Intro

Question 1

What is consensus?

Answer 1

The process of reaching an agreed output between n processes which all have an input in_i and a output out_i

Question 2

Rules for consensus

Answer 2

1. Termination
2. Agreement: All nonfaulty process must output the same
3. Validity: If all inputs are same the output of nonfaulty must be the same

Question 3

Synchronus Distributed System

Answer 3

1. Messages are recieved within bound
2. Clock drift has bound
3. Step in process takes bounded amount of time

Question 4

Asynchronus Distributed System

Answer 4

1. No bounds on anything whatssoever
2. Internet is asynchronus system
3. Consensus can never be reached (weaker notion required)
   => Proof idea: Hard to distinguish very slow process from crashed one (other nodes would wait forever)

Question 5

Crash threat model

Answer 5

Every process is honest but can fail => n = 2f+1 processes needed to allow for f faulty processes

Question 6

Byzantine threat model

Answer 6

Processes can act arbitrarily (equivocation) => n = 3f+1 processes and f+1 rounds of agreement needed to allow for f byzantine processes (n^2 message compexity)

Question 7

BFT consensus

Answer 7

Hard to scale the number of consensus nodes only tested up to 20

Question 8

Cryptographic foundations of BTC

Answer 8

HashFunctions and their onewayness
Public Key crypto: Verifiable signatures

Question 9

RSA vs ECDSA

Answer 9

ECDSA keys smaller (better for BTC) and faster in keygen, compareable in signature generation but RSA significantly faster in signature verification

Question 10

Security of Payments

Answer 10

Fairness: Cant pay more than you own
Resistance to impersonation: Can’t authorize payments of other users
Non-repudiation: Users can’t deny payments that are registered by the system
Accountability: User that misbehaves can be punished

Question 11

Roles in Bitcoin

Answer 11

Miners: Do hash calculations and thus mine new blocks
Full-Nodes: Have a history of blocks and transactions and can respond to requests a user might have
(Lightweight-)Users: Regular users in the system that want to do transcations etc.

Question 12

UTXO Model

Answer 12

Every transaction input must be the output of another transaction.
Thus if a user wants to use to pay he might need to split a previous transcation or combine multiple Unspend Transactions Outputs(UTXO).

Question 13

Mining

Answer 13

Miners calculate hashes of so that Hash(prev_hash, nonce, F(Tx)) < Difficulty

Question 14

Bitcoin Security Pillars

Answer 14

1. All transactions are publicly announced and signed with the private key of the sender that should match the public key of the UTXO
   => Non-repudiations, Resistance to Impersonation, Non repudiation
2. Transactions can’t be changed, are checked before being able to be included in a new block, are only confirmed after six blocks are built on the block
   => Fairness
3. Penelty system in place which disconnects neighbour nodes for 24 hours => Accountability