Introduction to Cryptography and Ciphers
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Questions and Answers

What is the key in a cryptographic algorithm?

  • An algorithm used to encode messages
  • A method for breaking encryption codes
  • A secret string of numbers or characters for encryption (correct)
  • A public string of text used for decryption

Which technique involves trying to identify patterns in encrypted messages?

  • Frequency analysis
  • General weakness identification
  • Key deduction
  • Pattern recognition (correct)

How does a Caesar Cipher encrypt the message 'Attack at Dawn'?

  • By replacing each letter with the next letter in the alphabet
  • By shifting letters according to the Fibonacci sequence
  • By reversing the order of letters in the message
  • By rotating each letter by three positions in the alphabet (correct)

What does cryptanalysis attempt to deduce about an encryption algorithm?

<p>The weaknesses in the encryption method used (D)</p> Signup and view all the answers

In a substitution cipher, which of the following best describes what the substitution process entails?

<p>Replacing each letter with another letter (D)</p> Signup and view all the answers

What is one potential strategy for inferring meaning from encrypted communication without breaking it?

<p>Analyzing the size of the message (D)</p> Signup and view all the answers

How many different keys are possible in a Caesar Cipher with an alphabet of 26 letters?

<p>26 (B)</p> Signup and view all the answers

What is the primary goal of cryptography?

<p>To protect information from unauthorized access (B)</p> Signup and view all the answers

What is cryptanalysis primarily concerned with?

<p>Recognizing and breaking encryption codes (B)</p> Signup and view all the answers

Which of the following concepts is NOT an objective of information security?

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

In what way does a transposition cipher differ from a substitution cipher?

<p>It rearranges the position of letters (D)</p> Signup and view all the answers

What is characterizes an unconditionally secure algorithm?

<p>It remains secure regardless of the attacker's capabilities (B)</p> Signup and view all the answers

Which type of cipher uses multiple substitution alphabets?

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

What aspect of cryptanalysis involves analyzing cryptographic techniques?

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

Which of the following describes data integrity in information security?

<p>Confirming data has not been altered without permission (D)</p> Signup and view all the answers

What does non-repudiation in data security refer to?

<p>The assurance of data origin and integrity (B)</p> Signup and view all the answers

What is the result of removing spaces and punctuation from the plaintext 'Bob I will call'?

<p>BOBIWILLCALL (D)</p> Signup and view all the answers

What is the key matrix used in the Hill Cipher example?

<p>[3 3][2 5] (C)</p> Signup and view all the answers

How many keys are needed for symmetric encryption in a group of size 10?

<p>45 (C)</p> Signup and view all the answers

Which algorithm is considered a symmetric block cipher?

<p>3DES (A)</p> Signup and view all the answers

What is a unique feature of asymmetric encryption algorithms?

<p>They use different keys for encryption and decryption. (B)</p> Signup and view all the answers

Which encryption standard is a replacement for DES?

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

What fixed block size does the Advanced Encryption Standard (AES) use?

<p>128 bits (D)</p> Signup and view all the answers

Which of the following statements about symmetric algorithms is true?

<p>They use the same pre-shared key for encryption and decryption. (D)</p> Signup and view all the answers

What is the primary purpose of the initial permutation (IP) in the DES encryption process?

<p>To rearrange the bits of the plaintext block (D)</p> Signup and view all the answers

How many rounds of encryption does the DES algorithm perform?

<p>16 rounds (B)</p> Signup and view all the answers

What function does the round key serve during the DES encryption rounds?

<p>It is XORed with an expanded right half of the block (B)</p> Signup and view all the answers

Which of the following describes the structure used by DES in its encryption process?

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

What characteristic of the Electronic Codebook (ECB) mode makes it less secure?

<p>It encrypts each block independently (D)</p> Signup and view all the answers

What happens to the left and right halves of the data in each round of DES?

<p>They are swapped after each round (B)</p> Signup and view all the answers

What is the maximum block size that DES can encrypt?

