Hash Functions

Question 1

what is weak collision resistant?

Answer 1

given m1, it’s computational infeasible to find another m whose has value is the same as m1

Question 2

what is strong collision resistant?

Answer 2

computationally infeasible that any 2 numbers have the same hash value

Question 3

hash function provides what function

Answer 3

message authentication

Question 4

what is the result of hash function

Answer 4

given a message digest of any size, it outputs a fixed size hash of 512-1024 bits

Question 5

Hash function weakness

Answer 5

pigeon hole principle with birthday paradox can compute the probability that you can violate the weak collision and have 2 hash values for different m’s.

For an attacker to have the likelihood of finding another hash value same as victim’s, they will have to make sqrt(hashBitSize) guesses

Question 6

what characteristic would make hash values more difficult to exploit

Answer 6

longer hash values
today hash values are 128 bits, which means the guesser would have to make
X^(128/2) guesses to make likelihood of guessing the correct hash value > 50%. this is deemed computational infeasible

Question 7

What’s name of most common NIST-supported hash algorithm

Answer 7

SHA-1
originally 160 bits, but now 256-512 bits
the max message size must be less than 2^(128)

Question 8

Describe the SHA512 algorithm

Answer 8

entire block to be hashed includes original message, length of message, and 10…0 padding. cut into blocks of 1024.
An Injection vector is used, and first block is hashed and fed into next block to be hashed, and subsequently fed into next block before hashing
80 rounds in which each round includes circular shifts, and boolean AND,NOT, OR, etc. operations with the previous hash, key, and message block

Question 9

Advantages of Hash-based Message authentication

Answer 9

Hash functions are efficient to compute and hash library code is widely available

Question 10

Why not use SHA-1 for Hash-based Message authentication? what instead is used?

Answer 10

SHA-1 doesn’t rely on a secret key
HMAC is used because it incorporates secret key that’s prepended to original message, ipad and opad are XOR’d to eliminate irregularities in secret key

Question 11

Why is HMAC preferred over SHA to guard against collision attacks?

Answer 11

It’s much harder to guess the secret key that’s in HMAC as long as HMAC is one-way and collision resistant