Figure $6.$ ANSI X$9\mathrm{.}17$ Random Number Generator

The initial input to the random number generator Vi comes from a stream of random numbers based on keystroke input from the user. DTi input is the current date and time. EDE refers to triple encryption with $2$ keys K$1$ and K$2.$ Both the keystroke timing and the actual keys struck by the user when typing the passphrase are used to generate the randomized input stream Vi$.$ This random input is also combined with the previous session key to form the key input to the generator. This produces a random number Ri which is used to form the session key Ks$.$ The result is to produce a sequence of session keys that is effectively unpredictable.

$1.$ Why does triple encryption require the use of only $2$ keys K$1$ and K$2?$

$2.$ Why would it be impossible for an adversary to determine the session key Ks$?$

$3.$ The DTi input changes constantly every millisecond. Does it ever repeat itself?

$4.$ What do you think Vi$+1$ used for?

$5.$ What boolean logic function is represented by the circle with the plus sign inside of it$?$