Hey! I'm David, the author of the Real-World Cryptography book. Previously I was the security lead for Diem (Libra) at 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 does TOR works posted 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?

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# Morse code in a pop song posted 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 posted 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)

## Sed

\$ echo day | sed s/day/night/
night

http://www.grymoire.com/Unix/sed.html

## Awk

http://www.grymoire.com/Unix/Awk.html

## 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? posted 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 :)

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.

## Resume

• 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)
• 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...

## Preparation

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.

## Questions

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 ...

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 posted 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 =)

# Faster Python posted 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'"
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
5
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# Journal of Cryptology & University of Bordeaux posted 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 posted 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.

## Overview

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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