When discussing consensus mechanisms for different cryptocurrencies, one issue that often causes arguments is a lack of understanding (and definition) of the security model that they provide for the historical data in the ledger. While each consensus model aims to prevent various theoretical attacks, it’s important to understand the goals for the model.

Every security model has two main parts: assumptions and guarantees. If the assumptions used as inputs hold true, then so should the guarantees that are output by the model.

Let’s dig into the security model that appears to be offered to bitcoin users who run a full node.

In search of truth

“One of bitcoin’s strengths – the most important in my opinion even – is the low degree of trust you need in others.” – Pieter Wuille

The goal of distributed ledgers is to provide an ordered history of events, because in distributed systems you can’t simply trust a timestamp.

When a new participant on a blockchain-based network joins, they download any available blocks and consider every valid series of blocks that they see, starting from a hard-coded genesis block.

One of the greatest assumptions made by bitcoin’s security model is that the majority of miners are honest – that they are working to secure the blockchain rather than attempting to undermine it. In practice, this has held true throughout bitcoin’s history due to miner incentives, though some question if it will continue to hold true in the future.

Given this assumption, full node operators can be completely sure of several facts:

  • Nobody has inflated the monetary supply except for miners, and only according to a well-defined schedule.
  • Nobody ever spent money without having the appropriate private key(s).
  • Nobody ever spent the same money twice.

Full node operators can be reasonably sure of several other things. There is a strong guarantee that:

  • Any block in the chain was created within approximately two hours of the block’s timestamp.
  • They are syncing the “true” blockchain history.

At a more technical level, this requires a multitude of checks:

  • All blocks follow the consensus rules:
  • All transactions follow the consensus rules:
  • Many other rules that would take too long to cover here.

Thermodynamic security

Once a transaction is confirmed in a block it can’t be reversed without someone expending a minimum amount of energy to rewrite the chain.

As long as no attacker holds more than 50% of the network’s computational power, and honest nodes can communicate quickly, the probability of a transaction being reversed decreases exponentially with the number of confirmations it has received. There are other attacks, such as selfish mining, that can reduce this power requirement, though they appear to be difficult to perpetrate.

Source: "Bitcoin’s Security Model Revisited" by Yonatan Sompolinsky1 and Aviv Zohar
Source: “Bitcoin’s Security Model Revisited” by Yonatan Sompolinsky1 and Aviv Zohar

Looking at the current cumulative work performed by bitcoin miners, it would take nearly 1026 hashes to build an alternative blockchain from genesis with greater cumulative proof of work that full nodes would consider to be the “true” chain.

Bitcoin network - total computations
Source: http://bitcoin.sipa.be

To crunch some numbers on the cost involved in such an attack:
An Antminer S9 runs at 0.1 Joule per GH (109 hashes)
1026 hashes * 0.1 J / 109 hashes = 1015 joules
1015 joules = 2,777,777,778 kw hours * $0.10 kw/hour = $277,777,778 worth of electricity to rewrite the entire blockchain

Whereas at time of writing a single block must hit a difficulty target of 253,618,246,641 which would require approximately:
253,618,246,641 * 248 / 65535 = 1.09 * 1021 hashes
1.09 * 1021 hashes * 0.1 J / 109 hashes = 1.09 * 1011 joules
1.09 * 1011 joules = 30,278 kw hours * $0.10 kw/hour = $3,028 worth of electricity per block

This is why we can state that bitcoin is provably thermodynamically secure.

There are a few variables that you can tweak in the above calculation to decrease the cost, but we can be sure that it will require many millions of dollars worth of electricity alone in order to rewrite the entire blockchain. However, an attacker with this much hash power would at worst be able to reverse transactions back to 2014 – we’ll delve into the reason for this shortly.

Also note that this doesn’t take into account the costs required to obtain and operate sufficient mining equipment to carry out such an attack.

Sybil resistance

Because the bitcoin protocol considers the true chain to be the one with the most cumulative proof of work (not the longest chain as is often incorrectly stated,) the result is that a new peer joining the network only needs to connect to a single honest peer in order to find the true chain.

This is also known as “sybil resistance,” which means that it’s not possible for someone to launch an attack against a node by creating many dishonest peers that feed it false information.

Nodes

Pictured here is a near worst-case scenario in which your node is being massively Sybil…



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