Exam 1

Question 1

name an advantage of perfect secrecy

Answer 1

key cannot be cracked or discovered

Question 2

name a disadvantage of perfect secrecy

Answer 2

difficult to communicate the key pad/ difficult to maintain syncronization

Question 3

what does the CIA stand for

Answer 3

Confidentiality
integrity
authentication

Question 4

what properties should and ideal block cipher have?

Answer 4

• encryption should be reversible ( Decryption should always return the original message and have a one to one mapping)
  -encryption/decryption should be easy with key and difficult otherwise
  -computing the mapping should be efficient

Question 5

what is a pseudo-random permutaion

Answer 5

key defines the permutation
permutation is random-looking
efficient ot compute

Question 6

what is the feistel network and how many block bits and key bits dows it require

Answer 6

A method of turning astrong PRF to a strong PRP in 4 easy steps
uses a 64 bit block and 56 bit ke

Question 7

what did claude shannon introduce

Answer 7

a substitution-permutation (S-P)

Question 8

what did claude shannon achieve

Answer 8

Confusion through substittion
Diffusion through permutation

Question 9

what is confusion and diffusion

Answer 9

confusion is obscuring the local correlation between plaintext and ciphertext
diffusion is hiding the plaintext statististics by spreading it oer a larger area of ciphertext

Question 10

what is the main limitation of ECB and what is the cause for this limitation?

Answer 10

message repetitions may show in cipher text due to the encrypted message blocks being independent

Question 11

what is CBC

Answer 11

Cipher Block Chaining breaks up repetition in plaintext by XORing each block with a random-looking value

it uses an initialization vector which is a one time vector sent along with ciphertext and gets sent unencrypted. it needs to be unpredictable

Question 12

what are the advantages of ECB

Answer 12

parallel encryption of blocks of bits
simple way of block cipher

Question 13

what is the main disadvantage of CBC

Answer 13

parallel encryption is not possible since every encryption requires a previous cipher

Question 14

what are the advantages of CTR

Answer 14

there is no relationship between cipher text and plain text
parallel execution of encryption is still possible

Question 15

what are the disadvantages of CTR

Answer 15

requires a lot of synchronisation for maintaining counters

can’t be used safely with small block length ciphers (3DES for example)

Question 16

what are osme otehr encryption modes (besides ECB, CTR, CBC)

Answer 16

Cipher feedback mode (CFB
output feedback mode (OFB)
XTS for random access applications
Offset codebook (OCD and Galios/counter (GCM for authenticated encryyption

Question 17

what are teh bit distributions for the rijndael AES cipher

Answer 17

128/192/256 bit keys
10/12/14 rounds
128 bit data

Question 18

what is the main difference between the AES and feistel network

Answer 18

unlike the feistel network, hte AES encryption uses the PRP round function

Question 19

what is the main similarity between AES and Feistel network

Answer 19

similar to feistel, AES uses a key schedule to make each round effectively different

Question 20

what are the 4 security services

Answer 20

authentication: assurance that the communicating entity is the one claimed

access control: prevention of the unauthorized use of a resource

data confidentiality: protection of data from authorised disclosure

data integrity: assurance that the data received is as sent by an authorized entity

non-repudiation: protection against denial by one of the parties in a communication

Question 21

what are some active attacks

Answer 21

masquerade: takes place when one entity pretends to be a different entity by capturing and replaying an authentication sequence

replay is the passive capture of a data unit and its subsequent retransmission

modification of messages

denial of service

Question 22

what are the 3 goals of crypto system designers

Answer 22

key space exponential in key length
infusible to brute force
make brute force the best strategy to crack the cipher