Startup Advisor, Entrepreneur, Full-Stack Developer, Cybersecurity Professional, Privacy Advocate
I first learned about Homomorphic Encryption (HE) about 5 years ago. Since then, I have hell-bent on learning how to leverage it within my applications. Most of the libraries I found use many different flavors, or “scheme types”, and are written in low-level languages such as C++. This is mainly because of performance, but has the side-effect of being very hard to adopt.
So how do we use a C++ library in a web app? Spoiler — WebAssembly.
Hopefully by the end of this article, you will have a better understanding of how to use HE in Web Applications. I will briefly go over the some of the technical jargon and illustrate the pain points behind using HE followed by some alternatives.
HE is a game changer for privacy. It allows for data to be manipulated without decrypting it. The concept dates back to RSA encryption — yes, even RSA has the capability of limited HE functionality. However, it took quite some time before the industry saw a change.
In 2009, Craig Gentry’s dissertation was published and sparked the proliferation of libraries with different capabilities and varying performance. The rest is history.
Today, many libraries can be found floating in the wild. I’ve listed a few prevalent ones and their respective supported scheme types, but there are notable others.
Finding the right library and scheme type for your application involves a fair amount of research. What differences do these implementations have? What dependencies do they require — how can I use this in my app? How does a scheme type affect performance — or even more confusing, what limitations does a scheme impose on encrypted data? What the heck is bootstrapping and do I need it?
Some of these questions need answers before you even start designing a privacy-preserving application.
To get started I will talk about Microsoft SEAL as it provided the best documentation when I was learning and is what I’m most familiar with. Full disclosure - I'm not sponsored nor affiliated with Microsoft. I will simplify and make very generalized assumptions so we can get on with it and not dwell on the details.
Let’s briefly cover how to encrypt data. First, you take an array (or a vector in C++), encode it to a special format to a plaintext, and then encrypt the plaintext to a ciphertext. Homomorphic evaluations occur on ciphertexts. To get a value back, you need to decrypt and decode.
const arr = [1,2,3...] const plain = encode(arr) const cipher = encrypt(plain) // Add the cipher to itself - element wise evaluate.add(cipher, cipher) const decrypted = decrypt(cipher) const decoded = decode(decrypted) // `decoded` contains [2,4,6, ...]
Whoa! Hold your horses — I skipped ahead and made this look easy. There are a few steps before you can even get to this point. Here’s an overview of the library:
Don’t worry if this is all foreign to you…bear with me…
Bootstrapping allows for infinite homomorphic evaluations on encrypted data. Without it, there is a limit on how many evaluations (multiplications, etc.) you may perform on a cipher before it becomes impossible to decrypt correctly due to a noise factor.
For now, it is hasn’t been implemented in SEAL; however it’s on their roadmap for CKKS. That being said, implementing bootstrapping can cause a significant penalty to performance, often an order of magnitude. In many cases, homomorphic algorithms for a given application don’t even need bootstrapping.
A homomorphic algorithm without bootstrapping is called a leveled algorithm. The number of levels of the algorithm (aka how many evaluations) is strictly defined by the encryption parameters you choose.
The first step is to choose a scheme that is appropriate to your application. Do you require integers or can it afford a margin of error? BFV should be used when you absolutely need accuracy. CKKS has its own advantages, but introduces a bit of error in the decryption. With sufficient parameters the error can be reduced to well within acceptable limits — it’s just harder to learn at first.
Once you’ve decided on a scheme, now it’s time to define the parameters. This question is perhaps the most difficult to answer because it depends on many factors. Then there are more questions: how do we test which ones work? Is there room for optimization? Do I have to build a new test application each time?
Yes, you might, but let’s go over a methodology. For now, ignore what these definitions mean.
Wow — that’s a lot of things! Whats more disheartening is that these are rather terrible generalizations. There’s still a lot to learn about optimizing and we haven’t even started coding a simple example… 😑
This is the problem which plagued me for days on end. Rather than having an immediate discussion on how to pick parameters, I will tell you a way where you can rapidly experiment on your own.
It uses WebAssembly at its core with light bindings to work in Node.js or modern browsers 😎. The npm package already has zlib and intrinsics baked-in to make the performance close to the native library. Installing is as simple as npm install node-seal or yarn add node-seal— no need to compile anything and no dependencies.
This is a great first step building web applications leveraging HE, but it still doesn’t solve the problem of quickly iterating on parameters to find what works and what doesn’t.
With no solution, I did what any other developer would do — I built an app for that 🎉
I’ve established HE is possible in client-side applications, but the next step is to process encrypted data.
Yes, you could — but you don’t have to. By becoming a beta user on my platform, you can create an API which operates under a set of parameters and public keys. No secret keys are needed unless you want to simulate decryption.
A helpful test bench simulates sending and receiving an encrypted request from the browser so you can focus more on building content rather than debugging. Oh and there’s also another basic code generator for Node.js so you can sending requests from your machine to the cloud and back 💪🏻.
Ultimately, learning HE is tough — there’s really no easy way around it. I’ve built a few tools to help you get started with with either client or server-side applications.
~All without ever decrypting your data~
A big shout out to the 🧠s at Microsoft SEAL!!!
Thanks for reading!
Previously published at https://medium.com/@s0l0ist/homomorphic-encryption-for-web-apps-a3fa52e9f03d