Calculate checksum by combined XORs

[charles-dyfis-net]

It appears that the checksum field can be calculated by taking a known checksum from a known source value, and XORing in a constant for each bit that differs from that known source value.

Note that these values are generated from an RC-A5II remote, rather than the original RC-A5; the only hardware I have on hand is VL-150 and VL-300 units, so I can't personally guarantee that the values being collected are legitimate for other lights.

(Yes, this ticket has been completely renamed/reworked; apologies about that).

[charles-dyfis-net]

This ticket will be reopened after the assertions contained have been more aggressively validated, ideally with working code that can validate correct checksums for arbitrary inputs.

[charles-dyfis-net]

Adding notes here, because it's as good as place as any. Note that the below should be considered hypotheses -- the assertions have been spot checked but not comprehensively vetted.

• The transformation for flipping a bit in the value field can be reflected in the checksum field by XOR'ing a constant that varies for a given bit.
  • Modifying the 128s bit in the value is not possible with the standard remote, because no brightness above 100 is allowed.
  • Modifying the 64s bit in the value XORs the checksum with 61.
  • Modifying the 32s bit in the value XORs the checksum with 134.
  • Modifying the 16s bit in the value XORs the checksum with 67.
  • Modifying the 8s bit in the value XORs the checksum with 185.
  • Modifying the 4s bit in the value XORs the checksum with 196.
  • Modifying the 2s bit in the value XORs the checksum with 98.
  • Modifying the 1s bit in the value XORs the checksum with 49.

[charles-dyfis-net]

In the group identifier:

• Modifying the 1s bit XORs the checksum with 110
• Modifying the 2s bit XORs the checksum with 220
• Modifying the 4s bit XORs the checksum with 137
• Modifying the 8s bit XORs the checksum with 35

[charles-dyfis-net]

In the channel identifier:

• Modifying the 1s bit XORs the checksum with 244
• Modifying the 2s bit XORs the checksum with 217
• Modifying the 4s bit XORs the checksum with 131
• Modifying the 8s bit XORs the checksum with 55

[charles-dyfis-net]

Reopening this ticket; https://github.com/charles-dyfis-net/gr-godox-remote contains an implementation of the above checksum algorithm in SQL, and tools demonstrating that its output is identical to values received over the air for a wide array of samples.