david wong

Hey ! I'm David, a security consultant at Cryptography Services, the crypto team of NCC Group . This is my blog about cryptography and security and other related topics that I find interesting.

How does TOR works January 2015

I've always wondered how TOR (The Onion Router) worked and was a bit scared of digging into it. After all, bitcoin is pretty hard to grasp, how would TOR be different? But I found out that TOR was actually a pretty simple concept!

The official explanation is top notch. To sum up, instead of sending a packet to the destination (google.com for example), you choose a route of TOR nodes that will lead to that destination (usually 3 nodes). And for efficiency purpose you will keep that route for 10 minutes)

The idea is similar to using a twisty, hard-to-follow route in order to throw off somebody who is tailing you


  • The first node only see who's sending the packet (you) and who it is for (the second node). It decrypts the payload and send it to the second node.
  • The second node only sees it came from the first node, decrypts the payload and send it to the third node
  • The third node sees it came from the second node, decrypts the payload and send it to the destination (in clear if you don't use ssl)

From TOR's FAQ:

Can't the third server see my traffic?
Possibly. A bad third of three servers can see the traffic you sent into Tor. It won't know who sent this traffic. If you're using encryption, such as visiting a bank or e-commerce website, or encrypted mail connections, etc, it will only know the destination. It won't be able to see the data inside the traffic stream. You are still protected from this node figuring out who you are and if using encryption, what data you're sending to the destination.

To be able to do this, this is where the encapsulation or rather onion routing technique is used.


As we know the route we are going to take, we can encrypt several time our packet. For example: we'll encrypt the packet router B will have to send to router C with the public key of router A. So when router A opens the packet and decrypts it with its private key, he only sees the encrypted payload destined to router B. He can then send it to router B and the latter will decrypt it and send the payload to router C and on and on. I think the picture is clearer than my explanations.

Voilà! Not that hard huh?

PS: here's a list of potential attacks on TOR in their design paper

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Morse code in a pop song January 2015

Amazing article on the verge about how the army created a song hiding a message ("19 people rescued. You’re next. Don’t lose hope") so that hostages of the FARC could hear it on the radio.

This is a genius idea for concealing a message! Not really crypto, but kinda cool none the less. I knew about Stenography and I also posted about transforming your message into spam as a way of hiding your message, but this is cool on a different level. Even the song is catchy ^_^

There was this disturbing video of a captive soldier in a North Vietnamese prison who when forced to do a fake interview, blinked the Morse Code 'T-O-R-T-U-R-E'.

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Sed and Awk tutorials January 2015

I just ran into amazing tutorials for sed and awk, two famous text processing tools on unix.

If you are on firefox I advise you to disable CSS (View > Page Style > No Style)


$ echo day | sed s/day/night/ 




What know?

There are still many text processing tools I need to learn, like strip, strings, xdd, grep, tr, xargs, cut, sort, tail, split, wc, uniq...

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How to prepare for interviews? January 2015

If you are a student looking for an internship in Cryptography, that might be the right post for you.

Those past few months, before landing an internship at Cryptography Services, I applied in many places around the world and went through several interviews. Be it irl interviews, phone interviews, video interviews... I traveled to some remote places and even applied to companies I really did not want to work at. But those are important to get you the experience. You will be bad at your first interviews, and you don't want your first interviews to be the important ones.

In this post I'll talk about how I got interviews, how I prepared and dealt with them, how I got propositions. I might be wrong on some stuff but I'm sure this will help some students anyway :)

Think about them

Put yourself in the employer's shoes, he doesn't want to read a cover letter, he doesn't want to spend more than a few seconds on a resume, he doesn't want to hurt his eyes on horrific fonts, too many colors and typos.


  • Ditch all those premade themes.
  • Learn how to do something pretty with Word/LaTeX/Indesign/CSS...
  • Okay, if you're really bad at building a nice layout, maybe you should use a premade theme, but then use a very simple/plain one.
  • No more than one page (you're a student, don't be cocky).
  • Use keywords.
  • Be simple. Don't write something that belongs in the cover letter.

