Cryptography: Stream Ciphers Overview
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Questions and Answers

Who invented stream ciphers in 1917?

  • Christof Paar
  • Gilbert Vernam (correct)
  • Thomas Eisenbarth
  • Jan Pelzl

    • Which of the following is NOT a topic covered in Chapter 2?

  • One-Time Pad (OTP)
  • Linear feedback shift registers (LFSRs)
  • Block ciphers (correct)
  • Random number generators (RNGs)

    • What is the role of Linear Feedback Shift Registers (LFSRs) in stream ciphers?

  • To provide asymmetric encryption
  • To construct hash functions
  • To facilitate block-wise encryption
  • To generate pseudo-random sequences (correct)

    • Which of the following statements about stream ciphers is true?

    <p>Stream ciphers can encrypt data one bit at a time.</p> Signup and view all the answers

    What additional topic is included in the content regarding modern stream ciphers?

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

    What is a primary characteristic of stream ciphers?

    <p>Encrypts bits individually</p> Signup and view all the answers

    What does the operation 'yi = xi + si mod 2' represent in stream ciphers?

    <p>The encryption function for stream ciphers</p> Signup and view all the answers

    In synchronous stream ciphers, what does the key stream depend upon?

    <p>The key and possibly an initialization vector</p> Signup and view all the answers

    Why is it important for the key stream in a stream cipher to be random?

    <p>To enhance security by preventing predictable patterns</p> Signup and view all the answers

    What is the main difference between synchronous and asynchronous stream ciphers?

    <p>The key stream in asynchronous ciphers depends on both the key and the ciphertext</p> Signup and view all the answers

    What is the main characteristic of a Cryptographically Secure Pseudorandom Number Generator (CSPRNG)?

    <p>Its output must be unpredictable.</p> Signup and view all the answers

    Which property makes the One-Time Pad (OTP) an unconditionally secure cryptosystem?

    <p>The key must be used only once.</p> Signup and view all the answers

    In a simple PRNG, what does the formula Si+1 = A * Si + B mod m represent?

    <p>The generation of the next number in the sequence.</p> Signup and view all the answers

    Which of the following is a necessary condition for a linear congruential generator to be secure?

    <p>The parameters must be large and unknown.</p> Signup and view all the answers

    What is typically the main application for CSPRNGs in cryptography?

    <p>For ensuring unpredictability in cryptographic keys.</p> Signup and view all the answers

    What happens when one uses the same key multiple times in a One-Time Pad?

    <p>It leads to potential vulnerabilities.</p> Signup and view all the answers

    Why are most simple PRNGs considered to have bad cryptographic properties?

    <p>They exhibit linearity in their output.</p> Signup and view all the answers

    Which of the following components are essential for encrypting with an OTP?

    <p>Individual bits of plaintext and a unique key for each bit.</p> Signup and view all the answers

    What operation is equivalent to modulo 2 addition in the context of stream ciphers?

    <p>XOR operation</p> Signup and view all the answers

    What is the key length and throughput of the AES symmetric cipher?

    <p>128 bits, 51.19 Mbit/s</p> Signup and view all the answers

    What is a characteristic of True Random Number Generators (TRNGs)?

    <p>Based on physical random processes</p> Signup and view all the answers

    Which of the following describes a Pseudorandom Number Generator (PRNG)?

    <p>Generates sequences from an initial seed value</p> Signup and view all the answers

    What is the primary purpose of a good random number generator in cryptography?

    <p>To create nonces and keys securely</p> Signup and view all the answers

    What statistical property should the output stream of a true RNG ideally have?

    <p>Pr(si = 0) = Pr(si = 1) = 50%</p> Signup and view all the answers

    Which stream cipher has the highest throughput according to the performance comparison?

    <p>RC4</p> Signup and view all the answers

    What occurs during the XOR operation when both input bits are 1?

    <p>Output is 0</p> Signup and view all the answers

    What is a significant drawback of most Pseudorandom Number Generators (PRNGs)?

    <p>They have bad cryptographic properties.</p> Signup and view all the answers

    What is a key disadvantage of the One-Time Pad (OTP) cryptosystem?

