Applied cryptography summary questions

The questions are randomised per student, and you will receive only correct/incorrect feedback for these questions. In each question in Problem 1, there is exactly one correct option.

In Problem 2, each student will receive at least one correct option, usually several. The system gives feedback incorrect for partially correct answers, but Problem 2 is accepted when the score is 12 or more and the points are shown with a yellow background. Points begin to accumulate only when more than half of the choices in the task are correct. The score does not directly indicate whether an individual choice is correct or not.

Question 1

Cryptographic primitives mainly refer to

prime numbers from which keys are formed.

basic mechanisms that appear in all cryptographic algorithms and protocols.

procedures that a practitioner of cryptology can treat as indivisible building blocks.

base numbers that generate other numbers in modular arithmetic.

Question 2

What does a cryptanalyst typically do (i.e. when performing cryptanalysis)?

Compares the strength of encryption and signature algorithms.

Breaks ciphertexts or keys.

Performs statistical analysis of public key usage.

Develops algorithms to be more efficient or stronger.

Question 3

What kind of “spice” is salt in cryptography?

It is needed in almost everything.

It is somewhat poorly named, as it is more like icing—visible rather than permeating as salt does.

It is random but not secret.

It is simply a shorter term for initialisation vectors used in many algorithms.

It is only used in very specialised applications and even then rarely important.

Question 4

A cryptographic hash is a primitive

that does not use a key but can be used to derive new keys from existing ones.

from which most encryption and signature primitives can be constructed.

that can replace random number generators.

that is in practice implemented using keyed primitives.

Question 5

A stream cipher can be constructed by suitably applying a block algorithm. Conversely, inside AES block encryption one can see a feature that resembles a stream cipher. Choose the most appropriate interpretation of this by eliminating the incorrect ones:

The key only affects the computation at the end via XOR, as in stream ciphers.

Confusion via byte substitution is achieved using a shift register.

In each round, plaintext is first transformed independently of the key and then XORed with a round key.

AES does not process plaintext directly; instead, it processes the key iteratively and XORs the resulting keystream with plaintext.

Question 6

Strong unforgeability under chosen-message attacks sounds like a demanding property for a MAC scheme. Yet it can also be achieved using a hash function with known collisions. How is this possible?

It is not really possible, since MACs require a key which hashes do not have; this applies only to password storage if salt is treated as a key, and even then it fails.

A collision involves two known inputs yielding the same hash, but this does not help a forger of a MAC built from the hash, who needs the same MAC value for two different messages under an unknown (though identical) key.

Collisions are known and few, so they can be avoided by appending constant bits to the MAC key.

As long as collisions have not been found in natural language texts, attackers cannot forge MAC values for them.

Question 7

By applying a private key, its holder can create a linkage between the corresponding public key and their identity,

which is necessary to prove who holds the private key.

which functions as a trust anchor in a certificate chain.

which allows others to know that the creator of the linkage knows the private key.

which allows others to trust claims about who holds the public key.

Question 8

In a public key infrastructure (as usual), certificate authorities guarantee that a client’s public key corresponds to that client’s identity. This binding is achieved by

recording both identity and public key in a public but secure database.

a digital signature computed by the authority over the identity and public key using its private key.

storing the identity publicly and the private key securely, which the authority uses when needed.

storing both identity and public key on the client’s smart card.

Select the correct statements. You may choose one or more options.

Question 1

A cipher that XORs plaintext bits with a keystream cannot be called a one-time pad if the keystream is derived from a shorter key.

A one-time pad always requires a device; it cannot be implemented mentally or with pen and paper.

The tasks of a private key include decryption and signature generation.

A public key may correspond to multiple private keys.

Trapdoors, sponge functions, and oracles are concepts specific to public-key cryptography.

When a hash function is used in HMAC, it is computed twice.

A central application of cryptographic hashes is keystream generation in stream ciphers.

The two main mathematical foundations of asymmetric cryptography are quantum curves and number theory.

Key exchange is implemented by combining several types of cryptographic protocols in sequence.

Oblivious transfer and zero-knowledge proofs are cryptographic protocols that do not require quantum computing.

Breaking ciphertext does not necessarily require cryptanalysis if the attacker has compromised key management.

Posting submission...