Recently technical and non-technical people alike started to believe that the best (and only) way to prove that something has happened in an information system is to use a blockchain. But there are other ways to achieve that that are arguably better and cheaper. Of course, blockchain can be used to do that, and it will do it well, but it is far from the only solution to this problem.
The way blockchain proves that some event has occurred by putting it into a tamper-evident data structure (a hash chain of the roots of merkle trees of transactions) and distributing that data structure across multiple independent actors so that “tamper-evident” becomes “tamper-proof” (sort-of). So if an event is stored on a blockchain, and the chain is intact (and others have confirmed it’s intact), this is a technical guarantee that it had indeed happened and was neither back-dated, nor modified.
An important note here – I’m stressing on “digital” events, because no physical event can be truly guaranteed electronically. The fact that someone has to enter the physical event into a digital system makes this process error-prone and the question becomes “was the event correctly recorded” rather than “was it modified once it was recorded”. And yes, you can have “certified” / “approved” recording devices that automate transferring physical events to the digital realm, e.g. certified speed cameras, but the certification process is a separate topic. So we’ll stay purely in the digital realm (and ignore all provenance use cases).
There are two aspects to proving digital events – technical and legal. Once you get in court, it’s unlikely to be able to easily convince a judge that “byzantine fault tolerance guarantees tamper-proof hash chains”. You need a legal framework to allow for treating digital proofs as legally binding.
Luckily, Europe has such a legal framework – Regulation (EU) 910/2014. It classifies trust services in three categories – basic, advanced and qualified. Qualified ones are always supplied by a qualified trust service provider. The benefit of qualified signatures and timestamps is that the burden of proof is on the one claiming that the event didn’t actually happen (or was modified). If a digital event is signed with a qualified electronic signature or timestamped with a qualified timestamp, and someone challenges that occurrence of the event, it is they that should prove that it didn’t happen.
Advanced and basic services still bear legal strength – you can bring a timestamped event to court and prove that you’ve kept your keys securely so that nobody could have backdated an event. And the court should acknowledge that, because it’s in the law.
Having said that, the blockchain, even if it’s technically more secure, is not the best option from a legal point of view. Timestamps on blocks are not put by qualified trust service providers, but by nodes on the system and therefore could be seen as non-qualified electronic time stamp. Signatures on transactions have a similar problem – they are signed by anonymous actors on the network, rather than individuals whose identity is linked to the signature, therefore making them legally weaker.
On the technical side, we have been able to prove events even before blockchain. With digital signatures and trusted timestamps. Once you do a, say, RSA signature (encrypt the hash of the content with your private key, so that anyone knowing your public key can decrypt it and match it to the hash of the content you claim to have signed, thus verifying that it is indeed you who signed it), you cannot deny having signed it (non-repudiation). The signature also protects the integrity of the data (it can’t be changed without breaking the signature). It is also known who signed it, owning the private key (authentication). Having these properties on an piece of data (“event”) you can use it to prove that this event has indeed occurred.
You can’t, however, prove when it occurred – for that you need trusted timestamping. Usually a third-party provider signing the data you send them, and having the current timestamp in the signed response. That way, using public key cryptography and a few centralized authorities (the CA and the TSA), we’ve been able to prove the existence of digital events.
And yes, relying on centralized infrastructure is not perfect. But apart from a few extreme cases, you don’t need 100% protection for 100% of your events. That is not to say that you should go entirely unprotected and hope that an event has occurred simply because it is in some log file.
Relying on plain log files for proving things happened is a “no-go”, as I’ve explained in a previous post about audit trail. You simply can’t prove you didn’t back-date or modify the event data.
But you can rely on good old PKI to prove digital events (of course, blockchain also relies on public key cryptography). And the blockchain approach will not necessarily be better in court. In a private blockchain you can, of course, utilize centralized components, like a TSA (Time stamping authority) or a CA to get the best of both worlds. But you don’t need a distributed consensus in order to prove something digital happened – we’ve had the tools for that even before proof-of-work distributed consensus existed.