Blockchains can be attacked in a variety of different ways. However, the most famous potential blockchain attack is the 51% attack against Proof of Work (PoW) blockchains. This attack takes advantage of the fact that PoW consensus is based on majority vote (with votes expressed as hashrate). If an attacker controls the majority of the vote, then they control the blockchain.
To protect themselves against 51% attacks, PoW blockchain networks try to attract as much hashrate as they can. However, hashrate is a finite resource, because computational power (and especially computational power that its owners are willing to devote to mining) is limited.
As a result, some blockchains have a lot of hashrate and protection and others do not.
As blockchain becomes more popular, these security limitations stand in the way of growth and scalability. Some blockchains are using sidechains to tap into the security of secure, stable blockchains, but the Polkadot Network is taking a different approach.
The Security of Sidechains
Cross-chain functionality has become common in an attempt to expand the functionality and scalability of a blockchain. Using sidechains “pegged” to a particular blockchain, it is possible to combine the stability of one blockchain (like Bitcoin) with the features of another (like support for smart contracts).
Sidechains also enable a blockchain to scale because - in most cases - the transactions performed on a sidechain are not recorded on the mainchain.
While sidechains have their benefits, security is a major limitation. Two sidechains that are pegged together are each responsible for their own security. If one of the networks is the victim of a 51% attack, the value of the associated cryptocurrency can fall, and the terms of the peg can change or fail entirely.
Polkadot’s Shared Security Model and Blockchain Security
The Polkadot network consists of up to one hundred parallel chains or “parachains”. This design allows the network to achieve the same scalability and functionality benefits as sidechains because Polkadot puts minimal constraints on how a parachain works.
The main requirement that Polkadot places on a parachain is that any state transitions performed on a parachain must be communicated to and verified by the Polkadot Relay Chain.
Under this model, the responsibility for securing all of the parachains in the Polkadot Network is transferred to the validators of the Polkadot Relay Chain. In fact, most parachains do not even need to implement their own security.
By centralizing the responsibility for validation with a single group of validators, Polkadot removes the need for different blockchains to compete for limited hashrate. Anyone with an investment in a parachain (creators, users, etc.) has incentive to join the group of Polkadot Relay Chain validators by acquiring and staking DOT tokens. By doing so, they further diversify the set of block creators, making it more difficult for an attacker to gain control over the consensus process.
Balancing Scalability and Security
Blockchain networks are designed to be decentralized, meaning that much of blockchain security is based on incentives. As long as it is in the best interests of the blockchain’s users to behave correctly, the blockchain can remain stable and relatively secure against attack.
Polkadot has built a system where different blockchains are incentivized to pool their resources to secure the system as a whole. This enables the network to scale securely in a way that isn’t possible using sidechains.