    <p>The key must be as long as the message.</p> Signup and view all the answers

    What does an LFSR consist of?

    <p>A shift register with a feedback path.</p> Signup and view all the answers

    Which equation describes the output of a linear feedback shift register?

    <p>$si + 3 = (si + 1) + si mod 2$</p> Signup and view all the answers

    What is the maximum output length of an LFSR with degree m?

    <p>$2m - 1$</p> Signup and view all the answers

    What is required for the One-Time Pad to be unconditionally secure?

    <p>Keys must be truly random and independent.</p> Signup and view all the answers

    What happens to the output sequence of an LFSR?

    <p>It repeats periodically.</p> Signup and view all the answers

    Which of the following is an example of a modern stream cipher mentioned?

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

    What is a necessary condition for the equations in the One-Time Pad to hold true?

    <p>Each xi must be either 0 or 1 with equal probability.</p> Signup and view all the answers

    What is a key characteristic of LFSRs when m output bits are known?

    <p>The feedback coefficients can be determined.</p> Signup and view all the answers

    Which statement is true about stream ciphers compared to block ciphers?

    <p>Stream ciphers are less popular in most domains.</p> Signup and view all the answers

    What type of LFSRs does the Trivium stream cipher utilize?

    <p>Three nonlinear LFSRs.</p> Signup and view all the answers

    What is one benefit of stream ciphers in constrained environments?

    <p>They often require fewer resources for implementation.</p> Signup and view all the answers

    What is the total register count for the Trivium cipher?

    <p>288</p> Signup and view all the answers

    Which component is NOT used in the Trivium cipher's output generation?

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

    Why are stream ciphers sometimes chosen over block ciphers for certain applications?

    <p>Less resource-intensive implementation.</p> Signup and view all the answers

    Which stream cipher is mentioned as an exception to the popularity of block ciphers?

    <p>RC4</p> Signup and view all the answers

    Study Notes

    Course Information

    • Course Title: Cryptography
    • Course Code: CCY6322-3
    • Instructor: Dr. Mohammad Zunnun Khan
    • Email: [email protected]
    • Year: 2024

    Stream Ciphers

    • Stream ciphers encrypt data bit by bit, often used in small, fast devices like GSM phones
    • Invented in 1917 by Gilbert Vernam
    • Contrast with block ciphers, which encrypt blocks of data
    • Encryption and decryption using XOR (modulo 2 addition)
    • Encryption formula: yi = esi(xi) = xi + si mod 2
    • Decryption formula: xi = esi(yi) = yi + si mod 2
      • Plaintext (xi), ciphertext (yi), and key stream (si) consist of bits
    • Synchronous stream ciphers depend only on the key (and potentially an initialization vector)
    • Asynchronous stream ciphers also depend on the ciphertext

    Stream Ciphers - Random Number Generators (RNGs)

    • RNGs generate the key stream for encryption, critical for stream ciphers' security
    • Cryptographically secure RNGs are essential in cryptography
    • True RNGs are based on physical phenomena, like radioactive decay
    • Pseudorandom number generators (PRNGs) generate a pseudo-random sequence based on a seed value
    • PRNGs have statistical properties, but are predictable

    Stream Ciphers - Linear Feedback Shift Registers (LFSRs)

    • LFSRs are a type of PRNG used in stream ciphers
    • Consists of flip-flops that generate a pseudo-random sequence using a feedback equation
    • Maximum output length is (2^m)-1, where m is the degree of the feedback equation

    Stream Ciphers - One-Time Pad (OTP)

    • Unconditionally secure; no computational method can break it
    • The key must be as long as the message to ensure perfect security, which is impractical
    • OTP encryption: eki(xi) = xi ⊕ ki
    • OTP decryption: dki(yi) = yi ⊕ ki

    Stream Ciphers - Trivium

    • A modern stream cipher
    • Combines techniques from various stream cipher forms
    • Often utilizes LFSRs in its implementation pattern

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    Understanding Cryptography - Stream Ciphers PDF

    Description

    This quiz covers the fundamental concepts of stream ciphers, exploring their historical background, encryption, and decryption mechanisms. It details the role of random number generators in enhancing the security of stream ciphers. Test your understanding of these critical components in cryptography.

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