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Questions and Answers
Which characteristic is NOT essential for a secure hashing algorithm?
Which characteristic is NOT essential for a secure hashing algorithm?
What is the primary goal when attacking an encryption system, according to the text?
What is the primary goal when attacking an encryption system, according to the text?
In symmetric encryption, what is a critical requirement for secure communication between sender and receiver?
In symmetric encryption, what is a critical requirement for secure communication between sender and receiver?
A cryptographic system that uses both substitution and transposition techniques is best described as which of the following?
A cryptographic system that uses both substitution and transposition techniques is best described as which of the following?
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Which of the following is a characteristic of a stream cipher?
Which of the following is a characteristic of a stream cipher?
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Which of the following correctly describes the relationship between cryptography, cryptanalysis, and cryptology?
Which of the following correctly describes the relationship between cryptography, cryptanalysis, and cryptology?
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In a symmetric encryption system, if Alice wants to send a secure message to Bob, what must they do before transmitting the ciphertext?
In a symmetric encryption system, if Alice wants to send a secure message to Bob, what must they do before transmitting the ciphertext?
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What is the primary purpose of a cryptographic hash function?
What is the primary purpose of a cryptographic hash function?
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Which of the following is the correct order of steps in a basic encryption/decryption process using symmetric encryption?
Which of the following is the correct order of steps in a basic encryption/decryption process using symmetric encryption?
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Why are single-key cryptographic algorithms also referred to as symmetric encryption?
Why are single-key cryptographic algorithms also referred to as symmetric encryption?
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In a ciphertext generated using a simple substitution cipher, the letters 'P' and 'Z' appear with high frequencies. Based on this information and typical English language letter frequencies, which of the following is the most likely substitution?
In a ciphertext generated using a simple substitution cipher, the letters 'P' and 'Z' appear with high frequencies. Based on this information and typical English language letter frequencies, which of the following is the most likely substitution?
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Why must a key phrase used in a key phrase substitution cipher contain unique letters?
Why must a key phrase used in a key phrase substitution cipher contain unique letters?
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A key phrase 'BOM' is used for a substitution cipher. How would the plaintext letter 'c' be encrypted?
A key phrase 'BOM' is used for a substitution cipher. How would the plaintext letter 'c' be encrypted?
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Consider the key phrase 'EXAMPLE'. After removing redundant letters, what would be the correct key phrase used for creating the substitution cipher?
Consider the key phrase 'EXAMPLE'. After removing redundant letters, what would be the correct key phrase used for creating the substitution cipher?
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Using the key phrase 'SPECTALUR', what ciphertext letter would the plaintext letter 'g' be replaced with?
Using the key phrase 'SPECTALUR', what ciphertext letter would the plaintext letter 'g' be replaced with?
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In a substitution cipher using the key phrase 'EZRA CORNELL', how many plaintext letters are substituted for themselves?
In a substitution cipher using the key phrase 'EZRA CORNELL', how many plaintext letters are substituted for themselves?
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What is the primary purpose of using a matrix with a key phrase in a substitution cipher?
What is the primary purpose of using a matrix with a key phrase in a substitution cipher?
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In asymmetric encryption, which of the following statements accurately describes the key usage?
In asymmetric encryption, which of the following statements accurately describes the key usage?
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What is the primary goal of the substitution technique in cryptography?
What is the primary goal of the substitution technique in cryptography?
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Which of the following statements accurately describes symmetric encryption algorithms?
Which of the following statements accurately describes symmetric encryption algorithms?
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What is a significant weakness of the Caesar cipher that makes it vulnerable to cryptanalysis?
What is a significant weakness of the Caesar cipher that makes it vulnerable to cryptanalysis?
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Why is it important for options in a multiple-choice question to be similar in length?
Why is it important for options in a multiple-choice question to be similar in length?
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The primary goal of a hashing algorithm is to generate ciphertext suitable for later decryption.
The primary goal of a hashing algorithm is to generate ciphertext suitable for later decryption.
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A secure hashing algorithm guarantees reversibility, which means one can always derive the original data from its hash.
A secure hashing algorithm guarantees reversibility, which means one can always derive the original data from its hash.
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For conventional encryption to be secure, only a strong encryption algorithm is sufficient; secure key exchange is not necessary.
For conventional encryption to be secure, only a strong encryption algorithm is sufficient; secure key exchange is not necessary.
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In a symmetric cryptosystem, different keys are utilized for encryption and decryption processes between sender and receiver.
In a symmetric cryptosystem, different keys are utilized for encryption and decryption processes between sender and receiver.
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In cryptanalysis, the main objective is usually to directly recover the plaintext from the ciphertext, bypassing the need to find the encryption key.
In cryptanalysis, the main objective is usually to directly recover the plaintext from the ciphertext, bypassing the need to find the encryption key.
