david wong

Hey! I'm David, the author of the Real-World Cryptography book. I'm a crypto engineer at O(1) Labs on the Mina cryptocurrency, previously 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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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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Elliptic Curve Cryptography posted March 2014

A video I found about Elliptic Curve Cryptography that talks about the Discreet Logarithm Problem and the Diffie-Hellman Handshake with ECCs. Class is in english, with bits of german and even some french :)

Such a nice lecture, Christof Paar makes me think of a younger Gilbert Strang, seems to be a great professor. I was captivated until the end and I started liking ECCs again :)

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Hacking Week posted February 2014

A teacher from my uni (and who was teaching Programming last semester) is organizing a Hacking Week next week. Signs up are still possible there : http://hackingweek.fr/contestant/list/

It should be a Capture The Flag kind of contest. It should be interesting, although I'm going to ski with some friends so I won't be able to be really into it...

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Initial Permutations in DES posted January 2014

I have to code a whitebox using DES encryption in a class. Which is pretty cool (I would have prefered doing it with AES but the other group got tails and we got heads).

Here is where the Stanford course I passed on Coursera shines. The explanation of DES on it is brilliant. I was wondering about the initial and final permutations that occurs in the algorithm though and Dan Boneh doesn't really talk about it besides saying it's not for cryptographic purposes.

I found a solution on a new sub-stackoverflow dedicated to Cryptography : http://crypto.stackexchange.com/questions/3/what-are-the-benefits-of-the-two-permutation-tables-in-des

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What is the best tool? posted December 2013

Constantly, when I start a new project, I try to look for better tools to do the job.

Lately I've been using CodeIgniter as a PHP MVC, jQuery as a javascript library, Bootstrap as a blueprint and TWIG as a template engine.

I've been noticing numerous people from the CodeIgniter community moving to Laravel, which seems to be pretty awesome. So I look at Laravel, and I think to myself "gosh this looks fun to learn, but I don't have time and I have a lot of projects in mind". And then as I read more and more about Laravel, I see people talking about how RoR is better. And then about how Django is better... This seems like a never ending search for a better technology.

I read somewhere that good coders code, great coders re-use. And more importantly, amazing coders ship. I have to ship code, I have to be productive, and I don't think I should be wasting too much time learning new technologies.

The difficult thing is to judge whether or not the time wasted in learning a new technology would be less than the time wasted coding with an outdated one.

So I want to learn, and I want to ship. And it's hard to do both.

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