Hash Function

takes any string as input
fixed-sized output(usually 256 bits)
efficiently computable

security properties:

collision-free
hiding
puzzle-friendly

SHA-256

Hash Pointer

Merkle Tree

Digital signature scheme

Only you can sign, but anyone can verify

• (sk, pk) := generateKeys(keysize)
• sig := sign(sk, message)
• isValid := verify(pk, message, sig)

  Public keys as Identities

  A useful trick as Bitcoin do

A Simple Cryptocurrency Example

• GoofyCoin
  1.Goofy, can create new coins whenever he wants and these newly created coins belong to him
  2.whoever owns a coin can transfer it on to someone else
  
  double‐spending attack — Alice is spending the same coin twice.
• ScroogeCoin
  Scrooge publishes an append‐only ledger containing the history of all the transactions that have happened.
  ScroogeCoin block chain.
  
  CreateCoins transaction.
  
  A PayCoins Transaction.