Proof of History (PoH)

Proof of History uses a sequential SHA-256 hash chain as a verifiable delay function (VDF). Each hash depends on the previous output, creating a cryptographic record of the passage of time. Transactions are inserted into this hash chain, giving them a provable timestamp and ordering without validators needing to communicate.

In traditional blockchains, validators must exchange messages to agree on transaction ordering — a process that takes time proportional to network latency. PoH eliminates this by providing a trusted clock: validators can independently verify the order by replaying the hash chain. This reduces consensus overhead from O(n^2) message passing to simple verification.

According to Solana Foundation documentation, PoH runs at approximately 800,000 hashes per second per core, producing a new time proof every ~400 milliseconds. Combined with Solana's Tower BFT consensus (built on PBFT) and Turbine block propagation, this enables Solana to achieve block times of 400ms and peak throughput exceeding 4,000 TPS in production, with theoretical capacity much higher.

PoH is Solana's most distinctive technical innovation, but it is also its most misunderstood. It is not a consensus mechanism — Solana's consensus is Tower BFT, a PoS variant. PoH is a clock that makes consensus faster by removing the need to agree on time.