Checkpointing

What is checkpointing?

Checkpointing was originally built in to Bitcoin in order to prevent dishonest people reversing transactions and taking back the money they had sent. Imagine someone sends you money and you dispatch goods only to find that they have taken the money back out of your account.

The basic checkpointing feature in Bitcoin makes sure that the software only connects to the blockchain defined by the developers. This is a safety feature to stop malicious users from trying to force clients on to a different blockchain. The blockchain has all the Bitcoin transactions of Bitcoin written into it. Every block in the blockchain has a unique string of characters called a hash. In the client the checkpoints are defined by providing the block number and its hash, this is then compiled and distributed. Clients will only accept the blockchain that has the corresponding block number and hash defined in the code. Checkpointing offers some protection against attackers but only up to the last checkpoint. So malicious users could try to orphan blocks after the last checkpoint and reverse transactions with 51% attacks.

What is a 51% attack?

As the network is decentralised the blocks that make it into the blockchain need to be agreed upon by the majority of the miners. So if someone has 51% of the network mining power they are able to dictate which blocks get accepted. Typically a 51% attack is used to force the network to accept alternative blocks to ones that have already made it into the chain. The result of this is to make the transactions in the original blockchain disappear. Typical clients will broadcast the transaction again but the attacker will make sure that their transaction disappears. The aim of this sort of attack is typically to reverse transactions. As these attacks orphan the original blocks, pools and miners also find that the coins they mined disappear. The victims of these attacks are the recipient of the attacker’s transaction and the miners whose mined coins from the original blocks will disappear.

What is Advanced Checkpointing?

Advanced Checkpointing allows us to send out checkpoints without having to redistribute the Feathercoin software. This works by having a ‘master node’ which checkpoints each block it sees on the network protecting it from the attacker. This checkpoint is then picked up by all the other clients on the network which will then ignore any blocks generated by a 51% attack. This protects merchants from transaction reversal and miners from losing their newly minted coins. With Advanced Checkpointing, Feathercoin becomes the most secure of all the Scrypt cryptographic currencies.

Why doesn’t Bitcoin or Litecoin have this protection?

51% attacks have generally been theoretical and in the past they have only happened a handful of times to newly born coins. However it appears that the cryptocoin landscape is becoming increasingly hostile as people invest in larger and more expensive mining farms and have access to large mining pools. The attacks that were theoretical are now becoming commonplace to coins which do not have a large amount of hash power.

Bitcoin and Litecoin have never been subjected to successful 51% attacks as their hashpower has so far been too great for the attackers to target. It would seem that as they have not suffered from these attacks then it is not a problem for them but we believe this complacency is the biggest threat. It may be true that Bitcoin is safe for now but all crypto currencies should have some concern for this issue as hash power should never be taken for granted. Litecoin could well find itself subject to such attacks especially as Scrypt hash power is becoming increasingly mobile with the advent of multipools which switch their user’s hashpower to the most profitable coins at the time.

Feathercoin in the past has been subjected to 51% attacks and even though we have enough hash power at the moment to thwart these attacks, that does not mean that the attackers will not have the resources to attack us again in the future. In fact it could be seen as irresponsible if Feathercoin did not come up with some kind of protection against these attacks as our hash power may drop or the attackers may get greater resources.

Is this a centralised solution?

ACP is a form of centralisation that we have put in place because we believe security comes first. Attacks on this new breed of crypto currencies are on the rise with the attackers becoming more and more resourceful. We are the first Scrypt based coin to respond to this threat by recruiting the best minds in this domain to ensure everyone is protected even those who engage in speculative mining and to whom we owe a debt of gratitude for their patience and understanding.

How will Feathercoin continue to innovate to ensure security and decentralisation?

This is the hard work that needs to be done and we will continue to listen to our community and innovate on the design and implementation of the ACP. We believe that Feathercoin should be powered by the community.

This is a form of centralisation as the checkpoint master node is deployed and maintained by the lead developer Peter Bushnell. Currently no other person has access to this system but it is planned to make the checkpointing system distributed over time. Distributing this system would allow several stakeholders like mining pools to have a vote on which block gets checkpointed. The controls in the system are very limited and only allow the developer to change the depth at which the blocks are checkpointed. This is an automated process and there is no facility to pick and choose which blocks get checkpointed.