Is the Bitcoin Taproot “upgrade” a slam dunk? Nothing to worry about? In this article, I am going to examine a potential drawback to the proposed Taproot upgrade. Making public keys visible on the blockchain. What danger could this pose for Bitcoin?

It's important to note that with the current version of Bitcoin your public key is not exposed on the blockchain as long as you do not re-use the same wallet address to send Bitcoin. If the Taproot upgrade goes ahead it will change that. With this new upgrade, public keys will become available on the blockchain. This came to my attention when listening to episode 180 of the Unchained podcast with Laura Shin.

In this episode, Laura is interviewing Jeremy Rubin  (a Bitcoin core contributor) about the proposed Taproot upgrade. At around the 8:40 timestamp of the interview, Jeremy mentions that one of the concerns people have about Taproot is that it will expose all public keys. Jeremy goes on to say that this may bring about "some small element of worse quantum security - IF quantum computers existed" he then goes on to explain further that "the general consensus among the Bitcoin development developers is that the quantum computing risk is pretty negligible at this point" he also mentions there will be "a lot of other problems to solve if that happens" and that at that point they will "probably have a migration plan". The impression left from this portion of the interview is that the threat from quantum computers is not something to worry about at this time and that by enabling the visibility of public keys on the blockchain some enhanced features for Bitcoin would be worth the trade-off.

But there is one problem..

Quantum computers do exist.

I am sure Jeremy Rubin is aware of that so I will cut him some slack and interpret his comments to mean that to our knowledge we don’t currently have quantum computers powerful enough to pose a threat to Bitcoins encryption. If we did making all public keys available on the blockchain would mean that your private keys could be derived from the public key with a powerful enough quantum computer. If that were to happen today Bitcoin would no longer be secure and your coins could be spent by someone else.

Even if a powerful enough quantum computer was only possible in 5-10 years why would we enable a potential quantum vulnerability in Bitcoin now? The simple answer is that by making the public keys available it will allow for new signature types when making transactions. It seems like the development community is looking for ways to make payment and transactions with Bitcoin more powerful and flexible. Perhaps this would make it easier for large players in the space like Square’s Jack Dorsey to implement more Bitcoin functionality in their upcoming Bitcoin wallets. But is the enhanced functionality worth the potential future security risk? Do we not have other cryptocurrencies that are fulfilling the role as faster means of payment for small transactions? Is it necessary to enhance Bitcoin's functionality in that way when it primarily acts as a store of value? And is a quantum vulnerability something that should be so readily dismissed?

To find out if the concerns of quantum vulnerability in the Bitcoin community are overblown let's consider for a moment where we are in the stream of time when it comes to quantum computing.

Consider some research and comments for those who are experts in the fields of cryptography and quantum computing:

First up we have Christian Paquin (Principal Program Manager , a cryptography specialist in the Security and Cryptography group at Microsoft Research) during a webinar from June 04, 2020 he said the following:

“Its as if Quantum computers have been designed to break cryptography” regarding how long it would take before we have a quantum computer powerful enough to be a threat to Bitcoin he said: “estimates say that the quantum computer could be built within 10-15 years” he also went on to say that “there are some people that think it may take just 5 years”.

Other experts in the field agree. A liberal ETA for a powerful quantum computer may be around 5 years. A more conservative estimate is that it could take over a decade. However, the progress that is being made on quantum computers is rapid, with many countries devoting large amounts of money and resources to quantum computing projects. Also, we should keep in mind that the possibility of a quantum breakthrough is not outside the realm of possibility. It's also not just private companies like IBM, Honeywell or IonQ that are developing quantum computers…there are governments and nation-states behind these efforts.

In a 2018 lecture the Author Andreas M. Antonopoulos (a blockchain expert) commented on the possibility and impact a powerful quantum computer might have on Bitcoin. When asked whether he thought the NSA or another powerful entity already a quantum computer capable of breaking Bitcoins code he gave the following anecdote:

“If you have a very powerful weapon, you do not use it until you have a very good reason to use it. One [illustrative example] is what happened in [the city of] Coventry when the British had broken Enigma. The most important secret was the fact that [Polish forces] had broken Enigma; otherwise, [the Germans] would change [the encipherment key settings]. To keep that secret, they had to do parallel construction. If they found out [someplace would be] bombed, they would capture a German soldier who knew about it and then say he told them, so that they would have a different [source of knowledge than the deciphered messages]. When they didn't have a different [source], they let the Germans bomb Coventry. Thousands of people died. To protect the secret [of Enigma being broken]. If the NSA has a quantum computer, they really don't want people to know. The quantum computer could also break the encryption keys on all the nukes and nuclear submarines, communication keys in the military intelligence networks and all of the commercial networks. I don't think they will use it to break Bitcoin, if you know what I mean.That is small fish to them.”

Is Bitcoin a small fish? As we know it has the potential to change the very foundations of currency and monetary value. If a vulnerability like that was exploited the resulting effects for the entire cryptocurrency could be catastrophic. We should remember that some of Bitcoins enemies may in fact be the ones who have the resources to build such a quantum computer while at the same time keeping it secret from the world.

If in the future a quantum computer powerful enough to break Bitcoins encryption is developed let's hope that we have enough time to update the nodes and the Bitcoin wallets and let's hope we have enough time to pass and implement that kind of upgrade. Let’s hope we don’t make it easier for them than we have to.

Twitter: @ty_kroh