Proof of space (PoS) is a type of

consensus algorithm A fundamental problem in distributed computing and multi-agent systems is to achieve overall system reliability in the presence of a number of faulty processes. This often requires coordinating processes to reach consensus, or agree on some data va ...

achieved by demonstrating one's legitimate interest in a service (such as sending an

email Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronic ( digital) version of, or counterpart to, mail, at a time when "mail" mean ...

) by allocating a non-trivial amount of

memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered ...

disk space Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a computer ...

to solve a challenge presented by the service provider. The concept was formulated in 2013 by Dziembowski ''et al.'' and (with a different formulation) by Ateniese ''et al.''. Proofs of space are very similar to proofs of work (PoW), except that instead of computation, storage is used to earn cryptocurrency. Proof-of-space is different from memory-hard functions in that the

bottleneck Bottleneck literally refers to the narrowed portion (neck) of a bottle near its opening, which limit the rate of outflow, and may describe any object of a similar shape. The literal neck of a bottle was originally used to play what is now known as ...

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 Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the economic ...

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. In 2014, Signum (formerly Burstcoin) became the first practical implementation of a PoS (initially as proof of capacity) blockchain technology and is still actively developed. Other than Signum, several theoretical and practical implementations of PoS have been released and discussed, such as SpaceMint and Chia, but some were criticized for increasing demand and shortening the life of storage devices due to greater disc reading requirements than Signum.

Concept description

A proof-of-space is a piece of data that a prover sends to a verifier to prove that the prover has reserved a certain amount of space. For practicality, the verification process needs to be efficient, namely, consume a small amount of space and time. For security, it should be hard for the prover to pass the verification if it does not actually reserve the claimed amount of space. One way of implementing PoS is by using hard-to-pebble graphs. The verifier asks the prover to build a labeling of a hard-to-pebble graph. The prover commits to the labeling. The verifier then asks the prover to open several random locations in the commitment.

Proof of storage

A proof of storage (also proof of retrievability, proof of data possession) is related to a proof-of-space, but instead of showing that space is available for solving a puzzle, the prover shows that space is actually used to store a piece of data correctly at the time of proof. Cryptocurrencies intended to assign value to store data use some form of this system; examples include Chives, Storj, Sia, Filecoin, and Arweave.

Proof of capacity

A proof of capacity is a system where miners are allowed to pre-calculate ("plot") PoW functions and store them onto the HDD. The first implementation of proof of capacity was Signum (formerly burstcoin).

Proof of space-time

A proof of space-time (PoST) is a proof that shows the prover has spent an amount of time keeping the reserved space unchanged. Its creators reason that the cost of storage is inextricably linked not only to its capacity, but to the time in which that capacity is used. It is related to a proof-of-storage (but without necessarily storing any useful data), although the Moran-Orlov construction also allows a tradeoff between space and time.

Uses

Proofs of space could be used as an alternative to proofs of work in the traditional client puzzle applications such as anti-spam measures and denial of service attack prevention. Proof-of-Space has also been used for malware detection, by determining whether the L1 cache of a processor is empty (e.g., has enough space to evaluate the PoS routine without cache misses) or contains a routine that resisted being evicted. Proofs of space have been developed further in several concept papers and in one live cryptocurrency implementation.

Signum (formerly Burstcoin)

Proof of capacity consumes disk space rather than computing resources to mine a block. Unlike PoW, where the miners keep changing the block header and hash to find the solution, proof of capacity (as implemented by Burstcoin, and developed further by Signum) generates random solutions, also called plots, using the Shabal cryptographic algorithm in advance and stores it on hard drives. This stage is called plotting and it may take days or even weeks depending on the storage capacity of the drive. In the next stage - mining, miners match their solutions to the most recent puzzle and the node with the fastest solution gets to mine the next block.

SpaceMint

In 2015, a paper proposed a cryptocurrency called SpaceMint.Park et al
SpaceMint: A Cryptocurrency Based on Proofs of Space.
''Cryptology ePrint Archive.'' Retrieved 31 October 2016. It attempts to solve some of the practical design problems associated with the pebbling-based PoS schemes. In using PoS for decentralized cryptocurrency, the protocol has to be adapted to work in a non-interactive protocol since each individual in the network has to behave as a verifier.

Chia

In 2018, a proposed cryptocurrency Chia presented two papers presenting a new protocol based on proof of space and proof of time. In February 2021, Chia published a white paper outlining its business and has since launched its mainnet and Chia coin (XCH) using the Proof of Space Time concept. The spacetime model of Chia also depends on "plotting" (generation of proof-of-space files) to the storage medium to solve a puzzle. Unlike many proof-of-storage cryptocurrencies, Chia plots do not store any useful data. Also, Chia's proof-of-time method for plotting has raised concerns over shortened lifespans of

solid-state drives A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It i ...

due to the intensity of write activity involved in plot generation (typically, plotting occurs on an SSD and then the finished plots are transferred to a

hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magne ...

for long-term storage).

Spacemesh

In 2019, a paper was released by Tal Moran and Ilan Orlov proposing a new protocol based on proof of space-time. Spacemesh is unique due to its use of a blockmesh, rather than a blockchain. While a blockchain is a chain of single blocks, a blockmesh makes use of a

directed acyclic graph In mathematics, particularly graph theory, and computer science, a directed acyclic graph (DAG) is a directed graph with no directed cycles. That is, it consists of vertices and edges (also called ''arcs''), with each edge directed from one v ...

(DAG) composed of layers, with each layer containing many blocks.