Utreexo

Utreexo is a dynamic, hash-based cryptographic accumulator designed by Tadge Dryja to shrink Bitcoin's Unspent Transaction Output set from gigabytes of state down to a representation only about a kilobyte in size. Rather than storing every UTXO, a Utreexo node stores a tiny set of Merkle-tree roots that commit to the whole set, growing only logarithmically as the network expands.

How stateless validation works

Because the node no longer holds the full set, each transaction input must arrive with an inclusion proof showing that the output being spent really exists in the accumulator. The node checks the proof against its compact roots, validates the transaction, then updates the roots to reflect the spent and newly created outputs. "Bridge" nodes maintain the full proofs and attach them to transactions so that lightweight Utreexo nodes can verify without trusting anyone.

Why it matters

The growing UTXO set is one of the harder scaling pressures on full nodes, since it must live in fast storage to validate quickly. By trading a small amount of extra bandwidth (for proofs) against a dramatic reduction in RAM and disk, Utreexo could let constrained devices run a fully validating node. Crucially it is consensus-compatible: it changes how a node stores data, not the rules, so it needs no soft fork. Dryja, Davidson Souza and Calvin Kim authored draft BIPs describing it.

Utreexo and AssumeUTXO attack node resource cost from different angles, and both serve the goal of accessible full nodes.