To achieve its headline feature of making transactions and contract data private, Quorum builds on Ethereum's existing transaction model, rather than completely redefining it.

Public transactions and public contracts are visible to everyone on the network, and make use of Ethereum's existing infrastructure—Quorum doesn't implement anything different in this respect. As well as these public transactions, Quorum offers the ability to mark transactions as private, making them visible only to the intended recipients.

This privacy is achieved using public-key cryptography, specifically by setting the recipient's public key in a new, Quorum-specific transaction parameter, privateFor. This allows the transaction to be encrypted, and therefore read-only by the owner of the private key.