10^4 attempts (worst case) to bruteforce the first four digits using the early NACK, 10^3 attempts (worst case) to bruteforce the entire pin once you know the f

10^4 attempts (worst case) to bruteforce the first four digits using the early NACK, 10^3 attempts (worst case) to bruteforce the entire pin once you know the first four (this part only has to iterate on three digits of the second half, and compute the checksum to get the last digit).