Utreexo

Utreexo is a proposed accumulator design that compresses bitcoin's UTXO set, a database currently measured in gigabytes, into a handful of Merkle forest roots occupying well under a kilobyte. Introduced in a 2019 paper by MIT's Tadge Dryja, it lets a node verify spends against compact cryptographic proofs instead of a stored database.

Why it matters

The UTXO set is the state every full node must maintain, and unlike the block history it must be consulted for every input of every transaction. Its growth is one of the quiet long-term pressures on node costs. Utreexo would let a fully validating node run with almost no state, opening the door to full validation on phones and embedded devices, which strengthens decentralization by keeping verification cheap for everyone.

The tradeoff is bandwidth: someone must attach inclusion proofs to transactions, and those proofs add data to relay.

How it works

Every unspent output is a leaf in a forest of perfect Merkle trees, and the node keeps only the tree roots. To spend an output, a proof shows it belongs under one of the roots; the node verifies the proof, deletes the leaf, and adds new outputs, updating the roots as it goes. Bridge nodes that hold the full set can generate proofs on behalf of old wallets, so the scheme can be adopted gradually without a consensus change.