UTReeXO: lightweight compact nodes

UTreeXO introduces a new kind of lightweight Bitcoin node: the ‘compact node’. Unlike Bitcoin full nodes, UTreeXO compact nodes don’t need to store gigabytes of UTXOs, just a small cryptographic proof of less than a kilobyte. Compact nodes would nevertheless offer the same level of security and privacy as full nodes.

“Don’t trust. Verify.” it often sounds in the Bitcoin community. One of the core concepts of Bitcoin is that every user must be able to independently and independently verify the validity of transactions on the blockchain.

Bitcoin nodes download and check the Proof-of-Work of the blockchain, but also a list of unspent transaction outputs (UTXOs) from which the transaction history can be drawn. As time passes and the network is used more intensively, the size of both increases.

There are concerns that this will eventually make the hardware requirements for running a Bitcoin node too high for normal users and will detract from the decentralization of the network.


The blockchain is currently almost 300 gigabytes in size and that is growing steadily with an average block time of ten minutes, but predictably further. Part of this data can be removed after review through ‘pruning’ , a process in which only the most recent data is retained.

The list of now 60 million UTXOs, on the other hand, is smaller at about 4 gigabytes, but grows much more unpredictable. Since activity on the blockchain consists of UTXOs and each new user presumably spawns a multitude of new UTXOs, space requirements can become a problem over time.

Moreover, there is hardly any pruning in the information, because the complete transaction history is needed to calculate the balances and to check whether funds have been spent twice.


With UTreeXO, however, Bitcoin developer Thaddeus ‘Tadge’ Dryja thinks he has come up with a solution. Dryja works for MIT Media Lab’s Digital Currency Initiative and gained fame as co-author of the Lightning Network white paper.

UTreeXO is a new way to verify the UTXO list using cryptographic accumulators . Not the entire list of UTXOs is saved, but only a cryptographic proof from which the necessary information can be deciphered. Instead of gigabytes, the information fits in less than one kilobyte. So almost nothing. Dryja calls these types of Bitcoin nodes ‘compact nodes’.

UtreeXO and compact nodes are optional and do not require a hard fork . In principle, UTreeXO therefore has no consequences for normal full nodes and the operation of the network.

Bridge nodes

When compact nodes exchange new UTXOs, they can verify validity by comparing the results of their cryptographic accumulators: they must be equal. The extra data that is exchanged during this process requires about 20% more bandwidth. Bitcoin full nodes do not need the included cryptographic proofs and can simply verify the transactions of compact nodes as usual.

However, if a compact node wants to verify a transaction of a full node, that is a problem, because the cryptographic proofs that compact nodes need for verification are then missing. In that case, something or someone will still have to generate a cryptographic proof via an accumulator, on the basis of which compact nodes can verify.

This requires a new kind of Bitcoin node: Bridge nodes. As the name suggests, a Bridge node fulfills a kind of bridge function between the two types of nodes. Bridge nodes are actually full nodes that also provide cryptographic proofs from an accumulator with transactions. In this way, transactions from normal full nodes can be spread among the compact nodes via bridge nodes.

There is a catch though: it requires some people to voluntarily choose to set up a bridge node. Whether that will often be the case remains to be seen, because bridge nodes, due to their dual role, consume slightly more storage space and bandwidth than traditional full nodes, while there are no advantages in return.

Under development

For the time being, UTreeXO is not finished and is still being worked on. Tadge Dryja does not do this alone, but is helped by other open source developers via github. You can also follow the development yourself or make a contribution there.

The project recently received a nice boost from the 100x Group of bitcoin exchange BitMex. It provided a number of grants to bitcoin developers, including Calvin Kim, who also works on UTreeXO. He received an amount of $40,000 (+/- ???????34,000) to continue his open source work for the UTreeXO project in the coming year.

Want to read more about UTreeXO? Then also read the announcement by Tadge Dryja, the explanation by Calvin Kim or dive straight into the white paper.

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