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Questions and Answers
Which type of encryption was the only type in use prior to the development of public-key encryption in the 1970s?
Which type of encryption was the only type in use prior to the development of public-key encryption in the 1970s?
- Symmetric encryption (correct)
- Single-key encryption
- Asymmetric encryption
- Conventional encryption
What is the difference between cryptanalysis and brute-force attack?
What is the difference between cryptanalysis and brute-force attack?
- Cryptanalysis is the process of trying all possible keys to decrypt a message
- Cryptanalysis is the process of deciphering encrypted messages without knowing the key (correct)
- Cryptanalysis is a type of brute-force attack
- Cryptanalysis is a more sophisticated attack than brute-force attack
What does the congruence relation signify?
What does the congruence relation signify?
- The numbers a and b belong to the same residue class (mod m)
- The numbers a and b are congruent (mod m)
- The numbers a and b have the same remainder when divided by m
- The numbers a and b differ by a multiple of m (correct)
Which of the following best describes the process of encryption?
Which of the following best describes the process of encryption?
What is the role of the secret key in symmetric encryption?
What is the role of the secret key in symmetric encryption?
What is the ciphertext in symmetric encryption?
What is the ciphertext in symmetric encryption?
What are the two requirements for secure use of conventional encryption?
What are the two requirements for secure use of conventional encryption?
Which type of attack is the easiest to defend against?
Which type of attack is the easiest to defend against?
Which type of attack involves the opponent knowing certain plaintext patterns?
Which type of attack involves the opponent knowing certain plaintext patterns?
Which type of attack allows the opponent to choose the plaintext messages to encrypt?
Which type of attack allows the opponent to choose the plaintext messages to encrypt?
Which type of attack is less commonly employed as a cryptanalytic technique?
Which type of attack is less commonly employed as a cryptanalytic technique?
Which type of encryption system uses the same key for both the sender and receiver?
Which type of encryption system uses the same key for both the sender and receiver?
What are the two general principles on which encryption algorithms are based?
What are the two general principles on which encryption algorithms are based?
What is the objective of attacking an encryption system?
What is the objective of attacking an encryption system?
What is the most difficult problem for a cryptanalyst when attacking an encryption system?
What is the most difficult problem for a cryptanalyst when attacking an encryption system?
Which encryption technique involves replacing each letter of the alphabet with the letter standing three places further down the alphabet?
Which encryption technique involves replacing each letter of the alphabet with the letter standing three places further down the alphabet?
What is the decryption algorithm for the Caesar cipher, given a key value 'k' and ciphertext 'C'?
What is the decryption algorithm for the Caesar cipher, given a key value 'k' and ciphertext 'C'?
What is the range of values for the key 'k' in the general Caesar algorithm?
What is the range of values for the key 'k' in the general Caesar algorithm?
What is the average number of possible keys that need to be tried in a brute-force attack to decrypt a Caesar cipher?
What is the average number of possible keys that need to be tried in a brute-force attack to decrypt a Caesar cipher?
Which encryption algorithm employs a large number of keys, making brute-force cryptanalysis impractical?
Which encryption algorithm employs a large number of keys, making brute-force cryptanalysis impractical?
Which type of cipher uses an arbitrary substitution and has a key space greater than 4 * 10^26 possible keys?
Which type of cipher uses an arbitrary substitution and has a key space greater than 4 * 10^26 possible keys?
What technique can a cryptanalyst use to exploit the regularities of the language in a monoalphabetic substitution cipher?
What technique can a cryptanalyst use to exploit the regularities of the language in a monoalphabetic substitution cipher?
What is the first step in a cryptanalysis that uses frequency analysis in a monoalphabetic substitution cipher?
What is the first step in a cryptanalysis that uses frequency analysis in a monoalphabetic substitution cipher?
Which two letters are likely to correspond to the plain letters e and t?
Which two letters are likely to correspond to the plain letters e and t?
Which set of letters are likely to correspond to the plain letters a, h, i, n, o, r, s?
Which set of letters are likely to correspond to the plain letters a, h, i, n, o, r, s?
