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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# How I explained REST to my wife posted March 2014

Wife: You mean http like the beginning of what I type into the browser?

Ryan: Yeah. That first part tells the browser what protocol to use. That stuff you type in there is one of the most important breakthroughs in the history of computing.

http://www.looah.com/source/view/2284

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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

# 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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# Monty Hall visualization posted December 2013

The Monty Hall problem is to me one of the most fascinating probability problem (for it's simpleness and unintuitive results) that got my mind blown since I learned about it in high school.

One day in high school, in my Math class, the teacher told us about that famous problem. Monty Hall was an old and popular TV show in the states were you had to choose a door to open from three different ones. Behind one of them was a car, behind the two others were goats. Obviously, the goal of the game was to win the car (except if you were really into goats, but then I guess you could have bought a lot of those with a car).

Anyway, the tricky part was that when you made a choice, the host asked you to wait before opening it and would open another door, revealing a goat. Then he would give you the opportunity to waive your initial choice and swap door one last time.

Here lies the probability problem. Do you think you would have more chance of winning if you changed your choice?

My math teacher said yes, and I could not believe that, I remember loudly objecting, telling the teacher it was not possible, that it was not logical. I declined what seemed grotesque at the time, I refused to acknowledge such an unintuitive result, such a simple thing, my brain could do the calculation easily so why would you tell me I was wrong on such a trivial thing.

But yes, I was wrong. I knew I was wrong. I was upset at my own mind. I didn't understand how I could be so convinced that changing choice wouldn't change my chances of winning the car. The problem was simple, so simple. And yet my mind couldn't make its way around it.

After many years of training my brain to think differently about probabilities, I can know see how this problem works. 7 years after my first introduction to this problem, I can now grasp a part of it. I understand it, I know the probabilities enrolled in the resolution of this problem, I've learned them at uni and I made the effort to think about that problem quite a lot during those last years. I actually often ask that problem to my friends, to blow their mind. But still, 7 years after being introduced to that problem, I still have troubles finding its probabilities "natural". My brain still cannot process the fact that it HAS to work that way, that the world is turning in that direction and no others.

I hope I didn't send you to sleep with this. If you want to know more about the mathematician who published this result and got insulted by numerous math PHD for being wrong, you can take a stroll on the wikipedia page.

My technique to wire my brain on the right path? Thinking about a hundred doors, 1 car, 99 goats. I open one door, the host closes 98 others. It feels easier to process when told this way, but there is still a part of me, somewhere, that tells me it wouldn't change a thing. Even with 98 doors opened. What is wrong with my brain?

If you still don't believe me, there is a short and visually clear explanation here.

PS: this is one of my go to when I want to be amazed at how unintuitive or how little we know about how things work. If you like that kind of thing, you can also check the twin paradox or the biography of Milton H. Erickson.

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