Most used MDC & MAC

Question 1

What does MD5 stands for?

Answer 1

MD5 stands for message digest 5

Question 2

What does SHA stands for?

Answer 2

SHA stands for Secure hash algorithm.

Question 3

When MD5 must not be considered to be used?

Answer 3

MD5 must not be considered to be used if collision resistance is required.

Question 4

How do MD5 and SHA-1 compare related to speed?

Answer 4

MD5 is 25% faster than SHA1

Question 5

How do MD5 and SHA1 compare related to simplicity?

Answer 5

Both are simple in all aspects

Question 6

How do MD5 and SHA1 compare regarding to security?

Answer 6

SHA1 offers better security agains brute force and birthday attacks

Question 7

What are the commonly used modification detection codes (MDCs)?

Answer 7

MD5 and SHA1

Question 8

Which are the commonly used message authentication codes (MACs)?

Answer 8

DES-CBC-MAC (Data encryption standard in cipher block chaining mode) and HMAC (Hashed message authentication code)

Question 9

How does CBC-MAC (cipher block chaining message authentication code) work?

Answer 9

A CBC-MAC is computed by encrypting a message in CBC mode and taking the last ciphertext block or a part of it as the MAC. This MAC do not need to be signed any further as it has already been produced using a shared secret K.

Question 10

What does CBC-MAC stands for?

Answer 10

cipher block chaining message authentication code

Question 11

what does DES-CBC-MAC stands for?

Answer 11

Data encryption standard in cipher block chaining mode

Question 12

What does HMAC stands for?

Answer 12

Hashed message authentication code

Question 13

Can CBC-MAC be used to check non-repudiation?

Answer 13

No since it is not possible to say exactly who has created a MAC (sender and receiver have the key k).

Question 14

With which kind of cipher can CBC-MAC be used?

Answer 14

This scheme works with any block cipher.

Question 15

How is the security of CBC-MAC against birthday attacks?

Answer 15

Good as an attacker does not know k the birthday attack is much more dificult to launch (if not impossible) allowing shorter MACs.

Question 16

How can CBC-MAC security be strengthened?

Answer 16

both parts should agree upon a second key k’ different from k and performin a triple encryption on the LAST block: MAC = E(K;D(K’;E(K;Cn-1))) doubling the key space while adding only little computing effort.

Question 17

Why are symmetric blocks ciphers not used to create modification detection codes(MDCs)?

Answer 17

relatively small block size of 64 bits offers insufficient security against birthday attacks and they require more computing than cryptographic hash function so they are more slow.

Question 18

Why construct message authentication codes (MACs) from modification detection codes (MDCs)?

Answer 18

Cryptographic hash function generally executes faster than symmetric block cphers and there are no export restrictions to cryptographic hash functions.

Question 19

What is the basic idea of constructing message authentication codes (MACs) from modification detection codes (MDCs)?

Answer 19

To “mix” a secret key k with the input and compute an modification detection code (MDC).

Question 20

How is the construction of HMAC?

Answer 20

HMAC construction is H(K || p 1 || H(K || p 2 || m)) where p1 and p2 are padding patterns to fill up the key to one input block of the cryptographic hash function.