dedbeef77b69fac

Structure of Uplink:

Preamble | Header | Flags | Sequence # | Unique Device ID | Payload |  MAC  |  CRC-16  |
---------|--------|-------|------------|------------------|---------|-------|----------|
 aaaa    | 35f0   |       | a23        |db334d00          | dedbeef |       | 9a54ad20 |
 aaaa    | 35f0   |       | a24        |db334d00          | dedbeef |       | 85845cad |
 aaaa    | 35f0   |       | a25        |db334d00          | dedbeef |       | 92387857 |
 aaaa    | 35f0   |       | a26        |db334d00          | dedbeef |       | 77b69fac |

Pitflaws

  • MAC is at most 16 bits long
    • brute forcing can be done in 4 hours on a single frequency
    • Can be done in less time with multi frequency can be done in a mater of mins
  • Sequence Number is only 12 bits so at max you can repeat a packet less than 30 days
  • Can only be done after a uplink that contains a downlink request
  • Uses Frequency Modulation using GFSK
  • Uses Scrambling with the only inputs being
    • Sequence Number
    • Device ID
  • BCH (15, 11, 1) Error Correction
  • Has MAC

Structure of Downlink:

Preamble                    | ECC    | Payload                  | MAC | CRC-16 |
----------------------------|--------|--------------------------|-----|--------|
 2aaaaaaaaaaaaaaaaaaaaab227 |        |                          |     |        |