david wong

Hey! I'm David, cofounder of zkSecurity and the author of the Real-World Cryptography book. I was previously a crypto architect at O(1) Labs (working on the Mina cryptocurrency), before that I was the security lead for Diem (formerly Libra) at Novi (Facebook), and a security consultant for the Cryptography Services of NCC Group. This is my blog about cryptography and security and other related topics that I find interesting.

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Exams posted April 2014

We've been a group of 4-5 students spending each nights at the Crémi these few last days, this building of three floors where each floor has around 10 rooms full of computers.

We work, we eat, we play, and we crash each other computers.

There are a bunch of games installed on every computers but we mostly play SauerBraten, a quake-like.

sauerbraten

My 15-year-old self would have spent most of his days here playing, if only he knew that his future campus would have such a sacred place :)

How do we crash each other computer? We just ssh into their machine and launch a fork bomb:

 :(){ :|:& };:

It operates by defining a function called ':', which calls itself twice, once in the foreground and once in the background.

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Fast Fourier Transform posted April 2014

So, I've learned about Fourier every year in my bachelor of Mathematics and I'm learning about the efficient algorithm dealing with the Fourier Transform in my class of Algebra right now.

I found a really nice video explaining really quick what it is, concretely.

Here's wikipedia way of showing that fourier made by LucasVB, this crazy guy doing all those math gifs you've probably seen before :) more here

There's also a visualization in d3.js here: http://bl.ocks.org/jinroh/7524988

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Just learn Vim posted March 2014

The editor I'm using the most is Sublime Text 3. It's just super easy to use and super useful when you combine it with the right plugins and snippets.

But I love switching editors. I've used Frontpage, Dreamweaver, PHP Designer, Netbeans, Notepad++... and others I can't remember. I've recently tried the beta of Light Table and Brackets (that is truly amazing!), and I am eagerly waiting for Atom the open source IDE of github.

I also love spending time with Emacs. It's hard to master but I dig the "you don't need a mouse" aspect. One thing I found really annoying though is that most software use Vim by default. Wanting to master emacs, I didn't want to spend time learning Vim as well and I started tweaking the settings so that software X would use emacs by default. And that works well until... But then you run into some complications, for example I'm still trying to figure out how to do a git diff with emacs, or you run into a machine without emacs, and then it's either nano, which is shitty, or something else that is installed on the machine... and vim is (almost?) always installed by default.

So I decided to just learn Vim. And it was actually easier than it sounded and I feel like I'm going to avoid a lot of headaches now. Sometimes it's better to learn and adapt rather than try to use our own tools.

And if you're like me, you'll actually have a lot of fun learning vim :)

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Hashes, MACs, Signatures posted March 2014

I was very confused when I was introduced to signatures and macs because I thought they were just Hashes. I got to understand what it was and... it's actually super simple.

Here's a great explanation on the crypto stackexchange but here's mine:

  • I have a huuuge message that I want to transfer to a friend. I'm scared some of the words would change during transit. Solution? I just hash it and send the hash with the message. hash = Hash(message). A hash is pretty small (for example a md5 hash is 32 characters) so it's no trouble. My friend then receives the message and the hash, he can Hash(message) it and see if it gives him the same hash. If it doesn't then he knows that the message was changed and he can ask me for a new copy.

You can also call that an unkeyed hash, simply because it doesn't use a key. You just apply the algorithm to the message, no other arguments are given to the hash function.

  • Okay now, We had some problems because some bad guy has sent numerous bad messages to my friends pretending he was me. I still want to hash my message but I also want to tell my friend it was me who wrote it. So, like a symmetric cipher, I generate a key that I share with my friend. And I hash my message with that key Hash = HMAC(key, message). My friend can now hash it with the same key when he receives the message and see that we have the same hash.

We just used a (symmetric) keyed hash or a HMAC (Hash-based message authentication code). Note that we could have used a MAC based on a Cipher as well (CMAC).

  • So me and my friend have been writing many messages to a community of coders. We want to sign each messages with our name, but that's not enough, another bad guy is posting bad stuff signed with our names on different websites. So let's use a Hash that people can verify, like an asymmetric cipher, we generate both a secret key and a public key, we hash the message with our secret key and we post the message, the hash and the public key. Hash = Sign(secret_key, message). People can then verifiy that Hash with the public key. Voila ! We just used a Signature or how I like to call them a asymmetric keyed hash. It allows for integrity of data, thanks to the hash, authentification of the authors, thanks to the secret key (this is a MAC), non-repudiation thanks to the public key (and now we have a signature).

So if you got it right, Hash < Mac < Signature. They're all useful and you should use the one relevant according to the context.

I'll just copypasta the table on the stackoverflow answer, because it's a real nice summary:

Cryptographic primitive | Hash |    MAC    | Digital
Security Goal           |      |           | signature
------------------------+------+-----------+-------------
Integrity               |  Yes |    Yes    |   Yes
Authentication          |  No  |    Yes    |   Yes
Non-repudiation         |  No  |    No     |   Yes
------------------------+------+-----------+-------------
Kind of keys            | none | symmetric | asymmetric
                        |      |    keys   |    keys
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