$15.$ Which statements on classical cryptography fundamentals are correct

$$ The encryption function in the RSA cipher is simply only an exponentiation $($by the e component of the public key$)$ modulo n$,$ while the decryption function is exponentiation with a different exponent, namely d constituting the private key used for decryption.

$$ In asymmetric cryptography the relationship between the public key and private key is asymmetric, so for the encryption it is easy to compute a private key from the public key, while the opposite operation is difficult.

$$ In breaking DES cryptosystem differential cryptanalysis is more efficient than linear cryptanalysis.

$$ An encryption scheme is deterministic if a particular plaintext is mapped to the same ciphertext when the key used has not been changed.

$16.$ 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.

$19.$ Which of these statements on classical cryptography fundamentals are correct

$$ In modular arithmetic two numbers are equivalent if they have the same remainder when divided by a certain number.

$$ Different ways of using a block cipher for encryption are referred to as modes of operation $($which can be divided into deterministic encryption and probabilistic encryption, with the latter category covering both standard use as block ciphers and also as stream ciphers$).$

$$ Cryptography can be considered a part of cryptology and cryptanalysis fields.

$$ A shift cipher with a key equal to $4$ will replace A letter of the plaintext with D letter in the ciphertext

$20.$ Which of the below statements on classical cryptography fundamentals are correct

$$ The Euler's theorem states that if a and m are integers with the greatest common divisor equal to $1($are coprime numbers$)$ then a raised to the power of phi function on m $($Euler$\text{'}$s totient function$)$ is congruent to $1$ modulo n$.$

$$ The maximum period generated by an LSFR $($linear feedback shift register$)$ of degree m is $2$ to the power m minus $1.$

$$ The parameter t of the EEA $($extended Euler algorithm$)$ is r with index $1$ modulo r with index $0.$

$$ A shift cipher cannot be easily broken by a brute$-$force attack $($key search$),$ because the key space is very large, due to modular arithmetic properties corresponding to wrapping around