Cryptography

Question 1

Define Cryptograhpy

Answer 1

Cryptography is the (art and) science of keeping information secure This is usually done by encoding it

Question 2

What is Cryptanalysis?

Answer 2

Cryptanalysis is the (art and) science of breaking a code

Question 3

What is Cryptology?

Answer 3

Cryptology is the branch of math needed for cryptography and cryptanalysis

Question 4

Which 4 areas of IS can Cryptography help with?

Answer 4

Confidentiality: only authorized persons are allowed to decode a message Authentication: receiver of a message (e.g. a password) should be able to ascertain its origin Integrity: receiver of a message should be able to verify that it hasn’t been modified Non-repudiation: a sender shouldn’t be able to falsely deny that they sent a message -We’re talking about messages here, but the principles can be applied to any information

Question 5

What is plaintext?

Answer 5

The original message

Question 6

what is encryption?

Answer 6

Disguising the content of a message

Question 7

What is ciphertext?

Answer 7

the encrypted message

Question 8

What is Decryption?

Answer 8

Turning ciphertext back into plaintext is called decryption

Question 9

What is a cryptographic algorithm?

Answer 9

the mathematical function used for encryption and decryption

Question 10

What is a restricted algorithm?

Answer 10

If security is based on keeping the algorithm secret, then it’s a restricted algorithm

Question 11

Why are restricted algorithms a bad idea?

Answer 11

Every time a user leaves a group, the algorithm has to be changed A group must develop their own algorithm; if they don’t have the expertise, then it will be subpar

Question 12

What is a cryptographic key?

Answer 12

taken from a large range of possible values, the keyspace used as additional input for the en-/decryption function to do the en-/decrypting

Question 13

What is the benifit of having the security based in the key, not in the details of the algorithm?

Answer 13

The algorithms can be published and analyzed by experts for possible flaws Software for the algorithm can be mass-produced Even if an eavesdropper knows the algorithm, without the key messages cannot be read

Question 14

What is a cryptosystem?

Answer 14

An algorithm together with all possible plaintexts, ciphertexts, and keys is called a cryptosystem

Question 15

What are symmetic algorithms aka conventional algorithms?

Answer 15

that encryption and decryption keys are identical Anyone who has the key can decrypt a message However, before two persons/systems can communicate, they need to agree on a key

Question 16

What are Asymmetric algorithms aka public-key algorithms?

Answer 16

The key used for encrypting messages is the public key The key used for decrypting messages is the private key

Question 17

How do public-key algos work?

Answer 17

The public key can be published, so that anyone can encrypt messages to a person/system The private key is only known to the receiver This only works if the private key cannot be calculated from the public key (in any reasonable time)

Question 18

Name 4 important encryption techniques

Answer 18

Substitution ciphers Transposition ciphers Stream ciphers Block ciphers

Question 19

What is a substitution cypher?

Answer 19

In a substitution cipher each character in the plaintext is substituted for another character

Question 20

What is a Ceasar cypher?

Answer 20

One of the earliest techniques to be used in the famous Caesar cipher In the Caesar cipher each character is shifted (by three characters in the original cipher)

Question 21

What is a Multiplicative Inverse?

Answer 21

Multiplicative inverse: a multiplicative inverse of x is the number we need to multiply to x to get 1.

Question 22

What is the problem with Monoalphabetic ciphers?

Answer 22

Monoalphabetic ciphers are not very secure and can be easily broken by statistical means: Different characters have typical frequencies in languages

Question 23

Homophonic substitution ciphers?

Answer 23

Homophonic substitution ciphers try to obscure the frequencies by mapping a character to more than one code For example, “A” could correspond to 5, 13, 25, or 56; while for “B” this could be 7, 19, 32, or 42

Question 24

What is the problem with homophonic substitution ciphers?

Answer 24

While this makes analysis a bit harder, it doesn’t hide all statistical properties With the help of a computer can usually be broken in a few seconds

Question 25

What is a polygram substitution cipher?

Answer 25

Instead of encoding single characters, a polygram substitution cipher encrypts groups of letters For example, “ABA” could correspond to “RTQ’, while “ABB” could correspond to “SLL”

Question 26

What is the problem with polygram substitution ciphers?

Answer 26

hides the frequencies of individual letters However, natural languages also show typical frequencies for n-grams (although the curve is flattened)

Question 27

What is a polyalphabetic substitution cipher?

Answer 27

A polyalphabetic substitution cipher uses multiple simple substitution ciphers

Question 28

What is the Vigenère cipher?

Answer 28

Adds a key repeatedly into the plaintext using numerical codes: A=0, B=1, . . . , Z=25 This is done modulo 26, i.e. if the result is greater than 26, then we divide the result by 26 and take the remainder C = M + K mod 26

Question 29

What is the problem wiht the Vigenere cipher?

Answer 29

Code can be broken by analyzing the period Knowing which letters were encoded with the same key allows the application of frequency methods again

Question 30

what is a Rotor Machine?

Answer 30

mechanical encryption devices were these machines implemented a complex Vigenère cipher A rotor is a mechanical wheel wired to perform a general substitution A rotor machine has a keyboard and a series or rotors, where the the output pins of one rotor are connected to the input of another

Question 31

What is a Transposition cipher?

Answer 31

Substitution ciphers on their own are usually not very secure That is why they are combined with transposition ciphers Transposition ciphers on their own are also not very secure In a transposition cipher the symbols of the plaintext remain the same, but their order is changed

Question 32

Explain a Simple Columnar Transposition Cipher

Answer 32

the plaintext is written horizontally onto a piece of graph paper of fixed width while the ciphertext is read off vertically

Question 33

What is the advantage of a Double Columnar Transposition cipher?

Answer 33

A simple columnar transposition can be broken by trying out different width/column lengths Putting the ciphertext through a second transposition enhances security In contrast to a simple substitution cipher, where a second application does not increase security

Question 34

What is the difference between between stream ciphers and block ciphers In symmetric algorithms?

Answer 34

Stream ciphers operate on the plaintext a single bit (or single character) at a time Simple substitution cipher is an example Block ciphers operate on groups of bits (or groups of characters) An example of an early block cipher is Playfair

Question 35

What is the avalanche effect?

Answer 35

This means that a slight change in the input (e.g. toggling one bit) has a major impact on the output

Question 36

Name two algorithms based on SP networks.

Answer 36

Data Encryption Standard (DES) Advanced Encryption Standard (AES)

Question 37

How does the Advanced Encryption Standard work?

Answer 37

Each input byte is sent through an S-box Then the rows of the matrix are shifted In a more complicated process the values in each column of the matrix are mixed In a final step the current round key is added This constitutes one round in AES

Question 38

How does the Data Encryption Standard work?

Answer 38

First step is an initial permutation of the plaintext Then there are 16 rounds of identical operations: A block is broken up into a left and a right half In function f, the data is combined with the (current round) key and then run through an SP network After the 16th round, there is a final permutation