What is succinctness?

A succinct blockchain is a distributed ledger that can be verified only with constant size information.

If you are running a PoW chain, the concept is not really defined, since you can never be sure that you are actually following the longest chain: There can always be a longer chain out there that you or your peers do not know about. This is a fundamental problem of PoW, which can even be considered as a security weakness.

In PoS, the situation is better, since each block gives you the final information about the validator set of the next block. Thus, you can just follow all blocks from the genesis until the most recent, and be sure that you get the latest state and validator set. This process is called synchronization, a well known concept among validators.

Nevertheless, the problem with synchronization is obvious: You need to download and execute all blocks from genesis to get trustless information about the latest block, which is an ever increasing computation, i.e. it takes longer and harder with each new block.

In succinct blockchains, the consensus itself is wrapped with a zero-knowledge proof, essentially making the synchronization on your behalf. Instead of you performing the execution on every block, the ZK proof handles it on your behalf.

Even though the generation and verification of the proof includes more and more blocks, this does not affect the computation power required. Specifically, by using a specific type of proofs called SNARKs, Pulsar makes the verification time and proof size almost independent of the number of blocks, essentially a constant.

Now, take a second to think about what this actually means: Essentially, you can download any block of Pulsar, along with a constant sized proof, and perform a constant-time computation, to make sure that this particular block that you are accessing is valid, i.e. was not broken since the genesis. This is revolutionary, since this makes the number of blocks or validators that the chain has unimportant for the decentralization of the chain. New validators can join instantly, without trusting any third party, and any light client can verify the entire chain, even in a browser.

Of course, reality is not this simple, and it is not enough to generate a ZK proof over the consensus to make the chain succinct. This type of a solution introduces a long-range attack vector over the chain, which has an increasing chance of success with every new block. About ten years ago, Mina Protocol introduced a new consensus protocol, Ouroborus Samasika, in order to be able to solve this problem. Mina’s design is a fully succinct PoS, and it solves the problem of synchronization perfectly. You can run a Mina full node in a mobile phone or even in a smart refrigerator.

Then, what is the importance of Pulsar, if we already had a succinct chain for all this time?