Proof of Space and Time …
Proof of Space and Time … Time for Chai?
Our computers are wasteful! Why? Because our CPUs run nearly idle all of the time, and we never really use them to their full potential. Along with this, we have a vast amount of unused disk space — many exabytes of data — across the world. So could we put these wasteful resources to good use, and create a better cryptocurrency than the crumbling Bitcoin network? Well, the Chia blockchain aims to do this, and it aims to create green money in a digital world.
Proof of Space and Time
Well, Bitcoin is now a teenager. While it hasn’t aged that well in terms of the energy it consumes, it has shown the way for a world of trust. But, we all know that its proof of work method is flawed, and that there must be better ways to provide a consensus. One of these is the Chia blockchain network and which uses both Proof of Space (PoS) and Proof of Time (PoT). The PoS method makes uses of the under-allocation of hard-disk space.
With PoS, the ‘farmers’ prove that they have allocated unused disk space for the network. This makes the Chia blockchain network at secure as Bitcoin, but where consumes a great deal less energy. It also overcomes the “rich get richer” problem that we see in the Bitcoin network, and where miners get paid on finding the required hash for a block, and who can then invest these funds in improving their computing environment. The rich gets richer approach means than we can eventually end up with just a few rich nodes defining the consensus for the whole network — and which leads to the 51% consensus attack.
Chia
Chia was created in 2017 by Bram Cohen — the inventor of BitTorrent. It has since been released as an open-sourced decentralised network and which uses its own cryptocurrency: chia or XCH. Its core focus is to simplify the ecosystem for digital transactions. Behind the company is a paper written by Abusalah et al [here]:
Proof of Space
Within a Proof of Space, we use a two-phase approach and where Peggy (the prover) has a file of size N, but Victor only needs to store a small value. Peggy then has to write out this file. In the next phase, Peggy has to prove the Victor that she has completed the work, and has stored the file. A cheating Peggy will only have stored a part of the original file, and Victor must detect this. In a simple form, Peggy may send Victor a random behaving function (f(x)), and ask to compute a table of values (y=f(x)). The table is of such a size that Peggy cannot store it in memory.
For the challenge, Victor will pick a value of x, and then ask Peggy to show the value of y. As f(x) is non-invertible, the only way to do this is to have a look-up on the table. If Peggy has not stored the table, she will have to recompute all the values, in order to find the x value that maps to the y value. An example of a non-invertible function is a hash function. If I provide you with a hash value (h), then I might ask you to find the value of x that generates that hash value and where:
h = Hash(x) … what is x given h?
This is not an easy task, and we would have to try all the values of x to see which one worked. If Victor is continually asking Peggy, she would have to store all of the values in the table when first asked, in order to always answer Victor within good time limits.
Proof of Time
For Proof-of-Time, we want to prove that Peggy has spent a given amount of time on a task. This should show her commitment to performing tasks in the network. For this, in 2019, Chai opened a challenge for a Verifiable Delay Function (VDF), and hired the winners of the challenge, and published their first green paper:
In a VDF, Victor gets Peggy to do some work, and where there are no short-cuts. Victor then sends Peggy a challenge to show her working out, and send a proof back to Victor, and who can check the answer. Victor does not have to actually compute the answer but can check that Peggy has answered it correctly. To understand the method, you can view the core method here:
https://asecuritysite.com/principles/vdf
Conclusions
Bitcoin is like a Ford Model T compared with modern electric cars. It is Version 1.0, and we have advanced massively from there. The SWIFT network for payments, too, is an old method of passing financial payments and was very little inherent digital trust within its transactions. The challenges, such as Chai, want to change this, using cryptographic proofs and improved consensus mechanisms. It will be interesting to see where we end up. One thing that is for sure, is that we need to build a new digital world on maths, rather than on our old paper-based world.
References
[1] Abusalah, H., Alwen, J., Cohen, B., Khilko, D., Pietrzak, K., & Reyzin, L. (2017, December). Beyond Hellman’s time-memory trade-offs with applications to proofs of space. In International Conference on the Theory and Application of Cryptology and Information Security (pp. 357–379). Springer, Cham.