How to calculate CRC-CCITT (Kermit) polynomial

Question

I have a problem with CRC. My message in hex is: 80 00 00 03 an the crc is f5 1f. I thought, the polynom is 0x1021 (crc-ccitt kermit). How to find out, what polynom the right one is? And how can I see, if I need it reversed and if the initial value is 0xffff or 0x0000 or something else? How can I calc the polynom I am searching for? Do I have to convert the 80 00 00 03 and the f5 f1 to BIN and then divide both? I tried to clac myself with the prog reveng, but that was not successful. Maybe because I have not all params -.-

I found a php-function HERE

but that will not calc the right sum for me. Maybe it is not CRC-CCITT KERMIT in that php-function

Answer 1

0x1ff5 is indeed the CRC-CCITT (Kermit) of 80 00 00 03. (It is apparently stored in little-endian order in your stream.)

The PHP code you found is for the false "CRC-CCITT", which you can find in the RevEng catalog here.

The true CRC-CCITT (Kermit) parameters are here in that catalog. The bits are reversed, so you use the polynomial reversed, 0x8408, and you shift the bits down instead of up. The initial register contents are zero, and there is no final exclusive-or.