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?

    <p>Electronic Code Book (ECB) Mode</p> Signup and view all the answers

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

    <p>Cipher Block Chaining (CBC) Mode</p> Signup and view all the answers

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

    <p>Electronic Code Book (ECB) Mode</p> Signup and view all the answers

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

    <p>Counter (CTR) Mode</p> Signup and view all the answers

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

    <p>Cipher Block Chaining (CBC) Mode</p> Signup and view all the answers

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

    <p>$2^{128}$</p> 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?

    <p>10 years</p> Signup and view all the answers

    How are realistic adversaries modeled in cryptographic schemes?

    <p>Probabilistic algorithms running in time polynomial in n</p> Signup and view all the answers

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

    <p>Efficient and length-preserving</p> Signup and view all the answers

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

    <p>If F is pseudorandom, then the construction is CPA-secure</p> Signup and view all the answers

    What is a characteristic of a strong pseudorandom permutation?

    <p>Cannot be efficiently distinguished from a random permutation</p> Signup and view all the answers

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

    <p>It is a form of security where an adversary behaves the same, regardless of whether it observes an encryption of $m_0$ or $m_1$</p> Signup and view all the answers

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

    <p>It transforms a short, uniform bitstring into a longer, 'uniform-looking' output string</p> Signup and view all the answers

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

    <p>It is the analog of perfect secrecy for computationally bounded adversaries</p> Signup and view all the answers

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

    <p>A is given 1, outputs pairs of equal-length message lists</p> Signup and view all the answers

    What does CPA-Security for Multiple Encryptions involve?

    <p>Extending the CPA indistinguishability experiment to multiple encryptions using lists of plaintexts</p> Signup and view all the answers

    What is the property of pseudorandom functions?

    <p>They are 'random-looking' functions, and pseudorandomness is a property of a distribution over functions</p> 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

    <p>Weaker</p> 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.

    <p>t, \epsilon</p> Signup and view all the answers

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

    <p>large, small</p> Signup and view all the answers

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

    <p>pseudorandom</p> Signup and view all the answers

    Distinguisher has no access to the key k

    <p>keyed</p> Signup and view all the answers

    Strong pseudorandom ______ cannot be efficiently distinguished from a random ______

    <p>permutation</p> Signup and view all the answers

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

    <p>10 billion years</p> 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 ______

    <p>negligible</p> Signup and view all the answers

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

    <p>negligible</p> 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

    <p>semantic security</p> 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

    <p>EAV-security</p> 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

    <p>pseudorandom generator</p> Signup and view all the answers

    CPA-Security for Multiple Encryptions is stronger than ______

    <p>EAV-security</p> 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 ______

    <p>b′</p> 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 ______

    <p>negl</p> Signup and view all the answers

    ECB is deterministic and cannot be ______

    <p>CPA-secure</p> 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 )

    <p>chosen</p> 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 ______.

    <p>chosen</p> 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

    <p>chosen</p> Signup and view all the answers

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

    <p>overheads</p> 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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    Private-Key Encryption.pdf

    Description

    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.

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