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1. Which statements are correct

A) A block cipher according to Shannon should comprise two atomic operations, i.e. confusion (obscuring relationship between plaintext and ciphertext) and diffusion (single bits of ciphertext influenced by many bits of plaintext). B) In cryptography key space refers to possible keys used by a certain cryptographic protocol. C) In modular arithmetic under a mod 5 operation the numbers 7 and 12 are equivalent. D) Substitution cipher is an example of a historical cypher in which every plaintext letter is replaced by another fixed letter, hence it is an example of an asymmetric cipher.

2. Which statements on classical cryptography fundamentals are correct A) The DES protocol is a block cipher using 56 bits long keys which is considered not secure due to increases of computing power. B) Encryption and decryption functions need to be kept secret for the cryptographic protocol to remain secure. C) A field F is a set of elements with following properties: all elements of F form an additive group with the group operation + and the neutral element 0, all elements of F except 0 form a multiplicative group with the group operation x and the neutral element 1, when the two group operations are mixed the distributivity law holds. D) Fermat's little theorem (stating that if p is a prime number, then for any integer a, the number a to the power of p minus a is an integer multiple of p) is very important for private-key cryptography.

3. Which statements are correct A) Electronic code book mode (ECB) allows for a simple block cipher encryption of large input plaintext by breaking the plaintext into subsequent blocks. B) Insecure communication channels may include Internet, GSM, wireless networks (Wi-Fi). C) Letter frequency analysis attack is considered effective against substitution cipher because identical plaintexts map to identical ciphertexts. D) Double encryption using DES may seem secure but is prone to the meet-in-the-middle attack.

4. Which of the statements below are correct A) A condition for the affinity cipher to work with a key (a,b) is that for having a greatest common divisor for a and m (number of letters in alphabet) equal to 1, which will mean that the cipher's encryption function is injective. B) In cryptography unencrypted message is referred to as the plaintext, while the encrypted message is referred to as the ciphertext. C) An exhaustive key search attack (brute-force type of attack) is considered effective against substitution cipher. D) Unconditional security of a cipher can be referred to as information-theoretic security (meaning it cannot be broken even with infinite computing resources) and is only true for the OTP cipher, however under a condition that the truly random key (of length such as of the plaintext message) can be distributed between the parties, which can only be done unconditionally securely using the quantum key distribution.

5. Which of these statements are correct A) Cryptography can be generally divided into symmetric (public-key) and asymmetric (private-key) cryptography. B) A linear feedback shift register (LSFR) has an input bit being a linear function of its previous state and can be easily implemented from flip-flops, however due to security issues it was not used in any practical cryptographic scenario. C) Permutation is an example of diffusion in a block cipher. D) Stream ciphers encrypt individual bits using public keys.

6. Which of the following statements are correct A) In a shift cipher to account for wrapping around we would need to use mod K, where K is a value of a key, i.e. the number for letters shifting in the alphabet. B) Rijndael cipher won a competition call by NIST to become the AES cryptosystem in 2000 in competing against 70 other algorithm submissions. C) For the RSA cryptosystem to be secure the initial large prime numbers selected for the keys computing algorithm should be really large (i.e. at least 512 bits or even twice or 4 times as much). D) Public-key cryptography (asymmetric cryptography) is used solely to solve the problem of the key distribution.

7. Which of the statements below are correct A) The public-key cryptography has been introduced to be used for encryption, but if the asymmetric relation in terms of the complexity in computing keys is reversed, it can be used for authentication rather than for encryption. B) A probabilistic encryption scheme uses randomness to achieve a non-deterministic generation of a cyphertext. C) Security of block ciphers does not depend on combining multiple times of the confusion and diffusion operations, as these operations do not increase when combined in multiple numbers. D) In modular arithmetic mod 2 addition and subtraction are different operations.

8. Which statements on classical cryptography fundamentals are correct A) The square and multiple algorithm involves the following steps upon scanning the exponent bits left-to-right: 1) in every iteration one needs to square, 2) if current bit is equal to 1, then multiple it by inversion of x. B) The RSA cryptosystem was invented in 1977 and patented in the USA until 2020. C) Under mod 3 operation there are 2 equivalence classes. D) The AES irreducible polynomial is x to the power 8 plus x to the power 4 plus x to the power 3 plus x plus 1.