<p>64 bits (D)</p> Signup and view all the answers

What is the function of S-boxes in the DES encryption process?

<p>They provide substitution during rounds (B)</p> Signup and view all the answers

What is the effective key length of the DES encryption algorithm?

<p>56 bits (B)</p> Signup and view all the answers

Which encryption method is considered a replacement for DES?

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

Which of the following algorithms uses the product of two very large prime numbers?

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

What is a key advantage of using Blowfish as an encryption algorithm?

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

Which type of encryption requires both the sender and receiver to share the same key?

<p>Symmetric-key encryption (A)</p> Signup and view all the answers

In what context is the Diffie-Hellman algorithm primarily used?

<p>For key exchange (A)</p> Signup and view all the answers

Which algorithm utilizes elliptic curves for its encryption and key exchange processes?

<p>Elliptic Curve Cryptography (ECC) (B)</p> Signup and view all the answers

What is a characteristic feature of the RC4 encryption algorithm?

<p>Stream cipher with variable key size (B)</p> Signup and view all the answers

What characteristic of a key matrix is essential for the Hill Cipher?

<p>It must be square and have a non-zero determinant. (B)</p> Signup and view all the answers

How does the Rail Fence Cipher rearrange characters?

<p>In a zigzag pattern across multiple rails. (B)</p> Signup and view all the answers

What adjustment is made when the length of the plaintext in the Hill Cipher is not a multiple of the block size?

<p>Pad it with filler characters like 'X'. (D)</p> Signup and view all the answers

What is the main function of the transposition cipher?

<p>To change the order of characters in the plaintext. (D)</p> Signup and view all the answers

In a columnar transposition cipher, what happens if the number of letters is not an exact multiple of the column size?

<p>The last column is filled with infrequent letters. (C)</p> Signup and view all the answers

What is the Hill cipher based on?

<p>Linear algebra principles. (A)</p> Signup and view all the answers

Which of the following best describes the Rail Fence Cipher's output?

<p>A rearranged version of the plaintext without losses. (A)</p> Signup and view all the answers

Why is the Hill cipher considered a polygraphic substitution cipher?

<p>It works with blocks of letters for substitution. (B)</p> Signup and view all the answers

Flashcards

Cryptography

The science of protecting information by converting it into an unreadable format (ciphertext), only accessible to authorized individuals.

Cryptanalysis

The practice of breaking codes and analyzing cryptographic systems to identify and exploit vulnerabilities.

Plaintext

The original, readable information before encryption.

Ciphertext

The encrypted version of plaintext, unreadable without the decryption key.

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Cipher

An algorithm used to convert plaintext into ciphertext and vice versa.

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Unconditionally Secure Algorithm

A cryptographic scheme that is theoretically unbreakable, even with unlimited computational resources and time.

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Data Integrity

Ensuring that the data has not been tampered with by unauthorized parties.

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Non-repudiation

Guaranteeing that the sender of a message cannot deny having sent it.

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Encryption Algorithm

A standardized method used for encrypting data.

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Key

A secret input that determines which characters to replace in a cipher.

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Caesar Cipher

A cipher that shifts each letter in the alphabet by a fixed number of positions. For example, shifting by 3 would make 'A' become 'D', 'B' become 'E', and so on.

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Decryption

The process of converting cipher text back to plain text using the key.

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Columnar Transposition

A type of transposition cipher that rearranges characters in columns of a grid. The plaintext is written into columns and then read row by row to produce the ciphertext.

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Monoalphabetic Cipher

A substitution cipher that replaces each letter in the plaintext with a different letter based on a fixed key.

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Rail Fence Cipher

A transposition cipher that rearranges characters in a zigzag pattern across multiple rails (rows). The text is read row by row to create the ciphertext.

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Hill Cipher

A polygraphic substitution cipher based on linear algebra. It involves replacing blocks of plaintext with corresponding blocks of ciphertext using a matrix transformation.

