For your ATM debit card, you have a $4$-decimal-digit personal secret code. In the absence of any clue, a brute-force attack takes time-$'t'$ to crack the code on an ATM terminal. Therefore $'t'$ is the secure-time for a customer to report in case the card is misplaced. Your Bank has decided to facilitate an increased secure-time. Out of the following, which option should provide the largest rise in the value of $'t'$?
- Instead of 4-decimal-digits, maintain the personal secret code in 4-hexadecimal-digits.
- Instead of 4-decimal digits, maintain a 5-decimal-digit personal secret code.
- Reduce the processing speed of the ATM terminals to the half of their current speed.
- None of the above provides any improvement.