What is(n't) blockchain to infosec? Part Three: Not quantum at all

Blockchain is often presented as a solution to the problems that quantum computing will pose for cryptography and these claims are false.  Blockchain has no particular relationship to quantum computing whatsoever.  Some very brief backstory: encryption methods are invariably built on a  math problem that is believed to be difficult and quantum computers are a sophisticated new technology that promises to make some of these math problems less difficult.  Although there is considerable propaganda out there to the contrary, blockchain is built on top of the same cryptographic techniques as everything else, it will be compromised by quantum computing like everything else, and if it gets patched up it will likely be in the same way as everything else.

As we discussed briefly in Part One, blockchain is not an innovation in cryptography per se but a distributed program built with heavy use of existing cryptography.  Notably, blockchain makes heavy use of cryptographic hash functions and asymmetric (public and private) key encryption.  Any frequent user of a cryptocurrency is implicitly familiar with the latter as it is important to keep track of one's private keys, which essentially give ownership of accounts with currency, while the corresponding public keys represent one's identity on the ledger.  The public vs. private key algorithms used in most blockchains are not unique at all but are well-tested algorithms, even down to the level of particular implementations, that are applied many other places.

It is the public vs. private key algorithms that are threatened by quantum cryptography and as blockchain is using the same algorithms it is also threatened.  It is well beyond the scope of this post to give the details of how quantum computing works, but for those readers who want to do their own Googling we will hit some of the high points.  Asymmetric key cryptography is typically built around some form of a type of math problem, the discrete logarithm problem, which involves certain computations that are efficient in one direction but very difficult to invert.  The security of the cipher is dependent on this difficulty, and the danger of quantum computing is that it allows new algorithms that make this inversion much easier.

For the moment, though, there is no danger.  The engineering difficulty around building a practical quantum computer is vast and existing (extraordinarily expensive) prototypes contain just a few quantum-analog logic gates.  Even as they improve, there will be a long period where very few actors have real access to them - they will be a sort of cryptographic nuclear weapon.  There are also new cryptographic techniques, notably lattice-based cryptography, that may prove more resistant to quantum attacks.  Blockchain's could easily be fixed by swapping in these new parts, but it would be the new components resisting quantum cryptography and not any aspect of the blockchain algorithm itself.

To recap, blockchain has nothing to do with quantum computing and won't do anything on its own to protect you from quantum attacks.  Next in Part Four, I will talk about how blockchain doesn't do anything itself to protect your data from theft and rather presents significant new liabilities.