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. (B)</p> Signup and view all the answers

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

<p>Trivium (A)</p> Signup and view all the answers

What is a primary characteristic of stream ciphers?

<p>Encrypts bits individually (D)</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 (B)</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 (D)</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 (C)</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 (D)</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. (A)</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. (C)</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. (B)</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. (B)</p> Signup and view all the answers

What is typically the main application for CSPRNGs in cryptography?

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

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

<p>XOR operation (A)</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 (C)</p> Signup and view all the answers

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

<p>Based on physical random processes (C)</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 (B)</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 (A)</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% (D)</p> Signup and view all the answers

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

<p>RC4 (B)</p> Signup and view all the answers

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

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

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

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

What does an LFSR consist of?

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

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

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

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

<p>$2m - 1$ (B)</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. (A)</p> Signup and view all the answers

What happens to the output sequence of an LFSR?

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

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

<p>Trivium (C)</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. (C)</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. (C)</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. (D)</p> Signup and view all the answers

What type of LFSRs does the Trivium stream cipher utilize?

<p>Three nonlinear LFSRs. (D)</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. (C)</p> Signup and view all the answers

What is the total register count for the Trivium cipher?

<p>288 (B)</p> Signup and view all the answers

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

<p>Multipliers (A)</p> Signup and view all the answers

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

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

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

<p>RC4 (B)</p> Signup and view all the answers

Flashcards

Stream Cipher

A type of encryption that encrypts data one bit or byte at a time, using a keystream that is generated from a secret key.

Random Number Generator (RNG)

A device or algorithm that generates a sequence of seemingly random numbers. These numbers are used as keystream for encryption.

One-Time Pad (OTP)

The most secure encryption method, using a truly random keystream of the same length as the message. Every character is uniquely encrypted.

Linear Feedback Shift Register (LFSR)

A type of shift register where the output is fed back to the input, creating a sequence of bits based on a specific feedback function.

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Trivium

A modern stream cipher known for its efficiency and security. It combines multiple shift registers and feedback loops to generate a complex keystream.

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Block Cipher

A type of cipher that encrypts data in fixed-size blocks, typically multiple bits at a time.

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Key Stream (in Stream Ciphers)

Key stream is a sequence of random bits used to encrypt data in a stream cipher. It is generated using a secret key.

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Synchronous Stream Cipher

In stream ciphers, the key stream is generated solely based on the secret key and possibly an initialization vector (IV).

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Asynchronous Stream Cipher

In stream ciphers, the key stream is generated based on the secret key, the initialization vector (IV), and the ciphertext itself.

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Unconditionally Secure

A property of a cryptosystem that means it cannot be broken even with unlimited computational resources.

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Cryptographically Secure Pseudorandom Number Generator (CSPRNG)

A type of PRNG used in cryptography that provides unpredictable output. Given any sequence of output bits, the next bit cannot be predicted in a reasonable amount of time.

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Linear Congruential Generator (LCG)

A simple type of PRNG that uses a linear equation to generate a sequence of numbers. However, it is not secure due to its predictable pattern.

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Stream Cipher Throughput

A stream cipher's performance is measured by its throughput, or how quickly it can encrypt data. This is often given in megabits per second (Mbit/s).

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Modulo 2 Addition

Modulo 2 addition is a mathematical operation where the sum of two bits is calculated modulo 2. This operation, in cryptography, is equivalent to the XOR operation.

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XOR Operation

The XOR (Exclusive OR) operation is a logical operation that results in a '1' if only one of the two input bits is '1', and a '0' if both bits are the same (either both '1' or both '0').

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True Random Number Generator (TRNG)

A True Random Number Generator (TRNG) generates random numbers based on physical random processes, such as coin flips, dice rolls, or thermal noise. TRNGs can produce truly random sequences that are difficult to predict.

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Pseudorandom Number Generator (PRNG)

A Pseudorandom Number Generator (PRNG) produces a sequence of numbers that appears random but is generated by an algorithm from a starting seed value. PRNGs are often used for cryptography when true randomness is not required.

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Keystream

A keystream is a sequence of random bits that is used to encrypt a plaintext message in a stream cipher. The keystream is generated by the cipher's underlying algorithm and typically depends on the secret key and initialization vector (IV).

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Initialization Vector (IV)

An Initialization Vector (IV) is a random value used in conjunction with a secret key to initialize a stream cipher. It ensures that the same key can be used to encrypt different messages without producing identical ciphertexts.

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LFSR Degree

An LFSR is a shift register that generates new bits using a combination of its existing bits. The number of bits it uses is called its 'degree'.

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LFSR Maximum Output Length

The LFSR generates a sequence that repeats after a fixed number of cycles. The maximum length of this sequence is 2m-1, where m is the degree of the LFSR.

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Importance of Keystream Randomness

The quality of the keystream is crucial for the security of a stream cipher. A truly random keystream results in maximum security, making it virtually impossible to break.

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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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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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