Introduction to Cryptography
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Questions and Answers

Which characteristic is NOT essential for a secure hashing algorithm?

  • Secure against collisions
  • Fixed size output
  • Unique hash for each unique input
  • Reversible to obtain the original data (correct)
  • What is the primary goal when attacking an encryption system, according to the text?

  • To recover the plaintext message without the key
  • To recover the encryption key (correct)
  • To decrypt a specific ciphertext
  • To compromise the encryption algorithm itself
  • In symmetric encryption, what is a critical requirement for secure communication between sender and receiver?

  • Secure exchange and storage of the secret key (correct)
  • Use of different keys for encryption and decryption
  • Publicly known encryption algorithm
  • Frequent changes to the encryption algorithm
  • A cryptographic system that uses both substitution and transposition techniques is best described as which of the following?

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

    Which of the following is a characteristic of a stream cipher?

    <p>Processes input elements continuously, one at a time. (A)</p> Signup and view all the answers

    Which of the following correctly describes the relationship between cryptography, cryptanalysis, and cryptology?

    <p>Cryptology encompasses both cryptography (designing secure systems) and cryptanalysis (breaking them). (D)</p> Signup and view all the answers

    In a symmetric encryption system, if Alice wants to send a secure message to Bob, what must they do before transmitting the ciphertext?

    <p>They must securely exchange the secret key that will be used for both encryption and decryption. (D)</p> Signup and view all the answers

    What is the primary purpose of a cryptographic hash function?

    <p>To create a unique, fixed-size representation (digest) of a data set for integrity verification. (A)</p> Signup and view all the answers

    Which of the following is the correct order of steps in a basic encryption/decryption process using symmetric encryption?

    <p>Plaintext -&gt; Encryption -&gt; Ciphertext -&gt; Decryption (C)</p> Signup and view all the answers

    Why are single-key cryptographic algorithms also referred to as symmetric encryption?

    <p>Because the same key is used for both encryption and decryption. (A)</p> Signup and view all the answers

    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?

    <p>'P' for 'E', 'Z' for 'A' (D)</p> Signup and view all the answers

    Why must a key phrase used in a key phrase substitution cipher contain unique letters?

    <p>To ensure each plaintext letter has a unique ciphertext equivalent. (A)</p> Signup and view all the answers

    A key phrase 'BOM' is used for a substitution cipher. How would the plaintext letter 'c' be encrypted?

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

    Consider the key phrase 'EXAMPLE'. After removing redundant letters, what would be the correct key phrase used for creating the substitution cipher?

    <p>EXAMPL (C)</p> Signup and view all the answers

    Using the key phrase 'SPECTALUR', what ciphertext letter would the plaintext letter 'g' be replaced with?

    <p>H (C)</p> Signup and view all the answers

    In a substitution cipher using the key phrase 'EZRA CORNELL', how many plaintext letters are substituted for themselves?

    <p>0 (C)</p> Signup and view all the answers

    What is the primary purpose of using a matrix with a key phrase in a substitution cipher?

    <p>To further obscure the relationship between plaintext and ciphertext. (A)</p> Signup and view all the answers

    In asymmetric encryption, which of the following statements accurately describes the key usage?

    <p>The encryption key (KE) is different from the decryption key (KD), providing enhanced security. (D)</p> Signup and view all the answers

    What is the primary goal of the substitution technique in cryptography?

    <p>To replace plaintext letters or bit patterns with other letters, numbers, symbols, or bit patterns. (C)</p> Signup and view all the answers

    Which of the following statements accurately describes symmetric encryption algorithms?

    <p>They use the same key for both encryption and decryption. (B)</p> Signup and view all the answers

    What is a significant weakness of the Caesar cipher that makes it vulnerable to cryptanalysis?

    <p>The small key space makes it susceptible to brute-force attacks. (B)</p> Signup and view all the answers

    Why is it important for options in a multiple-choice question to be similar in length?

    <p>To prevent students from guessing the answer based on length. (C)</p> Signup and view all the answers

    The primary goal of a hashing algorithm is to generate ciphertext suitable for later decryption.

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

    A secure hashing algorithm guarantees reversibility, which means one can always derive the original data from its hash.

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

    For conventional encryption to be secure, only a strong encryption algorithm is sufficient; secure key exchange is not necessary.

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

    In a symmetric cryptosystem, different keys are utilized for encryption and decryption processes between sender and receiver.

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

    In cryptanalysis, the main objective is usually to directly recover the plaintext from the ciphertext, bypassing the need to find the encryption key.

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

    In a brute-force attack, an attacker needs to try, on average, all possible keys to successfully translate ciphertext into plaintext.

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

    An encryption scheme is considered unconditionally secure if the cost of breaking the cipher is less than the value of the encrypted information.

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

    In a 'chosen ciphertext' attack, the cryptanalyst has access only to the encryption algorithm and the ciphertext.

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

    Strong encryption relies solely on using cryptographic algorithms that have been mathematically proven to be unbreakable.

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

    Cryptanalysis focuses solely on exploiting vulnerabilities within the encryption algorithm itself, disregarding any information about the plaintext.

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

    In symmetric encryption, the encryption key ($K_E$) is distinct from the decryption key ($K_D$).

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

    The Caesar cipher is a type of transposition technique where the order of the plaintext is rearranged to form the ciphertext.

