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Rolling your own crypto: Everything you need to build AES from scratch

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# Rolling your own crypto: Everything you need to build AES from scratch (and then never use it for anything of consequence)

You often hear the phrase **"Don't roll your own crypto"**. I think this sentence is missing an important qualifier: **"...and then use it for anything of consequence"**. If you are building a product or service, or are trying to communicate privately, then you should absolutely pick a vetted, open source, off-the-shelf implementation and use it. If, however, your goal is to learn, then there is honestly no better way than simply hacking away on your own code!

Before we get into it, just a quick word about why the phrase is so often touted: Cryptography is hard to get right for a number of reasons. First, there is the mathematical side of things, where a slip-up can take something that takes the *lifetime of the universe* to break, into something broken in *minutes* by someone with a bit of compute power. Lesser known perhaps, but still equally serious, is the issue of side-channels. The code you write can be completely correct, but still leak secrets through cache timing attacks, or even measured fluctuations in power usage as the algorithms are running. These are not academic attacks either - they are possible with off the shelf hardware, and someone with enough know-how to pull it off. The folks who write the industrial-scale crypto libraries are well aware of both of these aspects, and trust me: it's better to just leave it to them when it really matters. They're a pretty smart bunch.

## Do~~n't~~ try this at home

With the warnings out of the way, let's talk about why you **would** want to build your own crypto. One reason is [just for fun](https://justforfunnoreally.dev/)!. Another might be that you actually want to *become* one of those people who work on the industrial scale crypto. Finally, you might be interested from the red-team perspective; learning so that you try to attack the poor souls who write the insecure code.

I'm by no means an expert, but I recently got interested in peering behind the curtain while reading Jean-Philippe Aumasson's book [Serious Cryptography](https://nostarch.com/seriouscrypto). The chapter on AES gave a great overview of the algorithm, as well as its *modes of operation*, but some of the details were still a little fuzzy. This post is my attempt to explain some of the parts that were only clear to me after reading the [spec](https://nvlpubs.nist.gov/nistpubs/fips/nist.fips.197.pdf), trawling through code, and spelunking through an underground network of wikipedia rabbit holes.

## Just give me the code!

There is a full, [open source implementation in C](https://github.com/francisrstokes/AES-C) that accompanies this article. I strongly recommend reading the source as a supplement to the article, as it necessarily explores all of the ideas in their full detail.

## AES

AES, or the "Advanced Encryption Standard", is a extremely widely used symmetric block *cipher*. Symmetric here refers to the idea that both the one encrypting and the one decrypting use the same key. Block refers to the way in which the stuff you're encoding (the *plaintext*) is turned into the *ciphertext* (the random-looking but reversible sequence of bytes that is the result of the encryption). Cipher refers to any algorithm for encryption. A block cipher operates on multiple bytes of plaintext at the same time, arranged into a 2D block.

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2022-07-19 12:40:52 UTC