When Assetto Corsa loads a KN5 file, it must decrypt it in RAM. You can dump the decrypted data from memory.
For a walkthrough on basic car file management and unpacking data.acd (which often accompanies .kn5 issues): how to decrypt kn5 files verified
Locate the target file (e.g., data.acd or loose .kn5 files in the content/cars directory). When Assetto Corsa loads a KN5 file, it