Crypto glossary

Proof of History

Proof of History (PoH) is a cryptographic clock used by Solana. It produces a verifiable sequence of hashes that proves events happened in a particular order and that time passed between them, helping the network order transactions efficiently.

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How the hash clock works

A hash function turns any input into a fixed-length fingerprint. PoH runs SHA-256 over and over, each time feeding in the previous output. Because each step depends on the last one, the sequence cannot be computed in parallel or skipped ahead: producing a million steps requires doing them one after another, which takes real time.

When a transaction or other event arrives, its data is mixed into the next hash. Anyone can later re-run the sequence, which can be verified much faster in parallel, and confirm that the event was recorded after everything before it and before everything after it.

An analogy

Think of a notary who stamps pages in a numbered logbook, where each stamp includes a fingerprint of the previous page. You cannot insert a page in the middle without breaking every fingerprint that follows, and the count of stamps shows roughly how much time has passed.

PoH is not the consensus mechanism

PoH does not decide which chain is valid or who may produce blocks. Solana uses proof of stake for that, with a voting system called Tower BFT that builds on the PoH clock. The clock lets validators agree on time and order without exchanging many messages, which is one reason Solana can produce blocks quickly. The design was described by Anatoly Yakovenko in Solana's 2017 whitepaper.

Trade-offs and misconceptions

A common misconception is that PoH alone makes Solana fast or secure. Speed comes from many components working together, including parallel transaction execution, and security comes from staked validators. The approach also demands capable hardware and bandwidth from validators, which shapes who can run one. Solana experienced several network outages in 2021 and 2022, a reminder that high-throughput designs bring their own operational risks.

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Frequently asked questions

Is Proof of History a consensus mechanism?

No. It is a timekeeping and ordering tool. Solana's consensus is based on proof of stake.

Does any other blockchain use Proof of History?

It is closely associated with Solana and networks derived from its technology; most other chains use different ways to order transactions.

Why can't Proof of History be faked quickly?

Each hash depends on the previous one, so the sequence has to be computed step by step, which takes time that cannot be shortcut with more computers.

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