Private-Key Encryption Security Quiz

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Questions and Answers

What is the main difference between perfect secrecy and computational secrecy?

Perfect secrecy is measured in terms of CPU cycles, while computational secrecy is measured in terms of time

Perfect secrecy leaks information with a tiny probability to eavesdroppers with bounded computational power, while computational secrecy leaks absolutely no information about an encrypted message

Perfect secrecy leaks absolutely no information about an encrypted message, while computational secrecy leaks information with a tiny probability to eavesdroppers with bounded computational power (correct)

Perfect secrecy guarantees security against realistic adversaries, while computational secrecy does not

What does a (t, ϵ)-secure scheme guarantee?

The scheme can be broken with probability at most ϵ in time t

Any adversary running for time at most t can break the scheme with probability at most ϵ

No adversary running for time at most t can break the scheme with probability at most ϵ (correct)

The scheme can be broken with probability at least ϵ in time t

Why is it more convenient to measure running time in terms of CPU cycles for (t, ϵ)-secure schemes?

It aligns with the practical implementation of cryptographic schemes (correct)

It allows for a more accurate measurement of time compared to traditional time units

It provides a standardized measurement across different computing systems

It simplifies the calculation of success probability for adversaries

Which block cipher mode is deterministic and cannot be CPA-secure?

Electronic Code Book (ECB) Mode (B) Signup and view all the answers

In which block cipher mode is a uniform initialization vector (IV) of length n chosen?

Cipher Block Chaining (CBC) Mode (B) Signup and view all the answers

Which block cipher mode has ciphertext length double the plaintext length in a CPA-secure construction?

Electronic Code Book (ECB) Mode (D) Signup and view all the answers

Which block cipher mode involves encrypting a message of length l using a length-preserving block cipher?

Counter (CTR) Mode (B) Signup and view all the answers

Which block cipher mode uses an initialization vector (IV) for encryption?

Cipher Block Chaining (CBC) Mode (A) Signup and view all the answers

What is the key space size for a private-key encryption scheme using a 128-bit key?

$2^{128}$ (B) Signup and view all the answers

For c = 1 and n = 64, how many years does a 4 GHz processor with 16 cores require at most to break the scheme?

10 years (B) Signup and view all the answers

How are realistic adversaries modeled in cryptographic schemes?

Probabilistic algorithms running in time polynomial in n (A) Signup and view all the answers

Which of the following is a characteristic of a pseudorandom function?

Efficient and length-preserving (C) Signup and view all the answers

In the context of distinguishing pseudorandom functions, what does the theorem state about the construction's security?

If F is pseudorandom, then the construction is CPA-secure (B) Signup and view all the answers

What is a characteristic of a strong pseudorandom permutation?

Cannot be efficiently distinguished from a random permutation (A) Signup and view all the answers

What is the definition of EAV-security in private-key encryption?

It is a form of security where an adversary behaves the same, regardless of whether it observes an encryption of $m_0$ or $m_1$ (B) Signup and view all the answers

What is the key characteristic of a pseudorandom generator used in EAV-secure encryption schemes?

It transforms a short, uniform bitstring into a longer, 'uniform-looking' output string (D) Signup and view all the answers

What is the role of semantic security in private-key encryption?

It is the analog of perfect secrecy for computationally bounded adversaries (B) Signup and view all the answers

In the experiment PrivKA,Π (n), what is A given and what does it output?

A is given 1, outputs pairs of equal-length message lists (D) Signup and view all the answers

What does CPA-Security for Multiple Encryptions involve?

Extending the CPA indistinguishability experiment to multiple encryptions using lists of plaintexts (B) Signup and view all the answers

What is the property of pseudorandom functions?

They are 'random-looking' functions, and pseudorandomness is a property of a distribution over functions (C) Signup and view all the answers

Computational secrecy requirement: Information about the encrypted message is leaked with a tiny probability to eavesdroppers with bounded computational power

Weaker Signup and view all the answers

Upper bounds the success probability of an adversary running for some specified time. A concrete definition of security takes the following form. A scheme is (t, ϵ)-secure is any adversary running for time at most t succeeds in breaking the scheme with probability at most ϵ.

t, \epsilon Signup and view all the answers

How large can t be. How small should ϵ be.

large, small Signup and view all the answers

CPA-secure private-key encryption scheme uses ______ function F

pseudorandom Signup and view all the answers

Distinguisher has no access to the key k

keyed Signup and view all the answers

Strong pseudorandom cannot be efficiently distinguished from a random

permutation Signup and view all the answers

An event that occurs with probability $2^{-60}$ each second is expected to occur once every ______