There is a lot of theory on C.V. making. I'm sure you can find better resources on how fabricate your resume.

Cover Letter

  • Be concise (I use bullet points for my cover letters and it saves the reader time)
  • Write well, make your friends read it, take a break and re-read it.
  • Be formal, polite, etc...
  • Don't hesitate to contact them again if they don't answer
  • Use my application 3pages to write your cover letter (shameless plug :D)

Side Projects

Side projects are important, like really important. If you don't have anything to show you're just as good as the next student, (almost) no one will ask for your grades.

If you don't know what side project to work on, check Cryptopals (I'll be working with the folks who made this by the way :P)

Also if you're looking for something in development, applied crypto, get a github. Many employers will check for your github.

And, you know, you could... you could write a blog. That's a nice way to write down what you're learning, to motivate you into reading more about crypto and to be able to show what you're doing.

Where to apply?

Check with your professors, they usually know a list of known companies that do crypto. In France there are Thales, Morpho, Airbus and many big companies that you might want to avoid, and also very good companies/start-ups like Cryptoexpert, Quarkslab, ... that you should aim for.

You can also ping universities. They will usually accept you without an interview but will rarely pay you. If you really want to do academic researches, you should check some good universities in Finland, France, California, Sydney, India... wherever you want to go.

If you want to do crypto or applied crypto you might want to look for a start-up/smaller companies as they are not too big and you might learn more with them. But even in big companies you might find sizable crypto teams (rarely more than 2 or 3), check Rootlabs, Cloudflare, Matasano...


Preparing for an interview is a really good opportunity to learn many things! If you know what the subject is about read more about it beforehand. If you don't know anything about the internship subject, read more about what your interviewers have done. If you don't know anything because you have been talking to a PR all along then you might want to rethink applying there.


In all the interviews I've done I just had one PR interview. I would advise you to refuse/avoid those, unless you really want to work for the company, as it's most always a waste of time /rant.

Most interviews in France were focusing on my side projects, whereas the ones I had in the US were way more technical. Younger and smaller companies did ask me things about me and my hobbies (one even asked me if I was doing some sport! I thought that was a neat question :))

Now, here's an unsorted list of questions I got asked:

  • Explain whitebox cryptography
  • How do you obfuscate a program?
  • How can you tell a cipher is secure?
  • How do stream ciphers work?
  • What is a LFSR?
  • name different stream/block ciphers
  • Explain Simple Power Analysis
  • Explain Differential Power Analysis
  • Explain Differential Fault Analysis
  • Any counter measures?
  • Explain the Chinese Remainder Theorem
  • How are points on an Elliptic Curve represented?
  • What prevents me from signing a bad certificate?
  • Imagine a system to send encrypted messages between two persons
  • How to do it with Perfect Forward Secrecy?
  • How does a compiler works?
  • What is XSS?
  • How could you get information on someone's gmail by sending him javascript in a mail?
  • Explain Zero-knowledge with the discrete logarithm example
  • You have a smartcard you can inject code on, what do you do to perform a DPA?
  • You've recorded traces from the smartcard, do you have to do some precomputations on those traces before doing a DPA?
  • Techniques to multiply points on an Elliptic Curve
  • Example of homomorphic encryption
  • Knapsack problem ...

Your questions

An interview should be an interaction. You can cheat your way through by trying not to talk too much, but it still should be a conversation because eventually, if you get the job, you guys will be having real work conversations.

You should be a nice dude. Because nobody wants to work with a boring, elitist dude.

You should never correct the interviewer. This feels like a stupid advice but correcting someone that knows more than you, even if you are right, might lead to bad things... Wait to be hired for that.

You should never let the interviewer tell you something you already know. This is an occasion for you to shine.

If you can't answer a question, ask the interviewer how he would have answer, this is a good opportunity to learn something.

Eventually, ask questions about the job, the workplace, the city. Not only they will appreciate it, it is showing that you are interested in their job, but it's also nice for you to see if the company is the kind of place you would like to work at (it also makes the table turns).

