Proof of History (PoH)

Proof of History is a high-frequency verifiable delay function (VDF): a sequential SHA-256 hash that runs continuously over its own output, where the previous output is the next input. Because it is sequential and preimage-resistant, it cannot be parallelized; proving that N steps passed requires running N hashes, but verifying it is fast and can be parallelized. Transactions are inserted into the sequence, leaving a provable timestamp of their relative position. It is not a complete consensus mechanism on its own: it is the time source on which consensus operates.

It matters because, by pre-agreeing the temporal ordering without validators having to communicate to set timestamps, it sharply reduces the messaging overhead of consensus. That enables very fast blocks and high throughput, a meaningful technical differentiator for networks used in payments and in real-world asset tokenization. In Solana, PoH combines with Tower BFT, a PBFT variant that uses the PoH clock, and with proof of stake to reach consensus and finality.

Anatoly Yakovenko published the PoH whitepaper in November 2017, and Solana's mainnet beta launched in March 2020. Solana is the only network that uses PoH as such. VDFs as a general concept have been researched in other settings, for instance the Ethereum Foundation explored them for randomness sources, but Proof of History is specific to Solana. For anyone assessing networks where settlement speed is critical, PoH explains why Solana reaches finality very quickly after only a few slots.