What is a systematic approach mentioned in the text to deduce plaintext equivalents?
What is a systematic approach mentioned in the text to deduce plaintext equivalents?
Which type of attack involves the opponent knowing certain plaintext patterns?
Which type of attack involves the opponent knowing certain plaintext patterns?
What is the objective of attacking an encryption system?
What is the objective of attacking an encryption system?
Which type of encryption system uses the same key for both the sender and receiver?
Which type of encryption system uses the same key for both the sender and receiver?
What is the first step in a cryptanalysis that uses frequency analysis in a monoalphabetic substitution cipher?
What is the first step in a cryptanalysis that uses frequency analysis in a monoalphabetic substitution cipher?
Which of the following is an essential element of a symmetric encryption scheme?
Which of the following is an essential element of a symmetric encryption scheme?
What is the process of converting from plaintext to ciphertext called?
What is the process of converting from plaintext to ciphertext called?
What is the requirement for a secure use of conventional encryption?
What is the requirement for a secure use of conventional encryption?
What is the objective of attacking an encryption system?
What is the objective of attacking an encryption system?
What is the purpose of a secure channel in a symmetric cryptosystem?
What is the purpose of a secure channel in a symmetric cryptosystem?
What is the notation used for the encryption algorithm in a symmetric cryptosystem?
What is the notation used for the encryption algorithm in a symmetric cryptosystem?
Which type of encryption system processes the input one block of elements at a time?
Which type of encryption system processes the input one block of elements at a time?
What is the objective of attacking an encryption system?
What is the objective of attacking an encryption system?
Which type of encryption was the most widely used prior to the development of public-key encryption in the 1970s?
Which type of encryption was the most widely used prior to the development of public-key encryption in the 1970s?
What is the main difference between cryptanalysis and brute-force attack?
What is the main difference between cryptanalysis and brute-force attack?
Which type of cipher uses an arbitrary substitution and has a key space greater than $4 \times 10^{26}$ possible keys?
Which type of cipher uses an arbitrary substitution and has a key space greater than $4 \times 10^{26}$ possible keys?
What is the objective of attacking an encryption system?
What is the objective of attacking an encryption system?
What is the decryption algorithm for the Caesar cipher, given a key value 'k' and ciphertext 'C'?
What is the decryption algorithm for the Caesar cipher, given a key value 'k' and ciphertext 'C'?
What is the average number of possible keys that need to be tried in a brute-force attack to decrypt a Caesar cipher?
What is the average number of possible keys that need to be tried in a brute-force attack to decrypt a Caesar cipher?
What technique can a cryptanalyst use to exploit the regularities of the language in a monoalphabetic substitution cipher?
What technique can a cryptanalyst use to exploit the regularities of the language in a monoalphabetic substitution cipher?
What type of encryption system uses the same key for both the sender and receiver?
What type of encryption system uses the same key for both the sender and receiver?
Which letters are likely to correspond to the plain letters e and t in the encryption?
Which letters are likely to correspond to the plain letters e and t in the encryption?
What is the range of values for the key 'k' in the general Caesar algorithm?
What is the range of values for the key 'k' in the general Caesar algorithm?
What are the letters with the lowest frequencies likely to correspond to in the encryption?
What are the letters with the lowest frequencies likely to correspond to in the encryption?
What is a systematic approach mentioned in the text to deduce plaintext equivalents?
What is a systematic approach mentioned in the text to deduce plaintext equivalents?
What is the average number of possible keys that need to be tried in a brute-force attack to decrypt a Caesar cipher?
What is the average number of possible keys that need to be tried in a brute-force attack to decrypt a Caesar cipher?
What is the role of the secret key in symmetric encryption?
What is the role of the secret key in symmetric encryption?
What is the key space for the triple DES algorithm mentioned in the text?
What is the key space for the triple DES algorithm mentioned in the text?
What is the key space for a monoalphabetic substitution cipher with an arbitrary substitution of the 26 alphabetic characters?
What is the key space for a monoalphabetic substitution cipher with an arbitrary substitution of the 26 alphabetic characters?
