Page 116 - Handbook of Deep Learning in Biomedical Engineering Techniques and Applications

104 Chapter 4 A critical review on using blockchain technology in education domain

chain. Some nodes can accept new rules, whereas others
follow old rules resulting in forks. Forks can be categorized
as soft forks and hard forks. Soft forks are backward compat-
ible. When a new rule was added in Bitcoin network to
support time-locked refunds, it resulted in a soft fork. The
chain of nodes that follow new rules (updates nodes) can
interact with those that follow old rules. Hard forks are not
backward compatible. Here nodes that follow new rules
follow a separate chain. This chain will not interact with nodes
that do not follow new rules. In Ethereum, a flaw in smart
contract resulted in Decentralised Autonomous Organisation
(DAO) attack. To overcome this flaw, hard fork was con-
structed to create a new chain. Joint consensus mechanism
can be used to overcome this attack.
(b) Double spending attacks: Here the digital currency is prom-
ised to (spent for) two parties but delivered to only one. This is
prevented in BC by using only unspent output of one transac-
tion as input to the next. Timestamping (chronological order)
of transactions is considered. As the number of confirmations
(verifications) increases, the probability of double spending
decreases. Double spending can occur due to attacks like
51%, selfish mining, and eclipse attacks.
• Finney attack: Consider a miner mines a block with his
transaction and maintains it secretly. When an uncon-
firmed transaction is accepted by the victim, then money
can be sent to him. Later the secretly mined block is
published, which results in double spending.
• Race attack: The attacker submits an unconfirmed trans-
action1 to a victim. Attacker also broadcasts another
transaction2 to the network. Although it gives an illusion
to the victim that trasaction1 is first, it is never submitted
by attacker to the BC network. Vector76 is a combination
of Finney and race attacks.
• Alternative history attack: Here the hacker sends coins to
sellers and simultaneously creates a transaction to return
the same. This may result in attacker creating a longer
chain on return of coins. This attack requires huge compu-
tation power from the attacker.
• 51% (majority) attack: If a group of miners control over
51% nodes in the BC network, double spending or denial
of service can occur. A malicious miner performing a
double spend will try to create a regular transaction
spending and also another spending in a private chain.
Double spending is different from regular transactions in
two ways: