Hash

Question 1

Hash function general use(6)

Answer 1

1. Digital Signatures
2. Public key encryption
3. Integrity verification
4. Message Authentication
5. Password protection
6. Key Agreement protocols and cryptographic protocols

Question 2

Hash functions main uses(6)

Answer 2

1. Identify files and detect modified files in cloud storage
2. Identify files in repo in Git
3. Detect modified files in Host based Intrusion Detection
4. Detect malicious data in Network based Intrusion Detection
5. Prove digital artefacts not modified
6. Blockchain ensure integrity of previous transactions

Question 3

Hash function overview + 3 desired properties + 3 other properties

Answer 3

• many to one function, NOT one to one
• different files hash to SAME value
• Any length to Short Fixed length
  1. Preimage Resistance
  2. Second Preimage resistance
  3. Collision Resistance
• 1 and 2 hardest to crack
• for n bit function, security in finding hash collision at n/2 bit strength

1. Fast for integrity, slow for password hashing
2. Long length, at least 256 bits long
3. Unpredictable, minute change in M affects many bits in hash

Question 4

Preimage resistance

Answer 4

• computationally infeasible to revert output to get input

Question 5

Second preimage resistance

Answer 5

• given input and digest it hashes to, computationally infeasible to find another input with same digest
  -attacker no control over first input

Question 6

Collision resistance

Answer 6

• no one able to produce two different inputs that hash to same output
• attacker can choose two inputs

Question 7

Hash VS Encryption

Answer 7

Hash: ensures Integrity
Encryption: ensures confidentiality

Question 8

Using SHA256 on website, sha256sum

Answer 8

1. Click button to download file
2. Use SHA256 hash algorithm to hash downloaded file
3. Compare digest with string on webpage to verify right file
   * NOT always mean files not tampered despite hash being real secured, website may be hacked

Question 9

MD5 dumpster fire

Answer 9

• Colliding Certificates with different identities

Question 10

Final Notes on Hash

Answer 10

• Use KECCAK
• do not create or believe in proprietary hashes
• Sign hash of large file for file integrity
• use SHA256, SHA512

Question 11

Linear Algebra recap

Answer 11

1. Form system of equations
2. Gaussian elimination to get RREF
3. Start from bottom row
4. Find free variables and solve