In a Proof of Replication, a storage miner proves that they are storing a physically unique copy, or replica, of the data. Proof of Replication happens just once, at the time the data is first stored by the miner.
As the storage miner receives each piece of client data, they place it into a sector. Sectors are the fundamental units of storage in Filecoin, and can contain pieces from multiple deals and clients.
Once a sector is full, a CommD (Commitment of Data, aka UnsealedSectorCID) is produced, representing the root node of all the piece CIDs contained in the sector.
Next, a process called sealing takes place.
During sealing, the sector data (identified by the CommD) is encoded through a sequence of graph and hashing processes to create a unique replica. The root hash of the merkle tree of the resulting replica is the CommRLast. (Learn more about merkle trees in our Content Addressing on the Decentralized Web tutorial.)
The CommRLast is then hashed together with the CommC (another merkle root output from Proof of Replication). This generates the CommR (Commitment of Replication, aka SealedSectorCID), which is recorded to the public blockchain. The CommRLast is saved privately by the miner for future use in Proof of Spacetime, but is not saved to the chain.
The encoding process is designed to be slow and computationally heavy, making it difficult to spoof. (Note that encoding is not the same as encryption. If you want to store private data, you must encrypt it before adding it to the Filecoin network.)
The CommR offers the proof we need that the miner is storing a physically unique copy of the client's data. If you store the same data with multiple storage miners, or make multiple storage deals for the same data with a single miner, each deal will have a different CommR.
The sealing process also compresses the Proof of Replication using zk-SNARKs to keep the chain smaller so that it can be stored by all members of the Filecoin network for verification purposes. We'll learn more about zk-SNARKs in a future lesson.