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According parity.io website:

Proof-of-Authority is a replacement for Proof-of-Work, which can be used for private chain setups.

And also:

The chain has to be signed off by the majority of authorities, in which case it becomes a part of the permanent record. This makes it easier to maintain a private chain and keep the block issuers accountable.

And also:

PoA networks are well-suited to regulated industries where entities responsible for maintaining the network (authorities) need to be known, rather than remain anonymous as in mining-based chains like Bitcoin and Ethereum.

It is underlined to use Proof-of-Authority for a private chain.

What is the reason? And Is it logical to use "Proof-of-Authority" for a "public" blockchain, in sense of keeping decentralization?

4

According white papaper of bitcoin - Screenshot Bitcoin

In Bitcoin or for any public blockchain privacy is maintained by not exposing which public key is associated to which user. So users can participate in the blockchain network without disclosing their identity. In PoA networks all the authorities need to disclose their identities while participating in the network.

In the case of the public blockchain, all the users can participate in the consensus process while in the case of PoA network the consensus process is controlled by pre-selected nodes(Authorities). This kind of blockchain is called as consortium blockchain.

Public blockchains are fully decentralized whereas consortium blockchains are partially decentralized.

Therefore, it is not logical and we can not use Proof Of Authority(PoA) as a consensus algorithm for public blockchain.

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    Moreover, in case of using a centralized consensus such as PoA, the pre-selected, known and trusted validators are able to remove all transactions history that collapse immutability property of blockchain. So, in my opinion, there is no reason to use consortium blockchain in which blockchain is updated and maintained by a centralized manner. – Questioner Sep 24 '18 at 12:48
  • Do you have any reference for the same? – Soham Lawar Sep 24 '18 at 12:52
  • The reference for ability of removing transactions history by PoA validators? Logically, they can do it since every new transaction and block must be confirmed by those validators. You can assume situation of rogue or collusion of PoA validators when they decide to remove / change all transactions history. – Questioner Sep 24 '18 at 12:57
  • "the case of PoA network the consensus process is controlled by pre-selected nodes(Authorities). This kind of blockchain is called as consortium blockchain.": What if this authority is given to all the nodes, How would it be a consortium blockchain then? – Arpit Bajpai Dec 18 '18 at 17:16

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