Exercise 2.9

$\def\i{\mathbf {i}}\def\ket#1{|{#1}\rangle}\def\bra#1{\langle{#1}|}\def\braket#1#2{\langle{#1}|{#2}\rangle}\mathbf{Exercise\ 2.9}$

In the BB84 protocol, how many bits do Alice and Bob need to compare to have a $90\%$ chance of detecting Eve's presence?