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

    If the Caesar cipher algorithm is expressed as $C = E(k, p) = (p + k) \mod 26$, decrypting the ciphertext involves the calculation $p = D(k, C) = (C + k) \mod 26$.

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

    In a Caesar cipher with a key of $k = 5$, the plaintext 'hello' would encrypt to 'MJQQT'.

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

    The primary advantage of the Caesar cipher in modern cryptography is its resistance to brute-force attacks due to the large key space.

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

    Enciphering refers to the general process covering both encoding and encryption techniques, while encryption specifically means translating letters or symbols individually.

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

    If using a Caesar cipher with a key of 7, the letter 'z' would be encrypted as 'g'.

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

    Asymmetric encryption offers the advantage of simplified key distribution compared to symmetric encryption.

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

    A Caesar cipher's security is primarily limited by its small key space, specifically having only 25 possible keys.

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

    In a monoalphabetic substitution cipher with a 26-letter alphabet, the number of possible keys exceeds $5 \times 10^{26}$.

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

    Monoalphabetic substitution ciphers utilize a different cipher alphabet for each message to enhance security.

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

    Qalqashandi's cryptanalysis technique involves writing down all plaintext letters and counting their frequency to decipher a message.

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

    Cryptanalysis based on letter frequencies is ineffective against short ciphertext messages because short messages rarely display standard frequency distributions accurately.

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

    In cryptanalysis, comparing the frequency of ciphertext letters to a geometric distribution is essential for deciphering the message.

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

    The effectiveness of frequency analysis in cryptanalysis is largely independent of the length of the ciphertext being analyzed.

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

    Flashcards

    Cryptography

    The branch of mathematics that transforms data for security.

    Cryptographic algorithms

    Methods used to encrypt and decrypt information.

    Plaintext

    The original message before encryption.

    Ciphertext

    The encoded message resulting from encryption.

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    Cryptographic Hash Function

    A one-way algorithm creating a unique digital fingerprint of data.

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    Hashing Algorithm

    A process that converts data into a fixed-size string for comparison, not for decryption.

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    Characteristics of Secure Hash

    A secure hash must be fixed size, unique, original, and secure from reversal.

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    Symmetric Encryption Requirements

    To use symmetric encryption securely, a strong algorithm and secure key exchange are needed.

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    Types of Cryptographic Operations

    Plaintext can be transformed using substitution, transposition, or product methods.

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    Cryptanalysis Goal

    The aim of cryptanalysis is to recover the encryption key, not the plaintext itself.

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    Enciphering

    The process of translating letters or symbols individually.

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    Encryption

    A collective term covering both encoding and enciphering processes.

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    Symmetric Encryption

    A method that uses the same key for both encryption and decryption.

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    Asymmetric Encryption

    A method that uses different keys for encryption and decryption.

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    Substitution Technique

    A method where letters are replaced by other letters, numbers, or symbols.

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    Caesar Cipher

    A simple substitution cipher that shifts letters by a fixed number of places in the alphabet.

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    Caesar Cipher Algorithm

    It includes a mathematical transformation of letters through a defined shift value (k).

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    Brute-Force Cryptanalysis

    A method to decrypt by trying every possible key until the right one is found.

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    Ciphertext Frequency

    The distribution of letters in a ciphertext expressed as percentages.

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    Substitution Cipher

    A method of encoding where each letter is replaced by another letter.

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    Key Phrase

    A phrase used to create the cipher alphabet in a substitution cipher.

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    Collision in Substitution Ciphers

    Occurs when a plaintext letter is substituted for itself.

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    Redundancy Removal

    The process of eliminating duplicate letters from the key phrase.

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    Matrix in Substitution

    A grid format where key phrase letters form columns, and leftover letters fill the rest.

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    Ciphertext Example

    An example demonstrating how letters are encoded using a key phrase.

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    Hashing Purpose

    Hashing algorithms are one-way processes for data comparison and cannot be reversed.

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    Symmetric Encryption Conditions

    Secure symmetric encryption requires a strong algorithm and secure key exchange.

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    Types of Cryptographic Processing

    Plaintext processing methods include block ciphers and stream ciphers.

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    Cryptanalysis Focus

    The goal of cryptanalysis is to recover the encryption key, not the plaintext.

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    Cryptanalysis

    The study of analyzing and breaking encryption algorithms or keys.

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    Brute-force Attack

    An attack method that tries all possible keys until successful decryption.

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    Unconditionally Secure

    A type of encryption where decryption is impossible regardless of resources.

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    Strong Encryption

    Encryption that is nearly impossible for unauthorized users to break.

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    Types of Attacks

    Different methods used to decrypt messages using known information.

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    Key in Caesar Cipher

    A number (k) indicating how many places to shift letters.

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    Ciphertext of Caesar Cipher

    The result of applying the Caesar Cipher to plaintext.

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    Monoalphabetic Substitution Cipher

    A cipher using a single alphabet substitution per message to encode text.

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    Permutations

    Different arrangements of a set where each element appears exactly once.

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    Key Space

    The total number of possible keys in a cipher system.

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    Cryptanalysis by Frequency Analysis

    The technique of analyzing letter frequency in ciphertext to break ciphers.

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    Frequency Distribution in English

    The standard percentage occurrence of letters in the English language.

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    Qalqashandi's Technique

    An ancient method for solving ciphers using frequency counting developed by Arabic scholars.

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    Counting Ciphertext Letters

    The process of tallying the occurrence of letters in a ciphertext for analysis.

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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.

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