10 billion years Signup and view all the answers

A scheme is secure if for every probabilistic polynomial-time adversary A, the probability of A succeeding in the attack is ______

negligible Signup and view all the answers

Both PPT adversaries and ______ probabilities of success are needed to allow practical encryption schemes

negligible Signup and view all the answers

Semantic security is the analog of perfect secrecy for computationally bounded adversaries and is the first definition of computationally secure encryption to be proposed

semantic security Signup and view all the answers

______ is a form of security in private-key encryption where an adversary behaves the same, regardless of whether it observes an encryption of m0 or m1

EAV-security Signup and view all the answers

A ______ is a polynomial-time deterministic algorithm for transforming a short, uniform bitstring called the seed into a longer, 'uniform-looking' output string

pseudorandom generator Signup and view all the answers

CPA-Security for Multiple Encryptions is stronger than ______

EAV-security Signup and view all the answers

The LR-Oracle Experiment involves A being given 1 and oracle access to LRk,b (⋅, ⋅) and outputs a bit ______

b′ Signup and view all the answers

The private-key encryption scheme has indistinguishable encryptions under a chosen-plaintext attack if for all PPT adversaries A there is a negligible function ______

negl Signup and view all the answers

ECB is deterministic and cannot be ______

CPA-secure Signup and view all the answers

Let F be a length-preserving block cipher with block length n. A uniform initialization vector (IV) of length n is ______. c0 = IV. For i = 1, … , l, ci := Fk (ci−1 ⊕ mi )

chosen Signup and view all the answers

Let m = m1 , m2 , … , ml where mi ∈ {0, 1}. n Let F be a length-preserving block cipher with block length n. A uniform initialization vector (IV) of length n is ______.

chosen Signup and view all the answers

Let m = m1 , m2 , … , ml where mi ∈ {0, 1} n Let F be a length-preserving block cipher with length n To encrypt a message of length l

chosen Signup and view all the answers

Fortunately, there exist secure constructions based on block ciphers that have lower ______

overheads Signup and view all the answers

Study Notes

Security Notions in Private-Key Encryption Schemes

One-time pad scheme with length l(n) has a view eav ~ (n) of A identical to the view of A in PrivKA,Π when w is chosen uniformly from {0, 1}l(n).
When w is generated by choosing k uniformly from {0, 1}n := G(k), the view of A is identical to the view of A in PrivKA,Π (n) and setting w.
In the experiment PrivKA,Π (n), A is given 1, outputs pairs of equal-length message lists, and a key k is generated using Gen.
The one-time pad does not have indistinguishable multiple encryptions in the presence of an eavesdropper.
Chosen-plaintext attacks allow an adversary to influence honest parties sharing the key to encrypt messages of its choice.
Chosen-plaintext attacks are modeled by giving the adversary A access to an encryption oracle.
The private-key encryption scheme has indistinguishable encryptions under a chosen-plaintext attack if for all PPT adversaries A there is a negligible function negl.
CPA-Security for Multiple Encryptions extends the CPA indistinguishability experiment to multiple encryptions using lists of plaintexts.
The LR-Oracle Experiment involves A being given 1 and oracle access to LRk,b (⋅, ⋅) and outputs a bit b′.
A private-key encryption scheme is CPA-secure for multiple encryptions if for all PPT adversaries A there is a negligible function negl.
CPA-Security for Multiple Encryptions is stronger than EAV-security.
Pseudorandom functions are "random-looking" functions, and pseudorandomness is a property of a distribution over functions.

Security Notions in Private-Key Encryption Schemes

One-time pad scheme with length l(n) has a view eav ~ (n) of A identical to the view of A in PrivKA,Π when w is chosen uniformly from {0, 1}l(n).
When w is generated by choosing k uniformly from {0, 1}n := G(k), the view of A is identical to the view of A in PrivKA,Π (n) and setting w.
In the experiment PrivKA,Π (n), A is given 1, outputs pairs of equal-length message lists, and a key k is generated using Gen.
The one-time pad does not have indistinguishable multiple encryptions in the presence of an eavesdropper.
Chosen-plaintext attacks allow an adversary to influence honest parties sharing the key to encrypt messages of its choice.
Chosen-plaintext attacks are modeled by giving the adversary A access to an encryption oracle.
The private-key encryption scheme has indistinguishable encryptions under a chosen-plaintext attack if for all PPT adversaries A there is a negligible function negl.
CPA-Security for Multiple Encryptions extends the CPA indistinguishability experiment to multiple encryptions using lists of plaintexts.
The LR-Oracle Experiment involves A being given 1 and oracle access to LRk,b (⋅, ⋅) and outputs a bit b′.
A private-key encryption scheme is CPA-secure for multiple encryptions if for all PPT adversaries A there is a negligible function negl.
CPA-Security for Multiple Encryptions is stronger than EAV-security.
Pseudorandom functions are "random-looking" functions, and pseudorandomness is a property of a distribution over functions.