9. Which of the following statements on classical cryptography fundamentals are correct A) GSM system uses 11 LSFRs coupled together in implementing a stream cipher in order to assure difficulty in breaking its security. B) Cryptanalysis can be divided into social engineering, implementation attacks (including side-channel attacks) and classical cryptanalysis (further divided into analytical attacks and brute forces attacks). C) DES protocol uses 16 subkeys and its effective key length is 56 because it drops bits number 8, 16, 24, 32, 40, 48, 56 and 64, because these bits serve as parity bits. D) In terms of basic algebraic structures fields generalize rings (fields include division operation and have a multiplicative inverse for every element other than 0), while rings generalize groups (rings include multiplication operation).

10. Which of the following statements on classical cryptography fundamentals are correct A) The only really nondeterministic true random numbers generators (TRNG) are quantum random numbers generators (QRNG). B) DES protocol was introduced to improve the security of the AES cryptosystem and successfully doing so became a world standard in practical encryption, later on improved by the 3DES cryptosystem. C) The Feistel network iteratively runs multiple times a simple round function, which allows for easier hardware implementation of a block cipher cryptographic system. D) Pseudo random numbers generators recursively compute consecutive random numbers from a seed, which is an input random (either truly or pseudo random) number.

11. Which of these statements are correct A) In general cryptographic encryption requires a key to encrypt messages (encryption key) and a key to decrypt messages (decryption key), which do not have to be identical, except for the symmetric-key cryptography. B) A pseudo random numbers generator, a hash function or a message authentication code (MAC) can be built from the block cipher. C) In the RSA cipher for Alice to encrypt the message to Bob, she needs to receive the public key of Bob, which must be sent to her by a reliable, however not necessarily secure, communication channel. D) A block cipher can be easily turned into a stream cipher while the opposite is not the case.

12. Which of these statements are correct A) The Kerckhoffs's principle states that cryptosystems should be secure even when only the decryption key is kept secret. B) In cryptography using finite sets is not very common. C) A group is a set of elements G together with an operation which combines two elements of G, having following properties: the group operation is closed and associative, there is a neutral element in the group and there is an inverse element in the group (a group is abelian or commutative if for all elements of the group the operation combining two elements does not depend on the order of elements). D) A stream cipher using a truly random key binary sequence is the unbreakable One-Time Pad cipher (under a condition that each key stream bit is used only once, i.e. the key binary sequence is as long as the message binary sequence and is used only once).

13. Which of the statements below on classical cryptography fundamentals are correct A) AES MixColumn Sublayer involves a non-linear transformation which can be represented by a 4x4 matrix multiplication. B) In a shift cipher the letters at the end of the alphabet will be replaced with letters from the beginning of the alphabet upon wrapping around corresponding to modular arithmetic. C) Possible block ciphers modes of operation include e.g. ECB, CBC, OFB, CFG and Counter Mode. D) Exhaustive key search is not always a brute-force attack.

14. Which of the below statements on classical cryptography fundamentals are correct A) The parameter t of the EEA (extended Euler algorithm) is r with index 1 modulo r with index 0. B) The Euler's theorem states that if a and m are a divisor equal to 1 (are coprime numbers) then a raised to the power of phi function on m (Euler's totient function) is congruent to 1 modulo n. C) 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. D) The maximum period generated by an LSFR (linear feedback shift register) of degree m is 2 to the power m minus 1.

15. Which statements on classical cryptography fundamentals are correct A) 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. B) 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. C) In breaking DES cryptosystem differential cryptanalysis is more efficient than linear cryptanalysis. D) 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 A) The output feedback (OFB) mode of operation of the block cipher can be used as a keystream generator. B) 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). C) In the RSA cipher the public key has one part, while the private key has two parts used in modular algebra computations. D) Cryptographically secure pseudo random numbers generators are nondeterministic.

17. Which of the following statements are correct A) AES cryptosystem operations are based on finite fields. B) Cryptography can be used to communicate securely over an insecure communication channel C) The key space for the affine cipher is equal to 286. D) The Euler's theorem can be used to simplify reduction of large powers modulo n.

18. Which statements on classical cryptography fundamentals are correct A) 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. B) 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. C) In simplified terms a field is a set of numbers in which one can add, subtract and multiple but not divide. D) 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 A) In modular arithmetic two numbers are equivalent if they have the same remainder when divided by a certain number. B) 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). C) Cryptography can be considered a part of cryptology and cryptanalysis fields. D) 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 A) 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's totient function) is congruent to 1 modulo n. B) The maximum period generated by an LSFR (linear feedback shift register) of degree m is 2 to the power m minus 1. C) The parameter t of the EEA (extended Euler algorithm) is r with index 1 modulo r with index 0. D) 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.