# Fast sync for Bitcoin

# What problem does Fast Sync solve?

When you start a new BTCPay Server, you need to synchronize your Bitcoin node from the genesis block.

Validating from the genesis blocks takes in general 1 or 2 days on affordable servers. (around 10$ per months)

However, on some low powered devices like raspberry PI, synchronization time will take around 2 weeks nowadays. (and it will linearly increase with time)

Fast sync provides a solution to decrease dramatically the synchronization time to minutes or few hours.

Current UTXO Set snapshots only work for Bitcoin Core 0.17.0 or higher. Do NOT use these sets on older versions of Bitcoin Core (0.16.3 or lower)

# How does Fast Sync solve it?

In order for Bitcoin software to operate, you do not need all the history of blocks from the genesis.

What you need is the state of Bitcoin up to a certain block (this state is called UTXO Set), and around 300 blocks before this point.

Fast Sync downloads the UTXO Set at a specific block on an external server, and deploy it on your node. We call this file a UTXO Set snapshot.

When your node start up again, it will only need to synchronize from the snapshot, to the latest blocks.

# What are the downsides of Fast Sync?

# Malicious UTXO Set

Fast Sync can be potentially abused:

  1. The attacker gives you an UTXO Set snapshot which does not follow Bitcoin consensus
  2. The attacker makes a purchase to your store.
  3. Nodes following the consensus would not recognize the payment as valid, but your node will.
  4. The coins the attacker sent you are just worthless

Other attacks can completely bring down your node.

# Lightning Network routing issues

As a merchant, you generally do not care about this issue. Merchants are mainly receiving payment, so they don't need to do any routing.

However, if you plan to send payments from your lightning node, you may have issues:

Because you do not have old blocks, then your lightning node won't see any channels which have been created prior to the snapshot.

If this is a problem for you, just use an older snapshot.

# How to verify that the UTXO Set snapshot follows the consensus?

# If you trust the owner of this repository....

The snapshots recognized as valid by the btcpayserver-docker repository you cloned can be found on utxo-sets (opens new window).

The script load-utxo-set.sh (opens new window) will download the utxo-set from the environment variable UTXO_DOWNLOAD_LINK.

If UTXO_DOWNLOAD_LINK is empty, NBITCOIN_NETWORK (with value set to mainnet or testnet) will be used to take a default UTXO_DOWNLOAD_LINK that we hard coded inside load-utxo-set.sh (opens new window).

Once the files are downloaded, the hash will be checked against those in utxo-sets (opens new window).

However: This only prove that if the owner of this git repository is honest, then the utxo-set are correct.

NOTE: Completing those steps does not mean that the UTXO set snapshot is legit. It only mean that you trust the owner of this git repositoy to have verified that it is legit.

# If you trust someone else...

This repository contains the signatures of some developers, for example sigs/NicolasDorier.utxo-sets.asc (opens new window) contains the hashes that NicolasDorier verified himself.

You need to verify with KeyBase command line (opens new window) that the signature is legit:

keybase pgp verify -i sigs/NicolasDorier.utxo-sets.asc

If you don't like command line, you can verify against keybase verify page (opens new window) by just copying and pasting the content of sigs/NicolasDorier.utxo-sets.asc (opens new window).

You can verify that the handle NicolasDorier refers to the person who controls NicolasDorier twitter, github and reddit handle on the keybase profile page (opens new window).

NOTE: Completing those steps does not mean that the UTXO set snapshot is legit. It only mean that you trust the owner of a Keybase account who has proved access to some social media accounts in the past.

# Don't trust, verify!

If you don't trust anybody, which should be the case as much as possible, then here are the steps to verify that the UTXO set is not malicious.

  1. You need another node that you own, under your control, that you synchronized from the genesis block. Let's call this node Trusty.
  2. You need to create a new node which use Fast Sync with the UTXO snapshot you want to verify. Let's call this node, Synchy.
  3. Wait that Synchy is fully synched.
  4. Now on Synchy and Trusty run at the same time:
 bitcoin-cli gettxoutsetinfo

If Synchy or Trusty are using BTCPay Server use:

bitcoin-cli.sh gettxoutsetinfo
  1. Verify that the output of Synchy and Trusty are identical (you can ignore disk_size).

NOTE: Completing those steps, under the assumption the software you are running is not malicious, correctly prove that the UTXO set snapshot is legit.


# Can I add my signature to this repository?

If you are a bitcoin developer or public figure, feel free to add your signature. For this you need:

  1. A keybase account (opens new window) linked to your social media accounts.
  2. Follow the steps described in the Don't trust, verify! section each snapshots you want to sign.
  3. Create a file with same format as utxo-sets (opens new window) with the snapshots you validated. (Let's call this file YOU.utxo-sets)
  4. Run the following command line
# Assuming your are inside the FastSync directory
keybase pgp sign -i YOU.utxo-sets -c -t -o sigs/YOU.utxo-sets.asc
rm YOU.utxo-sets
git add sigs/YOU.utxo-sets.asc
git commit -m "Add YOU utxo-set signature" --all

And make a pull request to btcpayserver-docker repository.

# Where can I download UTXO set snapshots

You should not need to do this, because load-utxo-set.sh (opens new window) do the hard work for you.

But if you really want, just browse on this listing (opens new window).

Select the snapshot you want, and download it by querying http://utxosets.blob.core.windows.net/public/{blobName}.

# How can I create my own snapshot?

Assuming you have a node running on a docker deployment of BTCPay Server, you just need to run save-utxo-set.sh (opens new window).

This script shows the steps to create an archive of the current UTXO Set It will:

  1. Shutdown BTCPay Server
  2. Start bitcoind
  3. Prune it to up to 289 blocks from the tip
  4. Stop bitcoind
  5. Archive in a tarball the blocks and chainstate directories
  6. Restart BTCPay
  7. If AZURE_STORAGE_CONNECTION_STRING is set, then upload to azure storage and make the blob public, else print hash and tarball

# How can I do this for my altcoin?

Your altcoin does not need it, almost nobody use it compared to bitcoin.

However, if you insist, follow what we did for Bitcoin, we can't hand hold you on this.

# Do you plan to destroy Bitcoin?

This feature may be controversial, because of the risk that almost nobody will follow the Don't trust, verify! step.

What if somebody start spreading a corrupted snapshot on wild scale?

I think this issue can be mitigated at the social layer. If several person start using social media for spreading their bitcoin-cli getutxosetinfo every 10 000 blocks, any corrupt snapshot would be soon detected. We plan to make expose the hash via BTCPayServer and make it easy for people to share.

# Why you don't just: Make BTCPayServer rely on SPV

All SPV solution brings a systemic risk to Bitcoin. If everybody relies on SPV to accept payment and miners want to change consensus rules, then you will have no leverage as individual, nor as a community to decide against.

Even with UTXO Set snapshots you continue to validate consensus rules from the block of the snapshot.

# Why you don't just: Make BTCPayServer rely on an external trusted node

Why not just hosting BTCPayServer on the raspberry pi, but the bitcoin full node on another machine?

For two reasons:

First, BTCPayServer is trying to bring down the technical barriers to operate payments on your own. Running on an external node means that the user need the technical skills to set it up.

BTCPayServer also relies on Bitcoin's RPC which is not meant to be exposed on internet. We can't see any simple enough solution which would allow normal people to run an external node somewhere else.

The second reason is about reliability: You want your service to be self contained. If you host a node on another server, and for some reason this server goes down, then your BTCPayServer hosted on the raspberry PI will also cease to function.