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Test your knowledge of security notions in private-key encryption schemes with this quiz. From one-time pad schemes to chosen-plaintext attacks and CPA-security for multiple encryptions, this quiz covers a range of concepts essential for understanding encryption security.

Private-Key Encryption Security Quiz

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Questions and Answers

What is the main difference between perfect secrecy and computational secrecy?

What does a (t, ϵ)-secure scheme guarantee?

Why is it more convenient to measure running time in terms of CPU cycles for (t, ϵ)-secure schemes?

Which block cipher mode is deterministic and cannot be CPA-secure?

In which block cipher mode is a uniform initialization vector (IV) of length n chosen?

Which block cipher mode has ciphertext length double the plaintext length in a CPA-secure construction?

Which block cipher mode involves encrypting a message of length l using a length-preserving block cipher?

Which block cipher mode uses an initialization vector (IV) for encryption?

What is the key space size for a private-key encryption scheme using a 128-bit key?

For c = 1 and n = 64, how many years does a 4 GHz processor with 16 cores require at most to break the scheme?

How are realistic adversaries modeled in cryptographic schemes?

Which of the following is a characteristic of a pseudorandom function?

In the context of distinguishing pseudorandom functions, what does the theorem state about the construction's security?

What is a characteristic of a strong pseudorandom permutation?

What is the definition of EAV-security in private-key encryption?

What is the key characteristic of a pseudorandom generator used in EAV-secure encryption schemes?

What is the role of semantic security in private-key encryption?

In the experiment PrivKA,Π (n), what is A given and what does it output?

What does CPA-Security for Multiple Encryptions involve?

What is the property of pseudorandom functions?

Computational secrecy requirement: Information about the encrypted message is leaked with a tiny probability to eavesdroppers with bounded computational power

Upper bounds the success probability of an adversary running for some specified time. A concrete definition of security takes the following form. A scheme is (t, ϵ)-secure is any adversary running for time at most t succeeds in breaking the scheme with probability at most ϵ.

How large can t be. How small should ϵ be.

CPA-secure private-key encryption scheme uses ______ function F

Distinguisher has no access to the key k

Strong pseudorandom ______ cannot be efficiently distinguished from a random ______

An event that occurs with probability $2^{-60}$ each second is expected to occur once every ______

A scheme is secure if for every probabilistic polynomial-time adversary A, the probability of A succeeding in the attack is ______

Both PPT adversaries and ______ probabilities of success are needed to allow practical encryption schemes

Semantic security is the analog of perfect secrecy for computationally bounded adversaries and is the first definition of computationally secure encryption to be proposed

______ is a form of security in private-key encryption where an adversary behaves the same, regardless of whether it observes an encryption of m0 or m1

A ______ is a polynomial-time deterministic algorithm for transforming a short, uniform bitstring called the seed into a longer, 'uniform-looking' output string

CPA-Security for Multiple Encryptions is stronger than ______

The LR-Oracle Experiment involves A being given 1 and oracle access to LRk,b (⋅, ⋅) and outputs a bit ______

The private-key encryption scheme has indistinguishable encryptions under a chosen-plaintext attack if for all PPT adversaries A there is a negligible function ______

ECB is deterministic and cannot be ______

Let F be a length-preserving block cipher with block length n. A uniform initialization vector (IV) of length n is ______. c0 = IV. For i = 1, … , l, ci := Fk (ci−1 ⊕ mi )

Let m = m1 , m2 , … , ml where mi ∈ {0, 1}. n Let F be a length-preserving block cipher with block length n. A uniform initialization vector (IV) of length n is ______.

Let m = m1 , m2 , … , ml where mi ∈ {0, 1} n Let F be a length-preserving block cipher with length n To encrypt a message of length l

Fortunately, there exist secure constructions based on block ciphers that have lower ______

Study Notes

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