No bullshit

I don't know if any of you were planning on bullshiting but we're in a technical field, avoid bullshiting!

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My internship at Cryptography Services in Chicago December 2014

I finally chose where I'll be doing my internship to finish my master of Cryptography: it will be at Cryptography Services (a new crypto team, part of NCC Group (iSEC Partners, Matasano, IntrepidusGroup)). I will be conducting researches and audits in the offices of Matasano in Chicago. I'm super excited and the next 6 months of my life should be full of surprise!

Cryptography Services is a dedicated team of consultants from iSEC Partners, Matasano, Intrepidus Group, and NCC Group focused on cryptographic security assessments, protocol and design reviews, and tracking impactful developments in the space of academia and industry.

I guess I'll soon have to create a "life in Chicago" category =)

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Faster Python December 2014

Zokis wrote some tests on python, showing that a difference in declarations and simple syntax do have implications in the size of the program and the rapidity of execution.

For example writing a, b = 0, 1 seems faster than doing a = 0 then b = 1 Using chained conditions like a < b < 0 seems faster than doing a < b and b < 0 etc... you can find all the tests here

The differences seem negligible though. dis and timeit were used to quantify the tests.

Also here are two useful python arguments:

python -c cmd : program passed in as string (terminates option list)

# python -c "print 'haha'"

-i : inspect interactively after running script; forces a prompt even
if stdin does not appear to be a terminal; also PYTHONINSPECT=x

# python -i -c "a = 5"
>>> a
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Journal of Cryptology & University of Bordeaux December 2014

I was trying to access the Journal of Cryptology on Springer but I had to pay. Thanks to __x86 I realized I had free access to Springer thanks to my university!

So this post is oriented to my fellow classmates. If any of you want to check something there, it's free for us! (Well until we graduate).

I was trying to check the papers that got Dan Boneh and Antoine Joux their Gödel prize:

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Pohlig-Hellman Algorithm December 2014

I'm reading through A Key Recovery Attack on Discrete Log-based Schemes Using a Prime Order Subgoup which is a Small subgroup confinement attack.

It deals with stuff I had no knowledge of, like Schnorr's Signature that I talk about in a previous post, or like what I'm going to talk about now:

The Pohlig-Hellman Algorithm is a method to compute a Discrete Logarithm (which is a difficult problem) on a multiplicative group whose order is a smooth number (also called friable). Meaning its order can be factorized into small primes.

y = g^x mod p
ord_p(g) = p - 1
p - 1 = q_1^(i_1) * ... * q_j^(i_j)

Here y is the public key, x is the secret key we're trying to compute.
The order of g, our generator, is p - 1 since p is prime.
p - 1 is smooth so it can be factorized into something like 2^2 * 3 * 7 (extremely convenient example!)

Following is an overview of the method, if you read an equation and feel like it comes from nowhere (and it should feel like that), I posted a very short paper containing the simple proofs of those bellow.


The idea that should come to your mind, if you're used to seeing that kind of problem, is that there might be a way to use the Chinese Remainder Theorem (abbreviated CRT) to our advantage. What if we could write x modulo the factors of p - 1 and then reconstruct the real x with the CRT? Well, let's do just that!

To write x modulo a factor q of p - 1 we can write y^((p-1) / q) which we know and can compute, and is also equal to g^(x_q * (p-1) / q)

(If you have a factor that appears multiple times in the prime decomposition of p - 1 (for example p - 1 = 2^5, then there is also a way to ease the computations by finding multiple unknowns (5 unknowns in our example))

We then have a discrete logarithm to compute, but a small one, that we can compute efficiently thanks to Shanks' method (baby-step giant-step) or Pollard's rho algorithm.

Then we have multiple ways of writing our x (modulo the factors of p - 1) and we find out what is x with CRT (I explained this step in my airbus write-up here).

