Cryptography

Question 1

What is a Symmetric algorithms (secret key)?

Answer 1

use the same key for both encryption and decryption.

has dificulties distributing keys, and maintaining large number of keys

Question 2

What is a Asymmetric algorithms (public Key)?

Answer 2

use different keys for encryption and decryption

is an encryption scheme where each person creates a pair of keys

Each person’s key is published freely while the private key is secret.

Question 3

What are stream ciphers

Answer 3

convert one symbol of plaintext directly into a symbol of ciphertext

Question 4

What are block ciphers

Answer 4

encrypt a group of plaintext symbols as one block

Question 5

What are the advantages of block encryption

Answer 5

High diffusion Immunity to tampering

Question 6

What are the differences between singe and multi use keys

Answer 6

Key is only used to encrypt one message Key used to encrypt multiple messages

Question 7

How can you avoid delays with having a large number of keys.

Answer 7

Key Distribution Center (KDC)

Question 8

AES uses a block size of? AES allows keys of sizes?

Answer 8

AES uses a block of 128-bits. AES allows keys of size 128-bits, 192-bits, and 256-bits, with 10, 12, 14 rounds, respectively.

Question 9

What is Electronic Code Book (ECB) ?

Answer 9

The simplest way of using a block cipher like AES to encrypt (with the same key) each block in the plaintext.

Question 10

What is Cipher Block Chaining (CBC) ?

Answer 10

Block encryption modes such as CBC attempt to hide ECB by chaining blocks together in some manner

Question 11

What is Public Key encryption?

Answer 11

public key system is the identification of a one-way function: easily computed, but difficult to invert without additional information.

Question 12

What are the uses of the public key encryption?

Answer 12

Encryption / Decryption Digital signature Session Key Exchange

Question 13

Efficiency of Encryption

Answer 13

Devising an asymmetric encryption algorithm depends on identifying a one-way function, easy to compute but hard to invert. • Public key systems largely solve the key distribution problem. • Asymmetric algorithms are generally much less efficient than symmetric algorithms.

Question 14

What are keyed Ciphers?

Answer 14

Encryption and decryption algorithms use a key K . The key can be a series of bits used in a mathematical algorithm (used for E/D) or the knowledge of how to manipulate the plaintext.

Question 15

What is breakable encryption?

Answer 15

An encryption algorithm is called breakable if, given enough time and information, a cryptanalyst can recover the plaintext. Most encryption algorithms are breakable since the analyst can try all keys systematically. Being breakable does not mean that it is feasible to break

Question 16

What is a substitution cipher?

Answer 16

is one in which each symbol of the plaintext is replaced with another symbol. 1-1 mapping

Question 17

What is a Caesar Cipher?

Answer 17

monoalphabetic cipher in which each letter is replaced in the encryption by another letter a fixed “distance” away in the alphabet

Question 18

What is Polyalphabetic Substitution Cipher?

Answer 18

Start with a key string: “monitors to go to the bathroom” and a plaintext to encrypt: “four score and seven years ago.” Align the two texts, possibly removing spaces:

A Vigenere cipher

Question 19

Possible read: Confusion and diffusion

Answer 19

Confusion:
o Transforming information in plaintext into cipertext in a key-dependent manner so that an interceptor cannot readily extract it
o Complicating the relationship between the statistics of the ciphertext and the value of the encryption key
• Diffusion:
o Spreading the statistics of the plaintext widely over the ciphertext by allowing each plaintext symbol to affect the value of multiple cyphertext symbols (e.g., letting a ciphertext symbol eaulas to average of multiple plaintext symbols so that a plaintext symbol will end up being used in the avergaing operation of multiple ciphertext symbols)
o Complicating the statistical relationship between the plaintext and ciphertext symbols.

Question 20

hash functions what do they do?

Answer 20

converts variable-sized text into a small datum, usually a
fixed size integer.

Qualities

it is difficult to construct a text that has a given hash (the one-way property),
it is difficult to modify a given text without changing its hash (Integrity),
it is unlikely that two different messages will have the same hash (the collision-free
property).

Question 21

What is diffusion?

Answer 21

Diffusion: if the hash function does not exhibit the avalanche effect to a slight change of input, then it has poor randomization, and thus a cryptanalyst can make
predictions about the input, being given only the output.

Question 22

What is HMAC?

Answer 22

Keyed-hash message authentication code: a message authentication code that uses a cryptographic key in conjunction with a hash function
• Runs data and authentication key through hash function twice

Question 23

Diagram for digital signature for messages

Answer 23