(I work for Guardtime; I’m happy to answer any questions.) Some online resources might not make it very clear, but the KSI blockchain is indeed a blockchain; for example, new blocks are released periodically (1 block per second on average), and the new blocks contain hashes of the previous blocks, among other things. Then, blocks get distributed around so that users can verify their data against these blocks, etc.

Most people associate "blockchains" with a protocol by which the "blocks" are inserted. Usually these things revolve around proof-of-work, proof-of-stake, proof-of-space, fairness criteria, etc. I think everyone gets that the Guardtime product is hashtree. From a formal verification perspective, the critical behavior is probably in the protocol, and most likely not the implementation of the datastructure or database.

One way to look at it is that KSI is a proof of stake blockchain where the stakeholders are (mostly) fixed. Often people would call that a private blockchain (or a consortium blockchain), to use the terminology from the following article:

https://blog.ethereum.org/2015/08/07/on-public-and-private-b...

Thanks for that clarification! It is a great pleasure to talk with an knowledgeable person like you.

> 1 block per second on average

What stops a double-spend or other attacks like that in a 1-second timeframe? The 10 minute threshold of Bitcoin was chosen under belief that the network would be settled after roughly 10 minutes.

The speed comes from the fact that private and consortium blockchains tend not use Proof of Work (at least not as the only algorithm; you could mix and match, if you wanted to). Instead, such blockchains tend to use voting-based protocols (e.g. Proof of Stake), where a certain subset of nodes are trusted with voting rights, who together vote to approve the next block. In some systems you also have automatic punishments built in to incentivise correct behaviour (i.e., if someone attempts to fork a blockchain --- for example by voting for 2 conflicting forks -- and if other nodes see any evidence of that, the node who attempted the fork could automatically lose their rights as a trusted validator, and may also automatically lose a lot of money or face legal fees.)