First of all why do we focus on ZK Technologies?

Because Vitaliks said so:

In general, my own view is that in the short term, optimistic rollups are likely to win out for general-purpose EVM computation and ZK rollups are likely to win out for simple payments, exchange and other application-specific use cases, but in the medium to long term ZK rollups will win out in all use cases as ZK-SNARK technology improves.

What does ZK mean?

Defintion:
In cryptography, a zero-knowledge proof or zero-knowledge protocol is a method by which one party (the prover) can prove to another party (the verifier) that a given statement is true while the prover avoids conveying any additional information apart from the fact that the statement is indeed true.

ZK = Scaling + Privacy
They are both a privacy enhancing technology, as they reduce the amount of information that needs to be provided between users, and a scaling technology, since they can allow proofs to be verified at a faster rate because they don’t contain the full amount of information for non-private systems.

BUT...

Newer ZK Technologies like ZK Starks are out!
The general consensus is that they are better than ZK Snarks.
Because of that I would like to keep an open mind and compare both technologies.

ZK-SNARK

Stands for zero-knowledge succinct non-interactive argument of knowledge.
First paper in 2012.
ZK-SNARKs at their base depend upon elliptic curves for their security.

Protocols:

• Loopring
• Polygon Hermez
• ZKSync
• ZKSwap

Pros:
+ Smaller proof size
+ Smaller verification time
+ Bigger developer community and libraries (longer in the game)

Cons:
- Require trusted setup (honest participants needed)
- Longer prover time
- Not secure by quantum computers
- Stong crypto assumptions
(The weaker the assumptions of a model, the stronger the model. Relaxing an assumption is to go from a strong to a weak, more realistic, assumption.)

ZK-STARK

Stands for zero-knowledge scalable transparent argument of knowledge.
First paper in 2018.
STARKs rely on hash functions.

Protocols:

• Starkware/StarkEx
• Immutable X (StarkEx)
• DYDX (StarkEx)
• Starknet
• Polygon Miden

Pros:
+ Quantum resistant
+ Assumptions: Collision resistant hashes (less likely to be attacked)
+ No trusted setup required
+ Vocal support from Ethereum foundation
+ More scalable in terms of computational speed

Cons:
- Far larger proof size = more gas
- Small developer community because it is new

Below, we have a couple tables depicting some of the high-level differences between the two technologies.

​

https://preview.redd.it/80775mgbwua81.png?595&format=png&auto=webp&s=9694bc897d6811517df38b6d3d05009447774303

Lessons to learn:

While ZK Snark are currently more popular and also mentioned by Vitalik I would like to suggest everyone to keep an open mind about other technologies.

ZK Starks do not need a trusted setup and are generally more secure.
On the downside the verfication cost is orders of magnitudes higher!!
Currently this is the killer but if the verfication cost can be reduced

......ZK Starks can be very serious competition.

Now who uses what?

l2beat.com gives an good overview of all relevant L2's.

• ZK Stark protocols are marked in blue (all use Starkware)Starkware uses Volition: This is a hybrid of ZK Rollups and Validiums. This gives the user the freedom to chose the technology based on their preference. (Cost, Security, Scalability)
• ZK Snark protocols are marked in green

​

https://preview.redd.it/out0k59cwua81.png?936&format=png&auto=webp&s=db11db33b19276af3332c3767372b3d1f24f451c