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Questions and Answers
What is the primary purpose of an encryption algorithm?
What is the primary purpose of an encryption algorithm?
Which of the following statements correctly describes a symmetric cipher?
Which of the following statements correctly describes a symmetric cipher?
What is cryptanalysis primarily focused on?
What is cryptanalysis primarily focused on?
Which of the following is considered an aspect of cryptology?
Which of the following is considered an aspect of cryptology?
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Which equation correctly represents the encryption process in symmetric encryption?
Which equation correctly represents the encryption process in symmetric encryption?
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What is the main goal of both brute-force and cryptanalytic attacks on encryption systems?
What is the main goal of both brute-force and cryptanalytic attacks on encryption systems?
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In a brute-force attack, how many keys does an attacker expect to try on average to find the correct key?
In a brute-force attack, how many keys does an attacker expect to try on average to find the correct key?
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Which of the following is NOT a classification of cryptanalytic attacks based on the information the attacker has?
Which of the following is NOT a classification of cryptanalytic attacks based on the information the attacker has?
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What is a key characteristic of cryptanalytic attacks compared to brute-force attacks?
What is a key characteristic of cryptanalytic attacks compared to brute-force attacks?
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What will be the consequence if either a brute-force or cryptanalytic attack successfully deduces the encryption key?
What will be the consequence if either a brute-force or cryptanalytic attack successfully deduces the encryption key?
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Which attack is described as being the easiest to defend against due to the limited information the opponent has?
Which attack is described as being the easiest to defend against due to the limited information the opponent has?
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What is required for an analyst to successfully deduce a key using a known plaintext attack?
What is required for an analyst to successfully deduce a key using a known plaintext attack?
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In a ciphertext-only attack, what type of information does the attacker primarily rely on?
In a ciphertext-only attack, what type of information does the attacker primarily rely on?
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What limits the practicality of a brute-force attack?
What limits the practicality of a brute-force attack?
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Which of the following statements accurately describes a completely insecure encryption scheme?
Which of the following statements accurately describes a completely insecure encryption scheme?
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Study Notes
Symmetric Cipher Model
- Symmetric ciphers use the same key for encryption and decryption.
- Plaintext is the original message.
- Ciphertext is the encrypted message.
- Encryption is the process of converting plaintext to ciphertext.
- Encryption algorithms take plaintext and a secret key as input.
- Decryption is the process of converting ciphertext back to plaintext.
- Decryption algorithms take ciphertext and the same secret key as input.
Basic Terminology
- Plaintext: the original message
- Ciphertext: the encrypted message
- Encryption/enciphering: converting plaintext to ciphertext
- Encryption algorithm: performs encryption; takes plaintext and a secret key as input
- Decryption/deciphering: converting ciphertext to plaintext
- Decryption algorithm: performs decryption; takes ciphertext and a secret key as input
- Secret key: the same key used for encryption and decryption
Ciphers
- Symmetric cipher: uses the same key for encryption and decryption
- Block cipher: encrypts data in blocks (typically 64 or 128 bits)
- Stream cipher: encrypts data one bit or byte at a time
- Asymmetric cipher: uses different keys for encryption and decryption
Symmetric Encryption
- Also called conventional, secret-key, or single-key encryption
- The same key is used for encryption and decryption
- All classical encryption algorithms are symmetric
- The only type of cipher before asymmetric-key ciphers (1970s)
- Most widely used
Mathematically
- Y = E(X) or Y = E(K, X) (encrypt plaintext X with key K to get Y)
- X = D(Y) or X = D(K, Y) (decrypt ciphertext Y with key K to get X)
- X = plaintext
- Y = ciphertext
- K = secret key
- E = encryption algorithm
- D = decryption algorithm
- E and D are known to the public
Cryptanalysis
- Objective: recover plaintext from ciphertext or the secret key
- Kerkhoff's principle: adversary knows all details of a cryptosystem except the secret key
- Brute-force attack: tries every possible key
- Cryptanalytic attack: exploits characteristics of the algorithm
Input, Conversion, Breaking into Blocks
- Input: "Hello World!"
- Convert to binary: binary representation of each character
- Break into 64-bit blocks: split the binary data into 64-bit chunks. Padding may be required to fill out the last block.
Cryptanalysis and Brute-Force Attack
- Objective: recovering the key used to encrypt ciphertext
- Two approaches
- Cryptanalysis: analyzing the algorithm and possible plaintext characteristics
- Brute-Force: trying every possible key
- If either attack succeeds, all future and past messages are compromised.
Cryptanalytic Attacks
- Categorized by the amount of information the attacker has:
- Ciphertext-only attack: only ciphertext available
- Known-plaintext attack: plaintext and its corresponding ciphertext available
- Chosen-plaintext attack: attacker chooses plaintext and obtains corresponding ciphertext
- Chosen-ciphertext attack: attacker chooses ciphertext and obtains corresponding plaintext
Brute-Force Attack Time
- Time needed for brute-force depends on the size of the key space.
- Larger key spaces require exponentially more time to try all possible keys. (e.g., 56-bit key is much quicker to break than a 128-bit key)
Ciphertext-only Attack
- Attacker only has ciphertext
- Attempts to decrypt ciphertext to plaintext, or determine the key
- Can sometimes work; an encryption system is insecure if it is vulnerable to this. (Often impractical)
Known-Plaintext Attack
- Attacker has some known plaintext and its corresponding ciphertext.
- Used to deduce the key or recover plaintext
- Helps deduce some characteristics of the encryption scheme
Chosen-Ciphertext Attack
- Attacker can choose ciphertexts and obtain the corresponding plaintexts.
- Used to discover vulnerabilities in encryption schemes
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Description
Test your understanding of the symmetric cipher model, including key concepts like plaintext, ciphertext, encryption, and decryption. This quiz covers the fundamental terminology and processes used in symmetric encryption. Challenge yourself to see how well you grasp these essential cybersecurity concepts!