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DES Encryption Rounds

DES encrypts data in 16 rounds, each involving permutations, substitutions, and bitwise operations.

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Feistel Structure in DES

DES uses a Feistel Network to encrypt data, splitting the plaintext into two halves and processing them alternately with round functions.

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Initial Permutation (IP) in DES

The initial step in DES, where the 64-bit plaintext block is rearranged in a specific order.

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Feistel Network Rounds in DES

The core of DES encryption, where the permuted block is divided into two halves and processed in 16 rounds.

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DES Key Schedule

A key schedule generates 16 round keys (48 bits each) from the original 56-bit key, using permutations, rotations, and compressions.

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Electronic Codebook (ECB) Mode

Encrypts each block of data independently, making it vulnerable to patterns in repeated plaintext blocks.

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Cipher Block Chaining (CBC) Mode

Links blocks by XORing each plaintext block with the previous ciphertext block before encryption, enhancing security by adding interdependency.

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DES Decryption

The process of reversing DES encryption to recover the original plaintext, using the same steps in reverse order.

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Symmetric-Key Cipher

A type of encryption that uses a single key for both encrypting and decrypting data.

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Block Cipher

An encryption method that processes plaintext in fixed-size blocks. Each block is encrypted independently.

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Encryption Key

The unique sequence of bits that determines how a cipher encrypts data. It acts like a key to unlock encrypted information.

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Asymmetric Encryption

An encryption algorithm that uses one key for encryption and a different key for decryption. The keys are mathematically related but difficult to derive one from the other.

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RSA (Rivest-Shamir-Adleman)

A type of asymmetric encryption algorithm known for its use in secure internet connections. It uses the product of two very large prime numbers for key generation.

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Diffie-Hellman

A type of asymmetric encryption algorithm used for secure key exchange. It allows two parties to establish a shared secret key over an insecure channel.

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Elliptic Curve Cryptography (ECC)

A method of encryption using elliptic curves, a type of mathematical curve. It's known for its efficiency and strong security, especially for digital signatures and key exchange.

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RC4

A stream cipher that focuses on encrypting data bit by bit, rather than blocking it. It's known for its speed and is commonly used in secure communication protocols like SSL.

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What is a key matrix in Hill Cipher?

A 2x2 matrix used as a key in the Hill Cipher encryption algorithm. Each element in the matrix represents a number, and the matrix is used to transform blocks of plaintext characters into ciphertext.

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What is DES (Digital Encryption Standard)?

A block cipher that utilizes a 56-bit key and works in 16 rounds. It's considered relatively weak due to its short key length.

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What is 3DES (Triple DES)?

An enhanced version of DES that applies the DES algorithm three times for increased security. It utilizes a 168-bit key length.

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What is IDEA (International Data Encryption Algorithm)?

A block cipher that uses 64-bit blocks and 128-bit keys. It operates on each block in eight rounds and is known for its strength and efficiency. It's used in applications like PGP.

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What is AES (Advanced Encryption Standard)?

A modern, robust symmetric block cipher with a fixed 128-bit block size and variable key lengths of 128, 192, or 256 bits. Considered highly secure and widely adopted by governments and businesses.

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How to calculate number of keys in Symmetric encryption?

The number of keys needed in a symmetric key system is calculated using the formula n(n-1)/2, where 'n' represents the number of users in the group. For example, in a group of 10 users, the number of keys required would be 45.

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Study Notes

Introduction to Cryptography and Cryptanalysis

  • Cryptography is the science of making and breaking secret codes. It is used to securely store and transmit data so only the intended recipient can access it.
  • Cryptology encompasses both cryptography and cryptanalysis.
  • Cryptography strives to protect information from unauthorized access.
  • Cryptanalysis aims to break codes and uncover vulnerabilities in cryptographic techniques.

