Flashcards in Hash Functions Deck (11):

1

## what is weak collision resistant?

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

2

## what is strong collision resistant?

### computationally infeasible that any 2 numbers have the same hash value

3

## hash function provides what function

### message authentication

4

## what is the result of hash function

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

5

## Hash function weakness

###
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

6

## what characteristic would make hash values more difficult to exploit

###
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

7

## What's name of most common NIST-supported hash algorithm

###
SHA-1

originally 160 bits, but now 256-512 bits

the max message size must be less than 2^(128)

8

## Describe the SHA512 algorithm

###
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

9

## Advantages of Hash-based Message authentication

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

10

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

###
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

11