Proofs + Video

You can read through the Algorithm (or watch the video bellow), but I don't really like to do that since I'm not really good at memorizing something if I don't understand the nut and bolts of it. So here's a really good paper written by D. R. Stinson that demonstrate where the equations of the algorithm come from. And here's an explanation + example of the algorithm:

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I've open sourced my tournament app December 2014

Since I've made my first tournament app in ~2005-2006 I received many request to make an open source version available. Back then I didn't really want to release something badly coded so I just kept it for myself and allowed people to go through my online app to create tournaments.


It caught up, it was translated in 8 different languages (sometimes badly translated though) and used all across Europe in real life and on IRC (I think there was something like 7000 different organizations that got created through the app). One day some dude skyped me and offered me 80€ for the sourcecode. I made 80€.


I then rewrote everything using new technologies I had learn or I wanted to learn. CodeIgniter, Zurb Foundation, jQuery, Sass... It was kind of a mess and I must have scared away all the users it had. Eventually I didn't renew the domain name and people started complaining and asking me to hand them the app.


I was sad that there was so many people asking for a tournament app, and that mine was not out there anymore. So yesterday when someone asked me if he could have the code I uploaded everything on Github. The code is old, and it's a mess. The sass is nowhere to be found. I even wonder if it's really secure. But it works, it's easy to setup, and if it gains traction I might want to get back into it. If there was one project I fell in love with, it was this one.


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Did Korea hacked Sony? December 2014

According to the US government, yes they did:

the FBI now has enough information to conclude that the North Korean government is responsible for these actions

What do security experts think about that?

Here's a piece from Marc Roger called No, North Korea Didn’t Hack Sony. So you can guess what the director of security operations for DEFCON and principal security researcher of Cloudflare is thinking.

What about Schneier? Read about it here

I worry that this case echoes the "we have evidence -- trust us" story that the Bush administration told in the run-up to the Iraq invasion. Identifying the origin of a cyberattack is very difficult, and when it is possible, the process of attributing responsibility can take months.

What about Robert Graham? his article's title is as usual pretty straight forward: The FBI's North Korea evidence is nonsense

So there is some kind of consensus that the FBI's announcement is abrupt and shady...

To dig further... Nicholas Weaver posted an interesting article. Kurt Baumgartner as well.

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Schnorr's Signature and non-interactive Protocols December 2014

Interactive Protocols

Interactive Protocols are basically a discussion between a Prover and a Verifier where the Prover has some piece of information he wants to prove, without giving out the information.

It is often illustrated with Peggy and Victor and their super tunnel.


Usualy it takes 3 steps:

  1. Prover sends a fixed value.
  2. Verifier sends a challenge.
  3. Prover answers the challenge.

The Verifier can then verify the answer based on the fixed value. If the answer is correct, the Verifier can assume the Prover knows what he's trying to prove. Sometimes you have to repeat the protocols multiple time to be sure, and not all problems have an Interactive Proof.

Classic examples of such proofs can be found on the Discrete Logarithm problem (we'll talk about that later) and the Hamiltonian Cycle problem.

Interactive Protocols are verified if they are :

  1. Complete: a Prover can successfully answer the challenge if he is honest.
  2. Sound : a dishonest Prover cannot convince the Verifier he knows the secret.

In the real definitions we use probabilities (an honest prover still has a small chance of making a mistake, a dishonest prover still has a small chance of convincing the Verifier).

We also often want a 3rd condition on our Interactive Protocols: we want it to be Zero-knowledge, no information about our secret should be leaked in this interaction.

Here are how you prove each one of them:

  1. Completeness: Can the Prover answer correctly thanks to his secret?
  2. Soundness: From the point of view of the Verifier. If the Prover can correctly answer two different challenges for the same fixed value (however he crafted the answers and the fixed value), does it mean that he must know the secret then?
  3. Zero-Knowledgeness: If you see a transcript of a recorded instance of this interaction, will you learn anything about the secret? (See if you can create fake transcripts)

There are also notions of weak Zero-knowledge, strong Zero-knowledge, dishonnest verifiers, etc...

But let's talk about something else.

Non-interactive Protocols

Since we said that a recorded transcript of a past interaction has no value (if it is zero-knowledge), then we could assume that there is no way of proving something by showing an old transcript, or by showing a transcript with yourself.