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In a brute-force attack, an attacker needs to try, on average, all possible keys to successfully translate ciphertext into plaintext.
In a brute-force attack, an attacker needs to try, on average, all possible keys to successfully translate ciphertext into plaintext.
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An encryption scheme is considered unconditionally secure if the cost of breaking the cipher is less than the value of the encrypted information.
An encryption scheme is considered unconditionally secure if the cost of breaking the cipher is less than the value of the encrypted information.
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In a 'chosen ciphertext' attack, the cryptanalyst has access only to the encryption algorithm and the ciphertext.
In a 'chosen ciphertext' attack, the cryptanalyst has access only to the encryption algorithm and the ciphertext.
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Strong encryption relies solely on using cryptographic algorithms that have been mathematically proven to be unbreakable.
Strong encryption relies solely on using cryptographic algorithms that have been mathematically proven to be unbreakable.
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Cryptanalysis focuses solely on exploiting vulnerabilities within the encryption algorithm itself, disregarding any information about the plaintext.
Cryptanalysis focuses solely on exploiting vulnerabilities within the encryption algorithm itself, disregarding any information about the plaintext.
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In symmetric encryption, the encryption key ($K_E$) is distinct from the decryption key ($K_D$).
In symmetric encryption, the encryption key ($K_E$) is distinct from the decryption key ($K_D$).
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The Caesar cipher is a type of transposition technique where the order of the plaintext is rearranged to form the ciphertext.
The Caesar cipher is a type of transposition technique where the order of the plaintext is rearranged to form the ciphertext.
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If the Caesar cipher algorithm is expressed as $C = E(k, p) = (p + k) \mod 26$, decrypting the ciphertext involves the calculation $p = D(k, C) = (C + k) \mod 26$.
If the Caesar cipher algorithm is expressed as $C = E(k, p) = (p + k) \mod 26$, decrypting the ciphertext involves the calculation $p = D(k, C) = (C + k) \mod 26$.
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In a Caesar cipher with a key of $k = 5$, the plaintext 'hello' would encrypt to 'MJQQT'.
In a Caesar cipher with a key of $k = 5$, the plaintext 'hello' would encrypt to 'MJQQT'.
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The primary advantage of the Caesar cipher in modern cryptography is its resistance to brute-force attacks due to the large key space.
The primary advantage of the Caesar cipher in modern cryptography is its resistance to brute-force attacks due to the large key space.
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Enciphering refers to the general process covering both encoding and encryption techniques, while encryption specifically means translating letters or symbols individually.
Enciphering refers to the general process covering both encoding and encryption techniques, while encryption specifically means translating letters or symbols individually.
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If using a Caesar cipher with a key of 7, the letter 'z' would be encrypted as 'g'.
If using a Caesar cipher with a key of 7, the letter 'z' would be encrypted as 'g'.
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Asymmetric encryption offers the advantage of simplified key distribution compared to symmetric encryption.
Asymmetric encryption offers the advantage of simplified key distribution compared to symmetric encryption.
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A Caesar cipher's security is primarily limited by its small key space, specifically having only 25 possible keys.
A Caesar cipher's security is primarily limited by its small key space, specifically having only 25 possible keys.
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In a monoalphabetic substitution cipher with a 26-letter alphabet, the number of possible keys exceeds $5 \times 10^{26}$.
In a monoalphabetic substitution cipher with a 26-letter alphabet, the number of possible keys exceeds $5 \times 10^{26}$.
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Monoalphabetic substitution ciphers utilize a different cipher alphabet for each message to enhance security.
Monoalphabetic substitution ciphers utilize a different cipher alphabet for each message to enhance security.
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Qalqashandi's cryptanalysis technique involves writing down all plaintext letters and counting their frequency to decipher a message.
Qalqashandi's cryptanalysis technique involves writing down all plaintext letters and counting their frequency to decipher a message.
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Cryptanalysis based on letter frequencies is ineffective against short ciphertext messages because short messages rarely display standard frequency distributions accurately.
Cryptanalysis based on letter frequencies is ineffective against short ciphertext messages because short messages rarely display standard frequency distributions accurately.
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In cryptanalysis, comparing the frequency of ciphertext letters to a geometric
distribution is essential for deciphering the message.
In cryptanalysis, comparing the frequency of ciphertext letters to a geometric
distribution is essential for deciphering the message.
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The effectiveness of frequency analysis in cryptanalysis is largely independent of the length of the ciphertext being analyzed.
The effectiveness of frequency analysis in cryptanalysis is largely independent of the length of the ciphertext being analyzed.
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Flashcards
Cryptography
Cryptography
The branch of mathematics that transforms data for security.
Cryptographic algorithms
Cryptographic algorithms
Methods used to encrypt and decrypt information.
Plaintext
Plaintext
The original message before encryption.
