Internet and Its Uses · 4 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 3% of your exam marks.
Blockchain and cryptocurrency are a newer addition; questions are growing as the topic becomes more established.
The clever property of a blockchain is that an attacker cannot simply change an old transaction and get away with it. Several mechanisms work together to make tampering nearly impossible.
If an attacker modifies a transaction inside an old block:
prev value.prev no longer matches the previous block's actual hash.This means a tampered block forces the attacker to rewrite every block after it, not just one. The longer ago the block was, the more work this is.
Adding a new block (mining) is computationally expensive on purpose. The network requires the new block's hash to meet a specific mathematical condition (e.g. starting with a certain number of zeros). To find a hash that meets the condition, a miner must try many random small adjustments (called the nonce) until one works.
For the Bitcoin network in particular, finding a valid block hash takes the entire global mining network roughly 10 minutes. To rewrite the last 6 blocks, an attacker would need to redo 60 minutes of the world's combined mining work, while also keeping up with honest miners who keep extending the legitimate chain. This is practically impossible for anyone short of a state-level adversary.
Many computers hold a copy of the blockchain. Even if one attacker quietly rewrites their own local copy, they still have to convince the rest of the network to accept it. The rule of consensus is that nodes accept the longest valid chain. An attacker's tampered chain will be shorter than the honest one (because the honest network is mining ahead), so the network simply ignores it.
To beat the honest network, an attacker would need to control more than half of the entire network's mining power (the "51% attack"). For large blockchains like Bitcoin, this would cost billions of pounds in hardware and electricity.
Each transaction is digitally signed by the sender using their private key. Anyone in the network can use the sender's public key to verify that the signature is valid (asymmetric encryption, topic 6). Without the private key, no one can forge a transaction from someone else's wallet.
A blockchain is tamper-resistant because: