21. Oktober 2020

One of Hal Finney’s lost contributions to Bitcoin Core will be ‚resurrected

Although Hal Finney is no longer with us, one of his 2011 proposals may soon arrive at Bitcoin thanks to an expired patent


In a February 8, 2011 publication on Crypto Trader scam, Finney said that reading a book entitled „A Guide to Elliptic Curve Cryptography“ by Hankerson, Menezes and Vanstone gave him an idea of how to speed up signature verification by 25%. In the next publication on the same day, Finney announced that he had already written the „test code“ and uploaded it to the Github repository. However, there was a problem with Finney’s proposal: his method had already been patented by someone else.

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The „Method for Accelerating Cryptographic Operation on Elliptic Curves“ (also known as GLV or Gallant-Lambert-Vanstone Four-Dimensional Scalar Multiplication) received a patent on September 19, 2006, probably at a time when Satoshi Nakamoto was already busy working on Bitcoin (BTC). To understand the invention, we have to go a little deeper into elliptic curve cryptography. The patent says:

The improvement comes from representing „the k-scalar as a combination of k-components, and an A-integer“. The mathematical operations performed in k represented in this form seem to be less computationally expensive, hence the gains in speed.

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The 2013 Finney proposal was implemented with the launch of the libsecp256k1 library, but was never enabled due to existing legal issues. This was the case until September 25, when the patent expired. According to Adam Back, co-founder of Blockstream, the code is now expected to be activated in the next Bitcoin Core update.

February 2011 seems to be the time when Finney focused more on optimizing Bitcoin signature verification. In a February 7, 2011 release, Finney said he was looking at „batch signature verification,“ which he believed could speed up the process by a factor of four. The idea behind this was that instead of verifying signatures one by one, he would verify them by blocks: hundreds or even thousands at a time. However, according to Blockstream co-founder Pieter Wuille (who was one of the authors of the libsecp256k1 library), when GLV is combined with batch verification, the gains disappear once approximately 1,000 signatures are reached:

„The profit from batch+GLV over single batch is less than single+GLV over single. And for very large numbers, the profit tends to be a ratio of 1 (so there is no profit). But at least up to thousands of signatures, it is still an advantage“.

In fact, it has been implemented for Schnorr signatures, where it offers twice the speed. Back indicated that he expects a forthcoming release of Schnorr signatures that include batch verification:

„By the end of this/next year, the published Schnorr signatures, including the activation and then the acceleration of the batch verification, will also be available“.

It is unlikely that Finney’s cryogenic capsule will allow him any movement, but if it did, we could see a smile on his face.