The strength of a cryptocurrency is the share of global processing power it can muster in service of its ledger. An attack by a significant computational resource (botnet, mining pool, government supercomputer, etc.) could potentially reverse recent transactions or cause a fork of the currency. The reason the global share matters is that computational power is fungible, and may be used in service of any cryptocurrency.
The less computational power dedicated to mining a crypto, the more vulnerable it is to attack. Alt-coins that do not command as much hash power as Bitcoin would be first in line.
Processing power over time is governed by the price per computational unit and by the price of electricity. For example, a spike in electricity prices or transistor prices would increase mining costs, and therefore transaction fees, possibly disrupting the usability of the currency. A sudden drop in electricity prices or transistor prices would reduce mining costs, increasing the chance of attacks on a currency’s ledger.
Such attacks could be orchestrated, not necessarily to steal the underlying wealth of currency holders, but to accomplish secondary effects beneficial to the attacker. For example:
- A large investor in a particular crypto may attack a competing (smaller) crypto intruding in the space. This implies a first-mover advantage in the cryptocurrency space and stratification of crypto, with one per defensible economic niche. Eventually, the market may consolidate into one global crypto.
- Momentarily disrupt the functioning of a crypto at a critical moment in its development, for maximum PR effect, to spread fear & uncertainty to potential investors and adopters.
- Momentarily disrupt recent transactions to create uncertainty for retailers accepting the currency & consumers using it.
- Permanently disrupt or fork a smaller currency.
You would only need to bid on computing power for a short time to cause a major disruption. Miners are very sensitive to transaction fees, so would quickly respond to any change in market demand. Furthermore, cloud mining operations, and cloud computing in general, provide an easy way to quickly spool up computing power for a short time, disrupt the target crypto, and spool down, thus minimizing costs.
Alt-coins based on a permissionless, global blockchain are very vulnerable to this type of attack. Eventually, even Bitcoin itself may come under attack should there be a revolutionary development in computational hardware, exploited by a group of early adopters, or a government willing to throw massive fiat to kill it once and for all.