What is the difference between public and private blockchains?

The distinction lies primarily in the accessibility and governance of the two types of blockchains. Public blockchains are designed to be open and available to anyone who wishes to participate, which means that anyone can join the network, read the blockchain, submit transactions, and become a validator. This openness encourages widespread participation and transparency.

In contrast, private blockchains, also referred to as permissioned blockchains, impose restrictions on who can participate. Access is limited to a select group of participants, requiring authentication for users to join the network. This feature is crucial for organizations that need to maintain control over their data and transactions, allowing them to create a more secure and private environment tailored to their specific needs.

The other options misrepresent essential characteristics of these blockchain types. The speed of blockchains can depend on numerous factors, including network size and design, rather than strictly whether they are public or private. While private blockchains indeed require user authentication, this is just one aspect of their design, not a defining distinction. The idea that private blockchains operate on a global scale while public blockchains do not is also misleading, as both types can have global implementations based on their design and use cases.