cryptographic hashing

Summary

Types of hashes

• unkeyed hash
  • simple hash function
  • eg. MD5, SHA1
• keyed hash(mac)
  • hash function using a key
  • eg. CBC-MAC, HMAC
• signature(public key mac)
  • signed using private key
  • relies on RSA’s homomorphic property

signature’s provide non-repudiation since only the sender has the private key

Hash properties

One pool birthday attack

• two messages have the same hash
• among messages and hashes

Two pool birthday attack

• number of collisions between two groups of bit chunks

Collision resistance/strength

Concept

Hash

• sent alongside the message
• check for data-origin authenticity

Birthday attacks

• finding a collision
• how many message do we have to check to have a high chance, of finding a collision

Alongside encryption

• encrypt-and-hash
• integrity of , but not of

• encrypt-then-hash
• ensures integrity of

• hash-then-encrypt

only suitable for

Storage structures

• hash array - matrix
• hash chain - linear
• merkle tree - tree

Storage operations

• insert(f)
  • insert file f to the structure
• verify(i,j)
  • check hashes for the files at index i to j
• update(i, f)
  • replace the file at i with f

Application

Non-collision resistant hash

• using xor properties