Hey everyone. Welcome back to the course. In the last lecture, we discuss some of the significant challenges being faced by the industries in the implementation of the blockchain. Now, let's go and understand some of the attacks over the blockchain. One of the significant attacks talked about in length with blockchain is 51% attack. This attack states that if a group of miners control more than 50% of the network's mining hash rate, or computing power, they can take over the network.

It was a speculative attack over the Bitcoin blockchain, but recently we have seen some of the major block chains being attacked using the 51% attack. One of the blockchain projects which came under this attack was Bitcoin gold. Bitcoin gold is one of the top 30 crypto currencies in the world. suffered a 51% attack, losing around $18 million. This attack states that if a single miner or a group of miners are having more hashing power than the other miners connected to the chain, which means 51% of the hashing power of the complete chain, then the miners can take over the network. The potential damage could be that the attacking miners can prevent new transactions, gaining confirmations, allowing them to halt payments between users.

Attackers would also be able to reverse the transactions that were confirmed while they were in control of the blockchain network, meaning they could double spend coins which blockchain is trying to avoid. Bitcoin was almost under attack in 2014 when one of the mining pool known as G hash.io. Briefly exceed 50% of the Bitcoin network computing power, leading the pool to reduce their share voluntary. The second major attack on blockchain is the eclipse attack. The attack is based on the distributed application architecture that partitions that task or workloads among peers without the need for a central coordinating server, or stable hosts. blockchains are based on peer to peer computer node connections.

And if you cripple a node in such a way that it is not able to talk to the other nodes, then it's possible to take advantage of that node and make double spending transactions using the cripple node. The Eclipse attack is likely to happen because of a design strategy flaw in the blockchain, such as peer identity and peer selection process. For example, Simple. Bitcoin has eight outgoing connection, which means that if I am connecting to Bitcoin, my computer will only know about eight different peers to whom I am connected. Now, if these eight peers are under attackers control, then I am just going to verify the transactions that they send me. Suppose the attacker creates a fake transaction on all these eight peers.

My view of the network is only available through these eight peers, which is why my computer will see this fraudulent transaction as a validated transaction coming from these connected peers, as all of them will be communicating the same transaction. Ethereum blockchain also faces the same problem, where they have 13 peers connected or 13 outgoing peer connections. The potential damage which can be done but this attack is that the attacker can Cause double spending of the transactions because a node they are eclipsing won't be able to see what is happening in the complete network. Moreover, attackers can attack the second layer protocols when an attacker can obtain the product and services without actually paying for the same to the victims. They will trick the victims into thinking that the payment channel is still open. While the non Eclipse part of the network says that the payment channel is closed.

Even smart contracts are not safe from this kind of attack, as the users will see the inconsistent view of the blockchain. Sybil attack is the third major attack over the blockchain. In this attack, the attacker attempts to fill the network with the client nodes which they control. If this happens, then whenever you are connecting To the blockchain, you are most likely to communicate with an attackers node. For example, Bitcoin never keeps track of the nodes for any purpose. If somehow attacker isolates a node from the owners network by spinning up his nodes, then the attackers can achieve the Sybil attack.

This kind of attack can make sure that the only attacker is reeling the blocks which he creates, an attacker can also refuse to relay the blocks by other nodes. Moreover, by isolating the node, the attacker can also perform much more sophisticated attacks in continuation of the Sybil attack. Another major attack over the blockchain is time jacking attack. This attack is an extension of the Sybil attack and it depends upon the functionality in which each node Is internally maintaining a network time counter. This counter is based on the median time of the nodes which are connected to the blockchain and shared when the node share the version message with the peers while establishing the connection. The network time inside the blockchain has a feature that if the network medium time is diverting 70 minutes from the system time, then the blockchain shifts its time counter from the network medium time to the system time.

If the nodes shift to the system time, then there will be a significant difference between the time counter each node is keeping the network time counter, which is utilized to validate new blocks. And as the precaution nodes reject any block timestamp that is greater than two hours from the current timestamp nodes in Take the network median time for the acceptance and the rejection of the blocks in a way putting upper and lower bounds for the acceptable range of the block timestamps. If the attacker can make time jacking attack, then the attacker would be able to create a new block with the timestamp set ahead of the real time. Let's take an example where an attacker is making time jacking attack over a single node where the attacker manages to slow down the target time. Let's take the system time ahead of the network time by 60 minutes and acceptance time for the blocks by miners as 120 minutes the attacker will create a new block with the timestamp set about 190 minutes ahead of the real time.

Now as we know the target node is shifted to the system time because the time for the target node Was diverting from the network median time. So the target will see the block timestamp as 60 minutes past network time, and therefore reject this new block. But as the timestamp is 190 minutes, and the network median time is about 70 minutes, the mining nodes will see this block below the 120 minutes time limit and accept this block. This will isolate the target from the network's normal transaction processing, as the network keeps on seeing the blocks within the time limit, but the target node is always rejecting the block because of a greater timestamp. The attacker would continue this attack indefinitely until the target node itself creates a block or clocks are reset by some intervention. This kind of attack can lead to double spending as the attacker can time jack a targeted node Although when the targeted node again sees the network time, the changes done by the attacker will be reverse.

Still, the attacker has a good time window available for attacking the data or doing a double spending transaction over the blockchain. That was all about some of the major attacks over the blockchain. In the next lecture, we are going to discuss how the blockchain is going to take over the world and what kind of effects the globe can experience by introducing blockchain as a global software technology. For any questions feel free to contact us at info at the rate v two blocks.com