Proof of Space (PoS) is a means of showing that one has a legitimate interest in a service (such as sending an email) by allocating a non-trivial amount of memory or disk space to solve a challenge presented by the service provider. The concept was formulated in 2013 by Dziembowski.
It is a cryptographic technique where provers show that they allocate unused hard drive space for storage space. In order to be used as a consensus method, Proof of Space must be tied to Proof of Time. PoT ensures that block times have consistency in the time between them and increases the overall security of the blockchain.
Proofs of Space are very similar to Proofs of Work (PoW), except that instead of computation, storage is used. Proof-of-space is different from memory-hard functions in that the bottleneck is not in the number of memory access events, but in the amount of memory required.
After the release of Bitcoin, alternatives to its PoW mining mechanism were researched and PoS was studied in the context of cryptocurrencies. Proofs of space are seen as a fairer and greener alternative by blockchain enthusiasts due to the general-purpose nature of storage and the lower energy cost required by storage, but have been criticized for increasing demand for storage. Several theoretical and practical implementations of PoS have been released and discussed, such as SpaceMint and Burstcoin.
Proof of space can be thought of as a way to prove that you are keeping some storage unused on your hard-disk drive. Users of the Chia blockchain will “seed” unused space on their hard-disk drive by installing software which stores a collection of cryptographic numbers on the disk into “plots.” These users are called “farmers.” When the blockchain broadcasts a challenge for the next block, farmers can scan their plots to see if they have the hash that is closest to the challenge. A farmer’s probability of winning a block is the percentage of the total space that a farmer has compared to the entire network.
Proof of time requires a small period of time to pass between blocks. Proof of time is implemented by a Verifiable Delay Function that takes a certain amount of time to compute, but is very fast to verify. The key idea of a VDF is that they require sequential computation, and since having many parallel machines does not yield any benefit, electricity waste is minimized. There will likely be relatively few VDF servers (“Timelords”), as the fastest one will always finish first and it takes only one fast and fair Timelord on the network to complete a block and move the chain forward.