Introduction

Private-public key pairs are the cornerstone of much of the cryptographic security underlying everything from secure web browsing to banking to cryptocurrencies. Private-public key pairs are asymmetric. This means that given one of the numbers (the private key), it's possible to derive the other one (the public key). However, doing the reverse is not feasible. It's this asymmetry that allows one to share the public key, uh, publicly and be confident that no-one can figure out our private key (which we keep very secret and secure).

Asymmetric key pairs are employed in two main applications: In authentication, where you prove that you have knowledge of the private key; and encryption where messages can be encoded and only the person possessing the private key can decrypt and read the message.

In this introduction on digital signatures, we'll be talking about a particular class of keys: those derived from elliptic curves. There are other asymmetric schemes, not least of which those based on products of prime numbers, including RSA keys [1].

We're going to assume you know the basics of elliptic curve cryptography (ECC). If not, don't stress, there's a gentle introduction in a previous chapter.

Let's get started

This is an interactive introduction to digital signatures. It uses Rust code to demonstrate some of the idea presented here, so you can see them at work. The code for this introduction makes use of the libsecp256k-rs library.

That's a mouthful, but secp256k1 is the name of the elliptic curve that secures a lot of things in many cryptocurrencies' transactions, including Bitcoin.

This particular library has some nice features. We've overridden the + (addition) and * (multiplication) operators so that the Rust code looks a lot more like mathematical formulae. This makes it much easier to play with the ideas we'll be exploring.

WARNING! Don't use this library in production code. It hasn't been battle-hardened, so use this one in production instead

References

[1]: RSA (cryptosystem). https://en.wikipedia.org/wiki/RSA_(cryptosystem). Accessed 11 Oct 2018.