Ciphertext
Ciphertext
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Cryptographic Hash Function
Cryptographic Hash Function
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Hashing Algorithm
Hashing Algorithm
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Characteristics of Secure Hash
Characteristics of Secure Hash
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Symmetric Encryption Requirements
Symmetric Encryption Requirements
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Types of Cryptographic Operations
Types of Cryptographic Operations
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Cryptanalysis Goal
Cryptanalysis Goal
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Enciphering
Enciphering
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Encryption
Encryption
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Symmetric Encryption
Symmetric Encryption
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Asymmetric Encryption
Asymmetric Encryption
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Substitution Technique
Substitution Technique
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Caesar Cipher
Caesar Cipher
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Caesar Cipher Algorithm
Caesar Cipher Algorithm
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Brute-Force Cryptanalysis
Brute-Force Cryptanalysis
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Ciphertext Frequency
Ciphertext Frequency
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Substitution Cipher
Substitution Cipher
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Key Phrase
Key Phrase
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Collision in Substitution Ciphers
Collision in Substitution Ciphers
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Redundancy Removal
Redundancy Removal
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Matrix in Substitution
Matrix in Substitution
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Ciphertext Example
Ciphertext Example
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Hashing Purpose
Hashing Purpose
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Symmetric Encryption Conditions
Symmetric Encryption Conditions
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Types of Cryptographic Processing
Types of Cryptographic Processing
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Cryptanalysis Focus
Cryptanalysis Focus
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Cryptanalysis
Cryptanalysis
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Brute-force Attack
Brute-force Attack
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Unconditionally Secure
Unconditionally Secure
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Strong Encryption
Strong Encryption
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Types of Attacks
Types of Attacks
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Key in Caesar Cipher
Key in Caesar Cipher
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Ciphertext of Caesar Cipher
Ciphertext of Caesar Cipher
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Monoalphabetic Substitution Cipher
Monoalphabetic Substitution Cipher
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Permutations
Permutations
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Key Space
Key Space
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Cryptanalysis by Frequency Analysis
Cryptanalysis by Frequency Analysis
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Frequency Distribution in English
Frequency Distribution in English
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Qalqashandi's Technique
Qalqashandi's Technique
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Counting Ciphertext Letters
Counting Ciphertext Letters
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Study Notes
Introduction to Cryptography
- Cryptography is a branch of mathematics focused on transforming data.
- Cryptographic algorithms play a critical role in information security and network security.
- Cryptography is essential for secure data storage, transmission, and interaction between parties.
Cryptographic Algorithms
- Categories:
- Keyless: These algorithms do not use keys during transformations, like cryptographic hash functions.
- Single-key: The output depends on the input data and a single secret key (symmetric encryption).
- Two-key: These use different but related keys, such as private and public keys (asymmetric encryption).
- This study covers single-key algorithms, also known as symmetric or conventional encryption.
- Types of operations: substitution, transposition, product
- Number of keys: symmetric (single-key), asymmetric (two-key)
- Plaintext processing: block cipher, stream cipher
Definitions
- Plaintext: The original, unencrypted message.
- Ciphertext: The encrypted message.
- Encryption: The process of converting plaintext to ciphertext.
- Decryption: The process of converting ciphertext back to plaintext.
Definitions (continued)
- Cryptography: The study of encryption schemes (hidden writing).
- Cryptographic system (cryptosystem)/cipher: A scheme for encryption and decryption.
- Cryptanalysis: Techniques used to decipher messages without knowing encryption details. A cryptanalyst is a person who practices cryptanalysis.
- Cryptology: The combined discipline of cryptography and cryptanalysis.
Cryptographic Hash Function
- The most basic type of cryptographic algorithm, a one-way hash function, is a cryptographic hash function.
- A hash algorithm produces a unique "digital fingerprint" of a data set, called a hash or digest.
- Hashes represent data contents and cannot be reversed.
- Hashes are typically used for comparison purposes, not decryption.
- A secure hash has fixed size, uniqueness, originality, and security.
Simplified Model of Symmetric Encryption
- A secret key is shared between sender and receiver.
- Encryption algorithm transforms plaintext to ciphertext using the key.
- Decryption algorithm transforms ciphertext back to plaintext using the same key.
- Secure use relies on a strong encryption algorithm and secure key management.
Model of Symmetric Cryptosystem
- Shows the simplified flow of data in a symmetric cryptosystem.
- The diagram displays message source, encryption algorithm, key source, secure channel, decryption algorithm, and destination.
Cryptographic Systems
- Characterized by three dimensions:
- Operations used for transformation (substitution, transposition, product).
- Number of keys used (symmetric, asymmetric).
- Way the plaintext is processed (block cipher, stream cipher).
- Block ciphers process input in blocks, while stream ciphers process elements sequentially.