Don't fool yourself! Yes we can. We do this by using hash functions that we deem random enough.

The idea is that, by replacing the Verifier by a random oracle, we cannot predict the challenges and we thus cannot craft a fake transcript (we like to use random oracles instead of hashes, to prove some scheme is secure).

a random oracle is an oracle (a theoretical black box) that responds to every unique query with a (truly) random response chosen uniformly from its output domain. If a query is repeated it responds the same way every time that query is submitted.

What is interesting is that this protocol was used in a Signature Scheme.

Interactive Proof of a Discrete Logarithm

The most famous academic example of Interactive Protocol is done using the Discrete Logarithm problem.

we have <g> = G, with g of order q. The Prover wants to show he knows x in g^x = y.

  1. the Prover sends t = g^e
  2. the Verifier sends a challenge c
  3. the Prover sends d = e + cx

The Verifier can then compute y^c * t = g^(e + cx) and see if it equals g^d = g^(e + cx)

A transcript would look like this: (t, c, d)

Non-Interactive Proof of a Discrete Logarithm

Doing this with a non-interactive protocol, it would look like this:

(t, h(t), d) with h a hash function.

Schnorr's Signature

This is what Schnorr's Signature is doing:

  1. t = g^e
  2. c = H(m || t)
  3. d = e - x*c

he would then send (c, d) as the signature, along the message m. Which is basically a hash of the message with a proof that he knows the secret x.

To verify the signature you would use the public key y = g^x to compute y^c * g^d = t and then you would compute the hash. It would give you the proof that the signer knows x (authentication, non-repudiation) and that the message hasn't been tampered (integrity).

So this is one way of using Non-interactive proofs!

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FAQ on Whitebox Cryptography December 2014

A great FAQ, written by Marc Joye (Thomson R&D), on Whitebox Cryptography.


Thansk Tancrède for the link!

Q1: What is white-box cryptography?
A major issue when dealing with security programs is the protection of "sensitive" (secret, confidential or private) data embedded in the code. The usual solution consists in encrypting the data but the legitimate user needs to get access to the decryption key, which also needs to be protected. This is even more challenging in a software-only solution, running on a non-trusted host.
White-box cryptography is aimed at protecting secret keys from being disclosed in a software implementation. In such a context, it is assumed that the attacker (usually a "legitimate" user or malicious software) may also control the execution environment. This is in contrast with the more traditional security model where the attacker is only given a black-box access (i.e., inputs/outputs) to the cryptographic algorithm under consideration.

Q2: What is the difference with code obfuscation?
Related and complementary techniques for protecting software implementations but with different security goals include code obfuscation and software tamper-resistance. Code obfuscation is aimed at protecting against the reverse engineering of a (cryptographic) algorithm while software tamper-resistance is aimed at protecting against modifications of the code.
All these techniques have however in common that the resulting implementation must remain directly executable.

Or as Francis Gabriel writes here

Code obfuscation means code protection. A piece of code which is obfuscated is modified in order to be harder to understand. As example, it is often used in DRM (Digital Rights Management) software to protect multimedia content by hiding secrets informations like algorithms and encryption keys.

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Sécuday @ Lille on January 16th December 2014


SECURITY DAY will take place at the University of Lille 1, in France, on January 16th. People from Quarkslab (where I almost did my internship), ANSSI, Microsoft, ... will give talks. There is even one of my classmate Jonathan Salwan.

I'm trying to find a way to get there, so if you want to buy me a beer this might be the right place :D

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OneRNG December 2014

I like how people make an extreme effort to create "sure" source of random numbers.

OneRNG has released a new usb source. Everything is opensource (open hardware, open software), you can even create your own by following instructions on their websites.

OneRNG collects entropy from an avalanche diode circuit, and from a channel-hopping RF receiver. It even has a “tinfoil hat” to prevent RF interference — you can remove the hat in order to visually verify the components being used.

Now I'm wondering who is using that and for what

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