In password cracking, the exponential wall means that$1)$ the number of exponents that can be tested is limited mathematically$\mathrm{.}2)$ a string exponent is needed in the cryptographic key used to store passwords$\mathrm{.}3)$ the time for a brute force attach increases exponentially with the password length$\mathrm{.}4)$brute force cracking can be prevented by using an exponentially computed hash.