Cryptography Suite for zk-Rollups, zk-Bridges and much more.

Written by Mikhail Komarov on 25 Oct 2021.

This post is a response to the question we’ve received about isn’t it too much to try to build two pretty complex zk-bridges at the same time.

The answer is No.

Well, not a “Secret Sauce”, but just a project of ours behind those bridges. First one within a couple mentioned within our Twitter “About” section (“Home foundation for =nil; Crypto3 and =nil; `DROP DATABASE * projects”).

So, can I start?

As it can be noticed, all of the bridges of ours use pretty much the same approach: wrapping a heavy proof verification (or some heavy data set verification) into a verification of a lighter proof, which proves the successfull verification of the initial heavier proof.

This allows complicated data structures (in Solana’s case it is a “light-client” state, in Mina’s case it is a Pickles SNARK proof) to be succesfully verified on EVM by verifying a lighter proof which proves the initial data correct verification. So the whole construction stays formally secure and correct.

Such approach combined with the LPC commitment scheme gives the transparency properties, which in practice means the absence of a need for a proof system trusted setup, which in its turn makes such bridges completely trustless.

Sounds like the approach for a framework, right?

That is why we decided to wrap this approach into the framework based on the cryptography suite of ours.

Crypto3 C++ Cryptography Suite (https://github.com/nilfoundation/crypto3)

Yeah. Back then, in April of 2018, there were few more or less complex and complete and proven classic cryptography suites libraries for in C (OpenSSL, libsoduim, libtomcrypt) or C++ (Botan, libcryptopp, libsoduim C++ bindings, OpenSSL C++ bindings) and some early-stage Rust-based developments, which everyone was building their signatures and hashes on top of. More sophisticated ones included suites like libsnark, 5GenCrypto, and ZCash-originated developments, which were far from what we needed. Also none of cryptography suites mentioned contained threshold signature/encryption schemes, zero-knowledge proof systems, homomorphic signatures/encryption and verifiable delay functions and lots of other things all at once.

Just like that: https://github.com/filecoin-project/bellperson.git, https://github.com/cryptonomex/secp256k1-zkp, https://boringssl.googlesource.com/boringssl/ or https://github.com/libressl-portable/openbsd.git ?

Most of these patches/forks were made for some particular purpose and usually do not meet the requirements of handling enough of schemes and primitives at once. So patching those would result in the same thing - extensive refactoring, which sometimes gets even more complicated than implementing things from scratch.

Well, first of all, its size. It is not just a library, it is a suite, which consists of 39 libraries, representing literally every field of modern applied cryptography. Since the April of 2018 we’ve put pretty much everything you can think of inside of it: VDFs, signature schemes (including threshold ones with various DKGs), zero-knowledge proof systems (R1CS and PLONK-based ones), more traditional cryptography notions (block ciphers, hashes, message authentication codes, key deriveation functions etc). Full list is available in here: https://crypto3.nil.foundation/projects/crypto3/modules.html along with documentation to each module in here: https://crypto3.nil.foundation/projects/crypto3/pages.html.

Well, all the projects we’ve accomplished were either based on top or eventually ended up inside of the cryptography suite of ours. Those old Chia Network VDF and ProofOfSpace competitions results found their place among VDF’s implementations within the Crypto3.VDF module of ours In particular: https://github.com/NilFoundation/crypto3-vdf/blob/master/include/nil/crypto3/vdf/chia.hpp.

Our Filecoin-related effors (proof generation performance speed-ups and C++ Filecoin Proofs implementation) eventually found their place among the developments based on top of the cryptography suite: https://github.com/NilFoundation/crypto3-fil-proofs (not finished and will probably not be finished, but anyway).

Same thing happened with Mina-Ethereum, (https://github.com/nilfoundation/evm-mina-verification.git) and Solana-Ethereum, (https://github.com/nilfoundation/evm-solana-verification.git) zk-bridges projects of ours.

That is what for.

It is just a tool. Complicated but reliable, mature and time-proven. No special feelings in here.