What is the first step in a cryptanalysis that uses frequency analysis in a monoalphabetic substitution cipher?
What is the first step in a cryptanalysis that uses frequency analysis in a monoalphabetic substitution cipher?
What is the average number of possible keys that need to be tried in a brute-force attack to decrypt a Caesar cipher?
What is the average number of possible keys that need to be tried in a brute-force attack to decrypt a Caesar cipher?
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Study Notes
Types of Encryption
- Symmetric encryption was the only type in use before public-key encryption emerged in the 1970s.
- Symmetric encryption systems utilize the same secret key for both encryption and decryption by sender and receiver.
Cryptanalysis vs. Brute-Force Attack
- Cryptanalysis involves analyzing a cryptographic system to find weaknesses and break it, often using known patterns and frequency analysis.
- A brute-force attack systematically tries all possible keys until the correct one is found; it's often more straightforward but computationally intensive.
Congruence Relation
- The congruence relation in cryptography signifies that two numbers have the same remainder when divided by a given modulus, often used in modular arithmetic for encryption algorithms.
Process of Encryption
- Encryption is the process of converting plaintext into ciphertext, ensuring confidentiality of information.
Role of the Secret Key in Symmetric Encryption
- The secret key is essential for both encrypting and decrypting messages, ensuring that only authorized users can access the information.
Ciphertext in Symmetric Encryption
- Ciphertext refers to the encoded data produced from the encryption process, which is unreadable without the appropriate key.
Secure Use of Conventional Encryption
- For secure use, conventional encryption requires:
- A strong, unpredictable key.
- Keeping the key secret from unauthorized parties.
Types of Attacks
- The easiest attack to defend against is a ciphertext-only attack, where an attacker has only the ciphertext.
- Known-plaintext attacks occur when the attacker has certain plaintext segments and corresponding ciphertext.
- Chosen-plaintext attacks allow the attacker to select specific plaintexts to encrypt, gaining insight into the encryption algorithm.
Cryptanalytic Techniques
- Less conventional techniques in cryptanalysis can be quite rare compared to standard methods.
- Frequency analysis is a primary technique used against monoalphabetic substitution ciphers, exploiting letter frequency in a language.
Caesar Cipher Details
- The Caesar cipher replaces each letter with one three positions down the alphabet.
- The decryption algorithm for a Caesar cipher given key 'k' and ciphertext 'C' can be expressed as: Plaintext = (C - k) mod 26.
- The valid range for 'k' in a general Caesar algorithm is from 1 to 25.
Key Space Considerations
- The average number of keys to try in a brute-force attack against a Caesar cipher is 13, as there are 25 possible shifts.
- The key space for a triple DES algorithm is substantially large, making brute-force attacks impractical.
- A monoalphabetic substitution cipher has an enormous key space of more than 4 x 10^26 possible keys because it involves arbitrary substitution of letters.
Frequency Analysis
- Common letters 'e' and 't' in English are likely correlated with the most frequent letters in the ciphertext.
- Letters often corresponding to other common letters like 'a', 'h', 'i', 'n', 'o', and 'r' are strategically deduced through frequency patterns.
Objectives of Cryptanalysis
- The primary objective of attacking an encryption system is to access the plaintext without the key.
- Understanding the encryption algorithm and exploiting weaknesses improves the chances of successful cryptanalysis.
Essential Elements of Symmetric Encryption
- A secure channel is crucial in a symmetric cryptosystem for the safe exchange of keys and sensitive information.
- The notation for the encryption algorithm in symmetric encryption is often represented as E(K, P) where E denotes the encryption function, K the key, and P the plaintext.
Types of Cipher Processing
- Block ciphers process input in fixed-size blocks, as opposed to stream ciphers that handle data as a continuous stream.
Summary of Attacks and Encryption
- Continuing to understand the differences among encryption techniques and the various attacks strengthens knowledge in cryptography fundamentals.
- Established approaches to decrypting monoalphabetic ciphers include identifying patterns, often starting with frequency analysis.
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