70% of bitcoin network hashing power is from bitmain's asics. What if bitmain built backdoor access into the asics that can be triggered over the internet. They would effectively take over bitcoin and break it (assuming they had nefarious intention). What do you think?
It's a very common misunderstanding to think that majority attack (also known as 51% attack) gives attackers full control over Bitcoin. In reality, miners power is very limited, all they can do is decide which transactions are included in blocks and create forks by mining on top of previous blocks instead of the highest block. This leads to two malicious attacks - double spending, which is executed by forking blockchain to erase their own transactions, effectively rolling them back; and the second attack - black listing, when attackers decide to ban certain addresses and refuse to mine on top of blocks that included them in their blocks, which make those addresses frozen. Alternatively, attackers can mine empty blocks to freeze the entire network. However, attackers can not do things that violate network rules enforced by full nodes like creating more coins out of thin air or spending someone's coins without proper signatures. However, Bitcoin's protocol allows defense against those attacks, because every full node can change the network rules for themselves, so in case of majority attack nodes will be able to change mining algorithm to fire all existing miners. This is why it's important to keep the blocksize small, so regular users can run full nodes and keep miners in check, instead of trusting them like people trust banks.
