LECTURE NOTE 2

Question 1

making secret messages; The practice of creating secret messages by transforming plaintext (an ordinary readable message) into ciphertext (secret messages that are
difficult to interpret)

Answer 1

crypotography

Question 2

breaking secret messages
The process of deciphering secret messages, involving the recovery of plaintext from ciphertext without possessing the key. Also called code breaking

Answer 2

cryptanalysis

Question 3

The combined art and science of both creating and
deciphering “secret codes”.
It encompasses cryptography and cryptanalysis, representing the broader
field of secure communication techniques.

Answer 3

cryptology

Question 4

term that serves as a synonym for any or all of the
aforementioned concepts, with the precise meaning determined by the
context in which it is used.

Answer 4

Crypto

Question 5

what are the cryptographic systems are generally classified along three independent
dimensions

Answer 5

type of operations used for encryption, number of keys used, processing of plaintext

Question 6

what are the types of operations used for encryption

Answer 6

substitution, transposition, product systems/ciphers

Question 7

what are different types of number of keys used

Answer 7

symmetric, asymmetric

Question 8

what are the different types of processing of plaintext

Answer 8

block cipher, stream cipher

Question 9

each element in the plaintext is mapped into another element

Answer 9

substitution

Question 10

elements in plaintext are rearranged

Answer 10

transposition

Question 11

multiple stages/combinations of substitutions and
transposition

Answer 11

product systems/ciphers

Question 12

sender and receiver use same key

Answer 12

symmetric

Question 13

sender and receiver each use a different key

Answer 13

asymmetric

Question 14

operates on input data in fixed-size blocks. Processes each
block independently, generating an output block

Answer 14

block cipher

Question 15

process the input elements continuously, producing output
element one at a time, as it goes along

Answer 15

stream cipher

Question 16

only to the sender and receiver, is a critical
component of the encryption/decryption processes.

Answer 16

key

Question 17

a particular algorithm (cryptographic system)

Answer 17

cipher

Question 18

original message

Answer 18

plaintext

Question 19

encrypted or coded message

Answer 19

ciphertext

Question 20

convert from plaintext to ciphertext (enciphering)

Answer 20

encryption

Question 21

restore the plaintext from ciphertext (deciphering)

Answer 21

decryption

Question 22

critical information used in cipher known only to sender/receiver

Answer 22

key

Question 23

cryptosystem uses the same key to encrypt as to decrypt

Answer 23

symmetric key

Question 24

cryptosystem uses a public key to encrypt and a
private key to decrypt

Answer 24

public/Asymmetric

Question 25

composition and ordering of the exchanged messages

Answer 25

protocol

Question 26

What is a symmetric cryptosystem (private-key system)?

Answer 26

A system where the same key is used to both encrypt and decrypt messages. Correctness: Decrypting an encrypted message with the right key gives back the original message. Security: It’s hard to decrypt the message without the key.

Question 27

kerckhoff's principle

Answer 27

one should always assume that the adversary know the enryption/decryption algorithms and the resistance of the cipher to attacks must be based on only the secrecy of the key

Question 28

brute-foce attack

Answer 28

Try every possible key on ciphertext until readable text is obtained from the ciphertext On average, number of guesses is half the key space

Question 29

use knowledge of algorithm and/or plaintext patterns to “intelligently” decipher the ciphertext Exploit characteristics of algorithm to deduce plaintext or key Attacks differ based on amount of information known to attacker

Answer 29

Cryptanalysis

Question 30

the set of all possible keys that can be sued to generate a key

Answer 30

keyspace

Question 31

process of attempting to discover the plaintext or key

Answer 31

cryptanalysis

Question 32

what is a ciphertext-only attack?

Answer 32

an attacker only knows the ciphertext and tries to recover the plaintext or key

Question 33

what is a Known Plaintext Attack ?

Answer 33

an attacker knows some plaintext-ciphertext and tries to recover the plaintext of another ciphertext

Question 34

how can a known plaintext attack happen?

Answer 34

when an attacker learns or guesses plaintexts, such as from public releases or guessable message headers

Question 35

what is a chosen plaintext attack?

Answer 35

An attacker chooses plaintexts to be encrypted and gets the corresponding ciphertexts.

Question 36

What is a chosen ciphertext attack?

Answer 36

An attacker chooses ciphertexts to be decrypted and gets the corresponding plaintexts.

Question 37

What is an adaptive chosen text attack?

Answer 37

An attacker chooses each plaintext or ciphertext based on previous results, making the attack more powerful.

Question 38

What is computational security in encryption?

Answer 38

Encryption is secure if breaking it with current technology and resources is not feasible in a practical timeframe.

Question 39

What is perfect (information-theoretic) security?

Answer 39

Encrypted data reveals no information about the plaintext, regardless of the attacker’s computational power (e.g., one-time pad).

Question 40

all cassical ciphers are _______

Answer 40

symmetric

Question 41

exhaustive key search

Answer 41

try all possible keys and test to see if correct ; large keyspace prevents/helps against but it not sufficient

Question 42

what are substitution ciphers?

Answer 42

most common form of cipher; work by replacing each letter of the plaintext with another letter or even a random symbol. the most famous one is caesar cipher

Question 43

mono-alphabetic substitution (fixed replacement structure )

Answer 43

same letter is replaced with only one other ( always the same for given cipher message )

Question 44

caesar code decryption

Answer 44

Deciphering is done in reverse, with a left shift of 3. Replaces a letter another with an inverse alphabet shift: a previous letter in the alphabet

Question 45

shift cipher

Answer 45

shift by the number of postions. so the key is the number of postions to shift to

Question 46

a type of substitution cipher in which the mapping of the plaintext letters to ciphertext letters is an arbitarty permutation of the alphabet; 26! possible permutation key(s)

Answer 46

permutation cipher

Question 47

the study of the frequency of letters or groups of letters in a ciphertext; technique based on the idea that certain letters or letter combinations appear more frequently in a given language

Answer 47

frequency analysis

Question 48

what are transposition ciphers

Answer 48

the name given to an ecryption that involves rearrnaging the plaintext letters in a new order

Question 49

simple transposition

Answer 49

The key to the cipher is the number of rows and columns of a matrix. Encipher a message by writing the plaintext into the matrix by rows and reading the ciphertext out of the matrix by columns.

Question 50

a fundamental property of secure ciphers that aims to obscure the relationship between the ciphertext and the key; o leaks of information. You should not be able to find patterns

Answer 50

confusion

Question 51

refers to the process of spreading the influence of each plaintext element over a large portion of the ciphertext

Answer 51

diffusion

Question 52

another way to strengthen substitution ciphers by using different substitutions for different letter positions.

Answer 52

poly-alphabetic ciphers

Question 53

a polyalphabetic cipher in which the number of different substitutions r is also part of the key

Answer 53

Vigenère cipher

Question 54

when is cryptosystem secure and insecure

Answer 54

secure - best known attack is unknown insecure - any shortcut is known

Question 55

is it possible to make a completely unbreakable crytosystem

Answer 55

The Vernam cipher (one-time pad)

Question 56

rules of the one-time pad, which cannot be bronek if it used correctly

Answer 56

1. The key is as long as the message. 2. The key is random. 3. The key is never reused.

Question 57

two drawbacks of one-time pad

Answer 57

1. The key k must be as long as the message to be encrypted. 2. The same key must never be used more than once. (Hence the term “one-time”.)