$16.$ Which of the statements below are correct

$$ The output feedback $($OFB$)$ mode of operation of the block cipher can be used as a keystream generator.

$$ In cryptography an initialization vector $($IV$)$ does not always have to be random and can be a nonce $($i$.$e$.$ a number used only once, fulfilling the non$-$repeating property$).$

$$ In the RSA cipher the public key has one part, while the private key has two parts used in modular algebra computations.

$$ Cryptographically secure pseudo random numbers generators are nondeterministic.

$17.$ Which of the following statements are correct

$$ AES cryptosystem operations are based on finite fields.

$$ Cryptography can be used to communicate securely over an insecure communication channel

$$ The key space for the affine cipher is equal to $286.$

$$ The Euler's theorem can be used to simplify reduction of large powers modulo n$.$

$18.$ Which statements on classical cryptography fundamentals are correct

$$ Practical stream ciphers do not distribute the truly random key, but rather independently generate the keys with pseudo random numbers generators $($e$.$g$.$ linear congruential generator, LCG$)$ sharing the same seed $($a much smaller random number$),$ which becomes a key much simpler to distribute, however compromising the unconditional security of the OTP.

$$ Cryptographically secure pseudo random numbers generators' unpredictability is generally based on ciphers or other cryptographic methods $($e$.$g$.$ hashes$)$ or on hard $($asymmetric$)$ mathematical problems.

$$ In simplified terms a field is a set of numbers in which one can add, subtract and multiple but not divide.

$$ In the One$-$Time Pad scheme the key to encrypt is the same key to decrypt messages.