UTXO or Unspent transaction output is one of the central concepts that allow the proper functioning of the blockchain technology that Satoshi Nakamoto developed for Bitcoin, the world's first cryptocurrency.

Ddecades before it was created Bitcoin, the obsession with creating a purely digital money haunted many people's heads. But one of the main problems that everyone ran into head on was the famous double spending.

It was Satoshi Nakamoto who, thanks to a set of techniques, solved this great problem, being able to create the first purely digital decentralized money. In this article we will explain what UTXOs are and the great role they play in creating transactions so that your bitcoins can travel safely within the protocol blockchain.

UTXO, a concept to avoid double spending

As we have already explained in the article aimed at How do Bitcoin transactions work?, a transaction is made up of inputs and outputs. The set of inputs and outputs, together with currencies to be sent and cryptographic signatures, result in a transaction hash, usually called a HASH ID.

The entries are HASH ID of a transaction that received the purse and that have not been used previously, that is, they are UTXO, while the output is the destination address, to which UTXO will be created that you can later use in a transaction. The same address can have infinite UTXOs. This is why UTXOs are defined as a set of transactions.

When a person needs to send a transaction, it has to be nourished by UTXO. That is, of transactions that you have received and that have not been spent. This means that a person can use one or more UTXOs for the same transaction. In fact, those UTXOs can be part of one or more addresses in your wallet. And we go further, even a transaction could be created with UTXO from addresses of different wallets, as long as each one is signed with its corresponding clear private key.

All of this leads to one place: a UTXO can only be used once. And this is fundamental within the operation of blockchain technology, as it is part of the set of tools that guarantees that currencies are not used more than once (the famous double spending).

Such is the reliability of using this mechanism to identify unspent coins that this is the way in which in Bitcoin you can count how many coins there are in circulation. What you do is add up all the coins that are in the UTXO, that is, in the transactions that have not been spent.

Here is a simple example so that you understand the flow:

Imagine that Maria wants to pay Pedro a total of € 100 in bitcoins for a job. María hopes to receive payment from some people who owe her bitcoins and from that money she will pay for Pedro's work.

When Maria receives these payments, she has had some "Inputs" of money, and from those inputs she will make an "Output" to pay Pedro. Maria was paid her debts, which were a payment of € 75 and another of € 50.

When she pays Pedro, Maria has to use both entrances, since neither of them has enough, and she will put Pedro as the exit.
After that, Pedro will have a UTXO entry (he has received the payment).

Where do the remaining coins go?

As you can see, María had a total of € 125 divided into two UTXOs, but she only needed to send € 100. You have had to use both UTXOs, setting Pedro's address as Output or output, but your wallet will have done something else transparently to Maria. Your purse will have put an address for Maria as an outlet, to which you will have assigned the remaining € 25. It is what is known as direction of change.

By the way, there are some transactions that become a UTXO but are not generated by a previous UTXO. Read on to find out what they are.
With what has been described above you can understand how this model of "Tickets" y "Departures", can help us to establish relations of possession and granting of money. A relationships that in blockchain are marked by the use of cryptography and that provide us with absolute security that the money has reached its destination and that it is effectively under the control of the recipient.

UTXO and its importance in Bitcoin

In Bitcoin all transactions have this structure of inputs and outputs. In the "Entries" of a Bitcoin transaction, we can see the origins of the balance that we are managing with our wallet or purse. While in the "Departures", we can see where we send our money. In addition to this, we can also see the remaining money that is returned. All of this is visible and traceable by anyone, since the Bitcoin blockchain is public and transparent.

If you look closely, this model allows you to have total control over how money can be used and under what conditions it can be used. First of all, in order to use a balance in Bitcoin, that balance must be considered as a UTXO within your wallet. That is, someone must make a payment (an exit or UTXO) so that this payment becomes your entry and thus you have available balance to spend.

This is a recursive process. That is, it repeats from the moment the coin is generated as a result of a mined block. For example, a coinbase transaction it is actually a UTXO created by the miner to send that balance to an address under his control. Thus, this exit transaction becomes an inflow of money for the miner that can later be spent. The same process is repeated for the rest of Bitcoin users. With this we can see that UTXOs are an essential part of Bitcoin transactions and without them, their operation would be impossible.

Expanding the example of how a UTXO works on Bitcoin

We can see the operation of a UTXO in Bitcoin in the following way:

Daniel wants to pay for Luis's car, valued at 1 BTC, and a total of 1,5 BTC is available in his wallet. Daniel's balance is divided into two directions, A with 0,8 BTC and B with 0,7 BTC.

Thus, Daniel goes to Luis and makes the payment for 1 BTC. At this point, Daniel's wallet cannot send 1 BTC directly because the balance is split in two directions. So it takes both balances and converts them into the inputs of the payment transaction. He then takes the address of Luis and assigns to it the sending of 1 BTC, additionally assigns a total of 0,4995 BTC to the exchange address, and the rest remains as the mining commission.

Once Daniel submits his transaction, it will be processed and confirmed by the network, beginning its path to irreversibility. And at this point, it will be easy to see where the transaction UTXOs are.

The first UTXO that we can detect in the example are Daniel's balances. Those addresses with 0,8 BTC and 0,7 BTC of balance, are two UTXOs that are under their control and are the ones that allow them to make the payment to Luis. Since Daniel is in control of those balances, he can transform his UTXOs into "Tickets" for a new payment as he does in this example.

Immediately after the network confirms Daniel's transaction, the situation changes. Now, Daniel no longer has under his control the balances he had originally, and instead, his transaction has generated new UTXOs that overwrite the previous ones. The new UTXOs in question are represented first, by Luis's address and the 1 BTC he has received in it from Daniel. And second, for the exchange address and the 0,4995 BTC that Daniel has received from the network, because it is what remains of the payment he made. As we have said, the rest to complete the 1,5 BTC that Daniel had in principle remain as a commission payment for the miner.