Cryptography - Ciphers

  • A cipher is an algorithm or method used to transform plaintext into ciphertext, and vice versa.
  • Common ciphers include substitution ciphers, transposition ciphers, and polyalphabetic ciphers.
  • Substitution ciphers change the letters in the message.
  • Transposition ciphers rearrange the letters in the message without replacing them.
  • Polyalphabetic ciphers use multiple substitution alphabets.

Levels of Security

  • Unconditionally secure algorithms remain secure regardless of the attacker's resources or time. The one-time pad is an example.
  • Computationally secure algorithms rely on the attacker's limited computational resources. Modern cryptographic algorithms like RSA, AES, and ECC are computationally secure.

Cryptanalysis - Code Breaking

  • Cryptanalysis involves methods to break encryption codes.
  • Brute-force cryptanalysis tries every possible key until one works.
  • Ciphertext cryptanalysis analyzes ciphertexts without knowing the plaintext.
  • Known-plaintext cryptanalysis uses known corresponding plaintext and ciphertext pairs.
  • Chosen-plaintext cryptanalysis allows the cryptanalyst to encrypt chosen plaintexts.
  • Chosen-ciphertext cryptanalysis uses chosen ciphertexts to decrypt.
  • Meet-in-the-middle attacks try to find vulnerabilities in cryptographic systems.

Keys

  • The secrecy of the key is the most important factor in the security of encryption, not the algorithm.
  • Key length refers to the size of the key in bits.
  • Keyspace is the number of possible keys for a given key length. It is exponential in key length.

Symmetric Encryption

  • Symmetric encryption algorithms use the same key both for encryption and decryption.
  • Examples include 3DES, IDEA, and AES.
  • Block ciphers encrypt data in fixed-size blocks, while stream ciphers encrypt data bit-by-bit.

Asymmetric Encryption

  • Asymmetric encryption algorithms use different keys for encryption and decryption: a public key and a private key.
  • RSA and ElGamal are examples.

RSA Public Key Crypto System (Steps for Key Generation)

  • Select two large prime numbers, p and q.
  • Compute n = pq and Φ(n) = (q – 1)(p – 1).
  • Select an integer e such that 1 < e < Φ(n) and gcd(e, Φ(n)) = 1. Usually e=3 or 65537
  • Calculate d such that (ed mod Φ(n)) = 1, or ed = 1 + kΦ(n) for some integer k.

RSA Example

  • Public Key is (e.n)
  • Private Key is (d,n)

Diffie-Hellman Key Exchange Protocol

  • Diffie-Hellman is used to securely exchange keys over an insecure channel.
  • First, participants agree on a large prime number, p, and a primitive root, g, modulo p.
  • Then, each party chooses a secret integer.
  • This process generates identical shared secret keys on both parties.

ElGamal Encryption

  • ElGamal is an asymmetric encryption system.
  • It has public and private keys. There is a message and a random integer.
  • First, compute c1; then, compute c2.
  • The ciphertext is a pair of values.

Hashing Algorithms

  • Hashing algorithms take any input length and produce a fixed-length output.
  • Hashing is a one-way process (cannot reverse), but is relatively easy to compute.
  • Hashing is used to ensure data integrity.
  • Hashing uses a one-way function that converts data into a unique fixed-size hash.

Digital Signatures

  • Digital signatures ensure the integrity and authenticity of messages or documents.
  • Using a private key, it authenticates the message, proving it originated from a known source and preventing alteration.
  • It involves hashing the message, encrypting that hash.
  • Verification uses the recipient’s public key to decrypt the signature to verify originality and integrity

Public Key Infrastructure (PKI)

  • A PKI is an infrastructure that manages and distributes public key certificates.
  • A trusted third-party system called a Certification Authority (CA) issues digital certificates, which confirm the authenticity of public keys used for secure communication.
  • PKI ensures the interoperability of different vendors.
  • Various applications use PKI for authentication.

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Description

This quiz covers the fundamental concepts of cryptography and cryptanalysis. It explores various types of ciphers, including substitution and transposition ciphers, and discusses security levels. Test your knowledge on the science of encrypting and decrypting information.

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