Resmed CRC algorithm?

[wayward]

Out of curiosity, does anyone know what algorithm is used to generate the 64-bit CRC checksum files used for the EDF data on the S10 machines?  I ran RevEng against the 32-bit checksums for the TGT files and that turned out to be a pretty standard one.  But RevEng didn't find anything for the 64-bit checksums, and I got nosy.  Thanks!

[pholynyk]

Philip Koopman of Carnegie-Mellon has several web pages on CRC algorithms that may be useful: https://users.ece.cmu.edu/~koopman/crc/

I haven't looked into using the crc just yet. I did notice that each data record in an edf file has a crc16. It might be useful to crc check the identification.tgt as general check against a corrupt SD card, so I would be happy to have the crc32 polynomial that you discovered.

I hesitate to consider checking every edf file's crc64 because they are big, and there are a lot of them. It would be useful to check the STR.edf file because almost everything depends on it it, bur the STR.crc file was removed from the ten series machines.

[wayward]

No problem.  The values I got for the 32-bit checksums are below.  (Though the polynomials may be standard for CRC-32/ISO-HDLC.  "Refin" and "refout" refer to reflecting the input and output values.

width=32  
poly=0x04c11db7  
init=0xffffffff  
refin=true  
refout=true  
xorout=0xffffffff  
check=0xcbf43926  
residue=0xdebb20e3  
name="CRC-32/ISO-HDLC"

[wayward]

FWIW, there was an interesting post here: http://www. a DME-owned forum .com/viewtopic/t93676/Data--Settings-on-ResMed-S9-Escape.html

"They might just be seeding the CRC start value or using a non standard root polynomial.. which doesn't take long to hack out on a modern computer.
It could also be in a different byte order depending on the devices CPU.

Could also be stringing together 2 phases of a CRC32 variant or similar, which is quite possible considering there is both 4 and 8 byte crc's.. (a fairly common practice when firmware space is limited)"

[pholynyk]

Thank you, Wayward, I will incorporate that into the resmed importer.