Bitcoin Scripts and its relationship with UTXOs

The entire operation of the UTXO in Bitcoin is guaranteed by the Bitcoin Script, the programming language used to write all operations in Bitcoin. Each transaction has an associated script that allows us to:

  1. Validate that the balance used is really ours.
  2. Ensure that balances sent can only be spent by the person to whom we send them.

But how are these operations carried out? This is what we explain below:

Validating that the balance is ours

The first task to be able to use a balance in Bitcoin is to demonstrate that this balance is indeed ours. To do this, the first thing to keep in mind is that each UTXO that we transform into an entry (balance that we are going to spend) is actually the exit of a previous transaction that gave us access to those BTC. That is, all Bitcoin balances have a UTXO associated with it, and said UTXO has a blocking script associated with it.

This blocking script is a digital padlock that we must open in order to then be able to make use of the balance of said UTXO. The blocking scripts in Bitcoin are varied, but the most common is the P2PKH (Pay to Public Key Hash). Although there are also the P2SH (multisig), the P2PK (the most primitive of all). This blocking script can be opened with the private key that results in the address indicated in the UTXO.

So in order to unlock said script, what we must do is take the Bitcoin address, take our private key and generate the public key of said address. At the end we stamp our digital signature and with all this verified data, the balance of said address is unlocked so that we can use it. That is, the nodes will accept the transaction and put it in the mempool waiting for the miners.

On the contrary, if we fail to do that, the transaction is simply rejected by the nodes and we will not be able to use the balance. This process is called an unlock script. Of course, this cryptographic process is automatic and transparent for the users of a wallet, the whole process is carried out by the wallet itself, but now you know what happens inside it. In this scheme, this part of the operation is described in the scriptSig of each Bitcoin Script.

Creating the lock script

Proving that we can spend our UTXOs, we continue with the transaction creation process, creating a lock script for the new destination-directed UTXO. In this case, what the wallet does is create a script similar to the one that was created for us previously, which indicates that the only person who can spend this new UTXO is whoever has the private keys in their possession that control the addresses to where we have sent.

In this way, what is created is a validation chain, where bitcoins when passing from one address to another, are blocked to be used by the last person who takes possession of them.

As you can see, in Bitcoin everything has a well-defined why, and that allows all its parts together to allow the correct functioning of the cryptocurrencies. Being an open protocol, all nodes that want to be part of the protocol interpret the rules in the same way.

All the transactions are reviewed by all the nodes, being accepted those that accept the majority and confirmed those that the majority of the hash power (generated by the miners) accept. These controls democratically guarantee compliance with the protocol.

How much do you know, cryptonuta?

Is the UTXO model present in all cryptocurrencies?


The UTXO model as it is proposed in Bitcoin is not present in all cryptocurrencies. An example of this is Monero. In this currency, its privacy and anonymity protocol makes this model impossible to work. Instead, the creators of Monero found a cryptographic solution that allows you to hide your spending information and coin balances (UTXO) while at the same time controlling the misuse of transactions to perform improper operations such as double spending, among other privacy concerns. and anonymity.

Curiosities of the UTXO in Bitcoin

Now, in the previous example you have surely seen some curious things about UTXOs and their operation in Bitcoin, and we will give you an explanation in this section.

Transactions that do not come from a UTXO

As we indicated previously, there are some special transactions, some that are not generated with UTXOs, since the coins appear "magically". This particular case occurs in coinbase transactions, which are created by miners at the time of mining a block and are used to obtain the reward of the block.

Why not make a fractional payment using the balances in BTC?

Reading the previous example, you may have wondered why Daniel's Bitcoin wallet has not made an exact payment of 1 BTC using a single entry? The answer to this is that UTXOs represent the balance within Bitcoin indivisibly. That is, we cannot transform that balance into smaller values, unless we spend the balance creating a new transaction.

To make it easier, let's look at this example: if we want to pay 1 BTC with our wallet, but that amount is divided into 100 different UTXOs that have 0,01 BTC each, then our wallet will take the 100 addresses as inputs to our transaction and with it will make the payment. That is, the Bitcoin protocol marks those 100 UTXOs as "Spent". From there, it generates a new transaction with a new UTXO which is where we have sent the 1 BTC payment, and it leaves us without balance control in those addresses used.

Relationship with commissions in Bitcoin

Another utility of the UTXO has to do with the commissions or fee in Bitcoin. In the previous example, we explained that each time we carry out a transaction, the commission of that operation is equal to the difference between the entries and the exits. That is to say:

Total input - Total output = Commissions

This indicates that the construction of a transaction has from the outset a balance of commissions. The commission to the miners is not associated with any output, so the miners keep what remains unassociated. That is, the total input must always be greater than the associated total output.

The commission to the miners is not fixed, each one decides how much they want to pay depending on the rush they are in, but it is something that we see in all transactions within Bitcoin and its purpose is two things:

  1. Encourage the work of miners who, when generating a block, not only receive the reward from it, but also the commissions of all transactions confirmed by said block.
  2. Prevent the network from being vulnerable to malicious attacks such as DDoS, dusting attack, among others. This is because each transaction has a cost and making transactions in massive numbers has a high cost for the attacker.

UTXO on other blockchains and cryptocurrencies

The UTXO scheme is so good that the rest of the cryptocurrencies have decided to use it for their systems. Of course, each blockchain can adjust this concept differently. In this way, it can adapt to the additional functions that each blockchain protocol where they are executed may present. However, the basic operation is the same. Hence even privacy coins like Monero o Zcash, use this scheme adjusted to the particularities of your protocol.