symmetric key encryption

Question 1

what is kerchoffs principle

Answer 1

encryption is secure , even if attacker knows everything apart from the key

Question 2

what does frequency analysis do

Answer 2

Frequency analysis counts the number of times
each symbol occurs
each pair of symbols
etc.
and tries to draw conclusions from this

Question 3

problem with one time key pads

Answer 3

key needs to be as long as the message
key can only be used once

Question 4

describe block ciphers

Answer 4

Modern ciphers work on blocks of plain text, not just a single
symbol.
They are made up of a series of permutations and
substitutions repeated on each block.
The key controls the exact nature of the permutations and
subsitution

Question 5

how does AES CBC encrpyt plaintext

Answer 5

overall uses previous ciphertext to encrypt the next block of plaintext to make a new ciphertext (by xoring PT with CT )
first block of plaintext is xored

Question 6

what is a problem that occurs with AES CBC

Answer 6

computationally expensive, want to encrypte or decrypt a block that is later on in the chain you have to computer previous blocks before

Question 7

how does AES CTR encrpyt plaintext

Answer 7

generates nonce and then nonce is incremented with a counter and then encypted and then the encrypted nonce is xored with plaintext to give the new cipher text block

Question 8

what is a problem that occurs with AES CTR

Answer 8

vunerable to malleability attacks, since xoring is done after encryption
if you have PT and CT then you can change CT to get what you want
(known plaintext attack)

Question 9

What is the AES CBC encryption notation

Answer 9

IV = random number (sent in the clear)
C1 = encrypt(B1 ⊕IV )
C2 = encrypt(B2 ⊕C1)
···
Cn = encrypt(Bn ⊕Cn−1)

Question 10

what is the AES CTR

Answer 10

Plain text: B1, B2, . . . , Bn
IV : random number (sent in clear)
Cipher text: C1, C2, . . . , Cn where
C1 = B1 ⊕encrypt(IV )
C2 = B2 ⊕encrypt(IV + 1)
···
Cn = Bn ⊕encrypt(IV + n−1)

Question 11

What is the AES CBC decryption notation

Answer 11

Receive IV
Receive cipher text C1, C2, . . . , Cn
Plain text is B1, B2, . . . , Bn, where
B1 = decrypt(C1) ⊕IV
B2 = decrypt(C2) ⊕C1
···
Bn = decrypt(Cn) ⊕Cn−1

Question 12

how to decrypt AES CBC

Answer 12

Receive IV
Receive cipher text C1, C2, . . . , Cn
Plain text is B1, B2, . . . , Bn, where
B1 = decrypt(C1) ⊕IV
B2 = decrypt(C2) ⊕C1
···
Bn = decrypt(Cn) ⊕Cn−1

Question 13

what is a known plaintext attacks

Answer 13

if i know plaintext i can change ctr if i know ciphertext and plaintext i can change the ciphertext to what i want it to be
EncCTR (M1) ⊕ (M1 ⊕M2)