📌 What is a Public Blockchain?

A public blockchain is permissionless. Anyone can join the network and read, write, or participate within the blockchain. A public blockchain is decentralized and does not have a single entity which controls the network. Data on a public blockchain are secure as it is not possible to modify or alter data once they have been validated on the blockchain. Bitcoin and Ethereum are well-known examples of a public blockchain.

📌 What is a Private Blockchain?

A private blockchain is a permissioned blockchain. Private blockchains work based on access controls which restrict the people who can participate in the network. There are one or more entities which control the network and this leads to reliance on third-parties to transact. In a private blockchain, only the entities participating in a transaction will have knowledge about it, whereas the others will not be able to access it. Hyperledger Fabric of Linux Foundation is a perfect example of a private blockchain.

💡 Similarities between Public and Private Blockchains

• Both function as an append-only ledger where the records can be added but cannot be altered or deleted. Hence, these are called immutable records.
• Each network node in both of these blockchains has a complete replica of the ledger. Both are decentralized and distributed over a peer-to-peer network of computers.
• In both, the validity of a record is verified, thus providing a considerable level of immutability, until the majority of the participants agree that it is a valid record and reach consensus. This helps to prevent tampering with the records.
• Both blockchains rely on numerous users to authenticate edits to the distributed ledger thus helping in the creation of a new master copy which can be accessed by everyone at all times.

💡 Differences between Public and Private Blockchains

• The order of magnitude of a public blockchain is lesser than that of a private blockchain as it is lighter and provides transactional throughput.
• Level of access granted to participants—In a public blockchain, anyone can take part by verifying and adding data to the blockchain. In private blockchains, only authorized entities can participate and control the network.
• A public blockchain is decentralized, whereas a private blockchain is more centralized. Examples—Hyperledger and Ripple.
• Consensus algorithms such as Proof of Elapsed Time(PoET), Raft, and Istanbul BFT can be used only in case of private blockchains.
• Transactions per second are lesser in a public blockchains when compared to private blockchains. As the number of authorized participants is less in a private blockchain, it can process hundreds or even thousands of transactions per second.
• Public blockchains are trustless, and in a private blockchain setup, participants must not trust one another. In a private blockchain, the validity of records cannot be independently verified as the integrity of a private network relies on the credibility of the authorized nodes.
• A public network is more secure due to decentralization and active participation. Due to the higher number of nodes in the network, it is nearly impossible for "bad actors" to attack the system and gain control over the consensus network. A private blockchain is more prone to hacks, risks, and data breaches/manipulation. It is easy for bad actors to endanger the entire network.
• A public blockchain consumes more energy than a private blockchain as it requires a significant amount of electrical resources to function and achieve network consensus. Private blockchains consume a lot less energy and power.
• In a public blockchain, it is necessary to grant access to a centralized authority to oversee the entire network, thus making it a private blockchain at this point. In a private blockchain, anyone who is overseeing the network can alter or modify any transactions according to their needs.
• In a private blockchain, there is no chance of minor collision. Each validator is known and they have the suitable credentials to be a part of the network. But in a public blockchain, no one knows who each validator is and this increases the risk of potential collusion or a 51% attack