Bitcoin

Question 1

Difficulty in Bitcoin

Answer 1

Adjusted so that block generation time takes 10 mins
~ time = difficulty * 2^32/hashrate
If previous 2016 blocks took less than 2 weeks => difficulty increaseed
more => diff increased

Question 1

Merkle trees

Answer 1

Only merkle root is published in block:
If user wants to confirm transaction is in block he is provided with each node that he needs to compute the root => log2(n)

Question 2

Proof of Work Header

Answer 2

block version, hash of prev block, merkle root, time stamp, current target, nonce

Question 3

Common prefix

Answer 3

Each honest participants local chain without newest k block is prefix to local chain of other participant

Question 4

chain quality, chain growth

Answer 4

cq :If an honest majority exists the chain will contain atleast some blocks mined by an honest party
cg: blockchain will grow eventually

Question 5

Double spending if confirmation skipped

Answer 5

1. send tr_v to vendor and tr_a to the network
2. propagate tr_a to network with helper nodes
3. no detection required

Question 6

countermeasures to double spend

Answer 6

Listen to the network: Problem attack can get tr_v to neighbour nodes of vendor first
Get help in network: Observers in the network check if transaction is in network but requires multiple observers
Forward first double spending transaction

Question 7

selfish mining

Answer 7

Mine on your own “secret chain”. If other miners discover a block publish your own block and try to transmit it faster. Else adopt new block.

Question 8

Eclipse attack

Answer 8

Big Scale Attacker that has control over many IP addresses
=> DoS Bitcoin client and thus control their view of the blockchain
Populate tried tables with your own IP addresses and new table with non existing IP addresses => Victim can only connect to adversary
Will continue until client is restarted.
Splits the mining power, can increase advantage, can double spend

Question 9

Eclipse attack counter measures

Answer 9

1: make sure that the same address hashes to the same bucket at the same location
2: avoid recency bias in choosing new addresses
3: make sure that ip address exists before replacing ip at new or tried
4: add new buckets

Question 10

Partial eclipse attack

Answer 10

Bitcoin uses inventory based request:
inv, getheader, getdata, sendheader, senddata
Clients wait 20 mins for block and 2 for transaction
Try to send many invs to victim to populate fifo queue => no getheader messages. No receive version message => exhaust
Implications:
1. split mining power
2. increase advantage => only 34% of computing power needed
3. bypass double spending countermeasures
4. can double spend (using weak miner)

Question 11

Partial eclipse countermeasures

Answer 11

1. Dynamic timeouts
2. handling transaction ads: filter ip, randomly choose sender
3. update block advertisements. broadcast header instead of hash
   => header => getdata => block