[mah1]

System A uses 4-digit personal secret numbers (i.e. 0000{9999), and
on each login attempt it randomly chooses two of the numbers and asks
the user to cite them.
if attacker has made one observation of the credentials sarah used to successfully log into her account, and now he tries
to log in by himself and is faced with a fresh request of two digits.
Assume that he enters the requested digit if he knows it, and
otherwise makes a random guess. What is the probability that he
can enter them correctly?
if attacker has made two observations of the correct credentials.
What is the probability that he now knows all four digits of Alice's
PIN?