## Bridging Mina with =nil;.

How will a Mina Protocol's bridge become bi-directional?

Written by Mikhail Komarov, Aleksey Sofronov
Special thanks to Luke Pearson for discussion, adjustments and comments. on 28 Jun 2022

This post covers one of the applications of a =nil; ‘DROP DATABASE *-based Trustless Data Accessibility protocol - trustless bridging. In particular, it covers an integration with Mina Protocol, for which we’ve already described, developed and demoed a one-directional bridge.

In the end of April 2022 Mina Protocol’s in-EVM state verification finally saw the light of the day https://verify.mina.nil.foundation. Meanwhile, =nil; Foundation wasn’t sitting idle and announced a =nil; Database Management System called =nil; ‘DROP DATABASE * and its replication protocol extension which is capable of providing trustless I/O to various databases through the DBMS’s query language unified for all the databases (Bitcoin, Ethereum, Solana, Mina, others).

## And how is this relevant?

=nil; provides users (no matter what kind of users - applications or protocols) with on-demand state and query proofs for three different use cases:

1. Trustless Data Access. If a proof consumer is an end-user application or a frontend of any kind, it could be used for accessing data on the protocol that it lives on.
2. Trustless Bridging. If a proof consumer is a protocol (e.g. Mina), and a user of this protocol requested a state/query proof of another protocol (e.g. Ethereum or Solana), it could be used for trustless bridging of these two protocols.
3. Pluggable Scaling. If a proof consumer is a protocol and its user (an application or otherwise) it is of the same kind a user has requested a state/query proof from (e.g. some Avalanche user has requested a state/query proof of another independent, application-specific Avalanche deployment), then it results into increasing the throughput of the original protocol cluster deployment.

In this article, we’ll be describing the second use case applied to Mina.

## Okay. Haven’t you already done this? What’s so new about it?

Not so fast. What we’ve accomplished so far is a one-directional Mina-Ethereum bridge core (which is effectively an in-EVM Mina state transition verification).

Let’s briefly walk through the current design Since strainghtforward Kimchi proof verification on EVM is too expensive, we basically have to to wrap Kimchi proof verification algorithm in another proof (using =nil;’s Placeholder proof system), and create a ‘proof of a successful verification’ - this can then be cheaply verified within the EVM.

So the current step by step process is as follows:

1. Mina’s native state proof gets retrieved
2. Auxiliary proof in Placeholder proof system is being generated
3. Auxiliary proof is being submitted to EVM for the verification to happen

Application examples of a currently existing single-directional in-EVM Kimchi proof verification are:

1. Proving that a particular trade order was filled on Uniswap with Mina without revealing the actual trade
2. Proving the location with Mina with some transfer happening on Ethereum afterward
3. Proving the identity with Mina and using it as a second factor to the Ethereum-based action authorization
4. Others, you name it.

More detailed overview of a currently existing solution can be found in previous relevant blog posts: https://blog.nil.foundation/2021/11/01/mina-ethereum-bridge-design.html, https://blog.nil.foundation/2021/09/30/mina-ethereum-bridge.html.

The most important thing that the =nil; brings is the capability to fetch state and query proofs for various other protocols (like Ethereum, Solana or Avalance). And, since we can switch the proof system being used within =nil;, those query and state proofs can be bridged to any supported proof system. In Mina’s case specifically, we generate the proofs in Kimchi.

To be more presice we have a couple of options how to arrange this:

1. Reconstruct the query/state proof directly in Kimchi, which would look something like this: the user constructs a query (such as _db[“eth”].select(“accounts”).where(“key”).equals(“name”).and(“value”).equals(“100”)) and this would return a Kimchi proof, ready to be verified on Mina right away.

The downside of this approach is that proof generation will become much more expensive, because of the need to reconstruct the original proof in Kimchi - essentially incurring a linear double cost for each query.

2. Wrap’ the state/query proof (generated with the Placeholder proof system) by implementing the verification algorithm as a Kimchi circuit. This would also incur an additional proof generation for each query, but will generally be much cheaper as this circuit is constant-size, and avoids naively duplicating the original proof.

At a glance, the second option seems to be more viable.

This specification allows us to verify third-party protocol data proofs directly within Mina, which leads us to…

## Mina Bi-Directional Bridging!

Yes!

Let’s break this process down step-by-step.

1. First of all, we need a state proof for the third-party protocol we want to bridge to Mina, generated in the Placeholder proof system, and wrapped in a Kimchi proof. Let’s say this third-party protocol is Ethereum (but in reality, it could be any protocol supported by =nil;). So the user or application wanting to bridge Ethereum data to Mina has to retrieve this proof from =nil;.
2. Next, submit the retrieved proof to the Mina verifier. From Mina’s perspective, a =nil;-generated data proof will be identical to a native Mina’s zkApp-generated proof. A =nil;-flavored Mina’s zkApp, if you will.
3. After the proof is submitted to the Mina verifier, the user (or Mina-based application) receives the relevant Ethereum data - and voila! Ethereum’s data is being safely and trustlessly used in Mina.

Other protocols, supported by =nil; are also going to be available for Mina-compatible state/query proof generation and retrieval. For example, Solana’s integration is on its way.

Same process as for Ethereum from user story side:

1. Retrieve a state/query proof with Solana’s data generated with Placeholder and wrapped with Kimchi proof system from =nil;.
2. Submit such a proof to Mina’s native verifier just like you would do with a Snapp’s proof.
3. Feel free and safe to use Solana’s data within the Snapp.

## And a data proof from a combination of protocols?

Yes! It is a DBMS which supplies =nil; with data, remember?

So, in case a user wants to bridge Solana’s and Ethereum, the only adjustment to the user story would be to retrieve a proof of a composite query from =nil; with something like: SELECT FROM SOLANA WHERE a EQUALS b; SELECT FROM ETHEREUM WHERE c EQUALS d. Then =nil; would return a proof of such a query generated in Placeholder proof system and wrapped with Kimchi, which will be okay for Mina’s native verifier, so one can safely use other protocols data in Mina.

## Sold. When?

According to our best estimates, we plan to introduce the first working prototype of Ethereum’s state/query proof generator compatible with Mina’s proof system in Q4 2022.