To minimize complexities induced by such a toolchain usage, we decided to:

- Design the suite architecture from the scratch to keep the API clean out of backward compatibility.
- Design the API to be very similar to STL (hashing example. We even had a special Boost-dedicated version, intended to be proposed to Boost and then to standartization commitee (WG21) one day (https://github.com/nilfoundation/boost-crypto3), so in case it gets accepted it would be possible to do basic cryptography (we hadn’t proposed more advanced modules to the WG21) with STL right out of the box.
That is how would an Boost/STL-enabled AES-128 encryption look like:

`#include <boost/crypto3/block/aes.hpp> #include <boost/crypto3/block/algorithm/encrypt.hpp> #include <string> #include <cassert> using namespace boost::crypto3; int main() { std::string input = "\x6b\xc1\xbe\xe2\x2e\x40\x9f\x96" "\xe9\x3d\x7e\x11\x73\x93\x17\x2a" "\xae\x2d\x8a\x57\x1e\x03\xac\x9c" "\x9e\xb7\x6f\xac\x45\xaf\x8e\x51" "\x30\xc8\x1c\x46\xa3\x5c\xe4\x11" "\xe5\xfb\xc1\x19\x1a\x0a\x52\xef" "\xf6\x9f\x24\x45\xdf\x4f\x9b\x17" "\xad\x2b\x41\x7b\xe6\x6c\x37\x10"; std::string key = "\x2b\x7e\x15\x16\x28\xae\xd2\xa6" "\xab\xf7\x15\x88\x09\xcf\x4f\x3c"; std::string out = encrypt<block::aes<128>>(input.begin(), input.end(), key.begin(), key.end()); return (out == "3ad77bb40d7a3660a89ecaf32466ef97f5d3d58503b9699de785895a96fdbaaf" "43b1cd7f598ece23881b00e3ed0306887b0c785e27e8ad3f8223207104725dd4"); }`

- Make it a fool-proof by employing massive amount of compile-time correctness checks. So now it is as close as possible to the state “if it compiles, then it works correctly”. Every usage mistake which can be made will be prevented during the compilation.
- Make it comfortable for prototyping novel schemes/proof systems/hashes/other by keeping the implementation as close to formal constructions.

Hashes, for example, have their Merkle-Damgard or Sponge constructions defined with explicit reprensentations, which make it easy to map the implementation to the definition given within the paper or specification. Such representations, though, get elliminated during compilation so they do not affect the performance. So defining a new hash goes down to something similar to “I want a hash, based on Sponge construction with a compressor defined as follows (here goes the definition of a compressor)”. Poseidon hash is defined exactly this way. With, speaking in terms of software architecture, a stateless policy class, simply defining proper types inside.

More on prototyping new hashes can be found on the Crypto3.Hash “Implementation” page. Most of suite modules do have similar tutorials on https://crypto3.nil.foundation/projects/crypto3/pages.html.

Another example is an LPC or FRI commitment schemes which were given the same interface, so changing the commitment scheme within the proof system is as easy as changing the template parameter.

Sure. Any of currently existing bridges repositories (https://github.com/nilfoundation/evm-mina-verification.git or https://github.com/nilfoundation/evm-solana-verification.git) can be reused for the purpose of creating a new bridge.

All of them consist of several parts:

- Wrapping proof generator. Usually emplaced in
`bin`

directory. Uses RedShift (or any other) proof system and FRI/LPC commitment scheme (or any other) definitions from Crypto3.ZK. - Circuit definition done with Crypto3.Blueprint.
- Proof verification logic. In-EVM one most often. Sometimes other virtual machines get involved as well.
- Crypto3 libraries introduced as
submodules in
`libs`

directory.

This results in a executable (not matter if it is native or not) generating a RedShift proof for a circuit, defined with a Crypto3.Blueprint library, which complies with the virtualized verification logic input format, defined within the same repository in Solidity files. Same architecture for every zk-bridge core solution, the only difference is a circuit.

And, here we go. A reusable framework for completely trustless bridges.