Midterm

Question 1

ECB pros/cons

Answer 1

o Simple
o Don’t have to create/manage an IV
o Parallel encryption/decryption
o Reveals patterns in the plaintext – should not use

Question 2

CBC pros/cons

Answer 2

o Conceals plaintext patterns
o Requires sequential encryption
o Parallel decryption

Question 3

CTR pros/cons

Answer 3

o No need for padding
o Only have to implement encrypt function
o Parallel encryption/decryption
o Preprocessing able to generate the keystream in advance

Question 4

CFB pros/cons

Answer 4

o No need for padding
o Only have to implement encrypt function
o Parallel decryption

Question 5

OFB pros/cons

Answer 5

o No need for padding
o Only have to implement encrypt function
o Preprocessing able to generate the keystream in advance

Question 6

Why encryption padding?

Answer 6

So that all blocks are “full”
Disguise identical messages
Disguise message length

Question 7

Properties of cryptographic hash functions

Answer 7

Can be applied to a block of data of any size
Fixed-length output
Relatively easy to compute
One-way
Weak collision resistance
Strong collision resistance

Question 8

Collision attack

Answer 8

Two messages both hash to the same value

2^(n/2)

Question 9

Pre-image attack

Answer 9

Given a message m1, find a different message m2 such that hashes to the same value.
2^n

Question 10

Three ways to implement MACs

Answer 10

CBC-MAC (encrypt with CBC and use the last block as the MAC)
Hash the message and encrypt the digest
Hash the message with the shared key

Question 11

Why DH and RSA are secure

Answer 11

the discrete logarithm is a difficult problem

Question 12

Five steps a relying party uses to verify a certificate

Answer 12

o Integrity
o Expiration
o Revocation
o Usage constraints
• Basic Constraints
o Can the subject act as a CA?
o Is there a limit to the length of the certificate
chain?
o Limitation on key use – encryption or signing
o Ownership
• Does the entity presenting the certificate have access
to the associated
private key?
• Challenge for ownership of the key at the time of the
transaction