Hash Functions

Question 1

What are the properties for cryptographically secure Hash Functions?

Answer 1

• Preimage resistance i.e. one-way
• Second preimage resistance
• Collision resistance

Question 2

What is preimage resistance?

Answer 2

Given a hash of a message, it must be infeasible to calculate the message prior to the hash.

Question 3

What is second preimage resistance?

Answer 3

Given two different messages, it should be infeasible to have their hashed messages be exactly the same.

Question 4

What is collision resistance?

Answer 4

Make it impossible to find any message pair such that their hashed messages are equal.

Question 5

How do you make sure collisions are avoided in hash functions?

Answer 5

• Use a strong hash algorithm
• Increase the output length
• Use salted hashing
• Increase hashing space

Question 6

What is a Birthday Attack?

Answer 6

The output of a hash must be long enough, else you will find a collision after approximately 2^(n/2) attempts. This then means that your bit length needs to be double the size of the desired security margin.

Question 7

How does Merkle-Damgard work?

Answer 7

• Pads the input to ensure that the input is a multiple of the block size
• Initialise a hash state
• Divide the input into blocks
• Iterate the compression function for all message blocks
• Output final hash

Question 8

How does the compression function work in-depth for SHA-256?

Answer 8

Compression function mixes 512 bit blocks of message into the current state to produce a new state.
It then expands the message into 64 32-bit words. Each round then performs permutation and mixing

Question 9

What do all SHA functions have in common?

Answer 9

All SHA functions define constant values that represent H0.