What Is Proof Of History in Solana?
Proof of History (PoH) is a cryptographic technique used by Solana to create a verifiable record of time passing between events. It provides a global ordering of transactions without requiring validators to communicate timestamps with each other.
How PoH works
PoH uses a sequential SHA-256 hash chain. The output of each hash is used as the input to the next hash. Because SHA-256 cannot be parallelized, the chain proves that real time has passed between any two points. The number of hashes between events serves as a clock.
Each leader runs a PoH generator that timestamps transactions as they arrive. This produces a block with a cryptographic proof that the transactions were received in a specific order, at specific time intervals.
PoH is not a consensus mechanism
PoH does not determine which fork is correct — Tower BFT handles consensus. PoH is a clock that enables Tower BFT to work efficiently by providing a common time reference that all validators can verify independently.
Performance benefits
Without PoH, validators would need to exchange timestamps and agree on ordering through network messages. PoH eliminates this overhead:
- Validators verify the hash chain locally without network communication
- Block producers timestamp transactions without waiting for consensus
- The hash chain provides a natural ordering for all events in a block
FAQ
Is PoH the same as Proof of Work? No. PoH does not determine block producers or provide Sybil resistance. It is purely a timekeeping mechanism. Solana uses Proof of Stake for validator selection.
How fast is the PoH clock? The PoH generator targets approximately 400 ms per slot with the hash rate calibrated to the leader’s hardware.
Can PoH be faked? No. Generating the hash chain requires sequential computation. Verifiers can check the chain faster than it was produced (using GPU or parallel verification), so any tampering is detected.
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