Is this a checksum/CRC response or cryptography?! Thanks

I’ve been pointed here from the Cryptography Stack Exchange so hopefully you can help me here!

Long version of the story at the end….the short version is…

I want to identify this CRC/cipher/crypto and then I’ll code it in C.

It’s a challenge and response based exchange; a 4 byte challenge, and 4 byte response. They both change each time the exchange is carried out. Here’s a real example

    12   35   5B   74
    00010010    00110101    01011011    01110100    Challenge
    00111000    10010000    00001001    10110101    Response
    38 90 09 B5

I’m able to feed it challenges and observe the responses without penalty – so I have this data too

    00  00  00  00
    00000000    00000000    00000000    00000000    Challenge
    10110101    00100000    01100011    10111011    Response
    B5  20  63  BB


    FF  FF  FF  FF
    11111111    11111111    11111111    11111111    Challenge
    11010101    00010000    10100001    00011110    Response
    D5  10  A1  1E

If anyone can break/hack this for me I’ll love you forever etc etc

Thanks Joe

Long version The exchange happens between the engine control module, the instrument pack, and the body control module in my car…it takes place over the CAN networks and authorises the engine control module to start the engine. I really want to use this engine in a project car but need to get rid of this immobilisation issue (and body controller and instrument pack too) first.