Cryptanalysis and Brute-Force Attack
- Cryptanalysis aims to recover the encryption key, not the plaintext.
- Approaches include:
- Cryptanalysis- relies on the algorithm's nature and plaintext characteristics.
- Brute-force- attempts every possible key until decryption succeeds.
- On average, half of all possible keys must be tried to succeed in a brute force attack
- Knowledge of plaintext characteristics helps
- Ability to distinguish plaintext from garble important
Types of Attacks on Encrypted Messages
- Different attack types depend on the information available to the cryptanalyst
- Ciphertext-Only, Known-Plaintext, Chosen-Plaintext, Chosen-Ciphertext, and Chosen-Text attacks
Encryption Scheme Security
- Unconditionally secure - breaking it is completely impossible.
- Computationally secure - breaking it is too expensive or time-consuming.
Brute-Force Attack (cont'd)
- An attacker tries all possible keys to decode the ciphertext.
- Often, some knowledge of the expected plaintext is helpful for faster results.
- Automatic methods distinguish plaintext from garble.
Strong Encryption
- Strong encryption make it hard for unauthorized users to decrypt.
- Criteria for a strong encryption algorithm include:
- Appropriate algorithm selection.
- Use of adequate key lengths.
- Suitable cryptographic protocols.
- Well-engineered implementation.
- Absence of deliberate flaws.
Encryption Algorithms (Continued)
- Encoding- changes entire phrases to other phrases.
- Enciphering – changing individual symbols or letters.
- Encryption encompasses both encoding and enciphering.
Encryption Algorithms
- Symmetric encryption
- Asymmetric encryption
Substitution Technique
- A cipher that replaces letters or symbols with other letters or symbols.
Caesar Cipher
- A simple substitution cipher used by Julius Caesar.
- Shifts each letter a fixed number of places in the alphabet.
Caesar Cipher Algorithm
- Mathematical description of the Caesar Cipher algorithm
- Provides formulas for encryption (e.g., c = E(k, p) = (p + k) mod 26) and decryption (e.g., p = D(k, C) = (C − k) mod 26).
The Caesar Cipher - Examples
- Demonstrates the Caesar Cipher's encryption and decryption processes.
- Shows examples of encrypting and decrypting messages (different keys).
Monoalphabetic Cipher
- Describes a simple substitution method where each letter is replaced consistently.
- Provides numerical representations of letter-to-letter mappings.
- 26! possible keys
Cryptanalysis Based on Relative Frequencies of Letters
- Cryptanalysts use frequency analysis to decipher messages, particularly those written in a common language.
- Frequency analysis depends on word/symbol frequency in the ciphertext.
- Arabs made early significant advances
Relative Frequency of Letters in English Text
- The frequencies of letters in English text are not uniform.
- Analysts use standard distributions for cryptanalysis
- Analysts exploit the regularities of the language to aid decryption
Substitution using Key Phrase
- Substitutions using key phrases create cipher alphabets using a word.
- Requires unique letters in the key phrase, without redundancy.
- Strengthens the substitution cipher.
Substitution Using Key Phrase: More Examples
- Examples of substituting text using a keyphrase.
Substitution using a Key Phrase and a Matrix
- An enhanced substitution method using matrices.
- Matrix size (rows x columns) can vary (e.g., 5x6, 6x5)
Multiplicative Ciphers
- Monoalphabetic ciphers where each letter is replaced by a numerical value based on multiplication with a key.
- Relatively prime key and modulus values ensure a complete set of remainders.
- Example given: f(a) = a*k mod n , where k and n are relatively prime [GCD(k,n)=1].
- If k and n are not relatively prime, several plaintext will encrypt to the same ciphertext letter and not all letter will appear in the ciphertext alphabet; Example: if k = 12 and n =26
###Table of Multiplication modulo 26
- Tables showing multiplication modulo 26.
- Includes information for finding multiplicative inverses
Multiplicative Cipher - Encryption
- Examples of encrypting texts with a multiplicative cipher.
Multiplicative Cipher - Decryption
- Examples of decrypting texts with a multiplicative cipher.
- Explanation of how to find multiplicative inverses
Cryptanalysis of the Caesar Cipher
- Techniques for breaking the Caesar cipher, including starting with common English words and analyzing ciphertext patterns.
Polyalphabetic Substitution Ciphers
- Techniques to flatten the frequency distribution of ciphertext for improved security.
- Use of separate encoding methods (e.g., odd and even positions)
Polyalphabetic Substitution Ciphers (continued)
- Example shown of using two encryptions to provide stronger encryption.
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Description
Understand the basics of cryptography, a field of mathematics critical for information and network security. Explore cryptographic algorithms, including keyless, single-key, and two-key methods. Learn the definitions of plaintext, ciphertext, encryption, and decryption.