Proof-of-History (PoH)
Basics
- From their blog (30-7-2019):
"The core Solana innovation is Proof of History (POH), a globally-available, permissionless source of time in the network that works before consensus. POH is not a consensus protocol or anti-Sybil mechanism. Rather, POH is a solution to the clock problem."
"Basic principles of how PoH:
- Sha256 loops as fast as possible, such that each output is the next input
- Loop gets sampled and the number of iterations and state are recorded
The recorded samples represent the passage of time encoded as a verifiable data structure. Additionally, this loop can be used to record events.
- Any message that reference any of the samples are guaranteed to have been created after the sample
- Messages can be inserted into the loop and hashed together with the state, guaranteeing that a message was created before the next insert.
This data structure guarantees both time and order of events. With Proof of History, you can create a historical record that proves that an event has occurred at a specific moment in time. The PoH is a high frequency Verifiable Delay Function (VDF), which is a function that requires a specific number of sequential steps to evaluate, yet produces a unique output that can be efficiently and publicly verified.
Other blockchains require validators to talk to one another in order to agree that time has passed, but Solana requires all validators to maintain its own clock by constantly solving SHA256-based VDF. Because each validator maintains its own clock, leader selection is scheduled ahead of time for an entire epoch which lasts for thousands of blocks. Each validator is compensated for running the VDF because the block producer receives awards for producing a block."
- Used in Solana:
"In short: communication overhead. Since Proof of History (PoH) provides a trustless sense of time and ordering of messages, network nodes can trust the relative message times that they receive in the PoH broadcast without having to communicate with all the other nodes in the network. For many distributed networks, the time to finality (the guarantee that past transactions on a digital ledger are legitimate and will not change) scales as the square or even the cube of the number of nodes in the system due to the necessity to confer with other network participants on message ordering.
Because of our Proof of History implementation and our Avalanche communication design, Solana blockchain’s time to finality scales with the logarithm of the number of nodes. This translates into a high throughput network that can support tens of thousands of nodes, while retaining sub-second confirmation times."
- From Bitcoin.com (25-11-2019):
"Proof of History (PoH) is the creation of Solana, a high throughput blockchain that was initiated in 2018 and secured a $20M funding round led by Multicoin Capital. Although not technically a consensus mechanism, Proof of History plays a critical role in enabling network participants to reach consensus on time by using a Verifiable Delay Function, thereby avoiding “the longest chain” rule. While PoH serves as the network’s clock, TowerBFT is its watchtower, tasked with preventing malicious actors from cheating the time parameters. Any validator that votes for a certain block must wait for the next block to be produced, receiving confirmation from the Proof of History that “time has passed” before being entitled to vote again.
With Solana, Proof of Stake determines the network rules, which are enforced with the aid of PoH, TBFT, and validators. Although Solana is intent on using PoH alongside proof of stake in its native blockchain, Proof of History has broader applications. It can be deployed on Proof of Work chains, for example, to enhance the accuracy of the existing consensus mechanism."
