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.

Find all the pairs in a list that are summing to a known number posted May 2014

I got asked this question in an interview. And I knew this question beforehands, and that it had to deal with hashtables, but never got to dig into it since I thought nobody would asked me that for a simple internship.

I didn't know how to answer, in my mind I just had a simple php script that would have looked like this:

$arr = array(-5, 5, 3, 1, 7, 8);
$target = 8;

for($i = 0; $i < sizeof($arr) - 1; $i++)
{
    for($j = $i + 1; $j < sizeof($arr); $j++)
    {
    if($arr[$i] + $arr[$j] == $target)
        echo "pair found: ${arr[i]}, ${arr[j]}";
    }    
}

But it's pretty slow, it's mathematically correct, but it's more of a CS-oriented question. How to implement that quickly for machines? The answer is hash tables. Which are implemented as arrays in PHP (well, arrays are like super hash tables) and as dictionaries in Python.

I came up with this simple example in python:

arr = (-5, 5, 3, 1, 7, 8)
target = 8

dic = {}

for i, item in enumerate(arr):
    dic[item] = i

    if dic.has_key(target - item) and dic[target - item] != i:
        print item, (target - item)
  1. iterate the list
  2. assign the hash of the value to the index of the value in the array
  3. to avoid finding a pair twice, we do this in the same for loop:
    we do the difference of the target sum and the number we're on, we hash it, if we find that in the hash table that's good!
  4. but it could also be the number itself, so we check for its index, and it has to be different than its own index.

Voilà! We avoid the n-1! additions and comparisons of the first idea with hash tables (I actually have no idea how fast they are but since most things use hash tables in IT, I guess that it is pretty fast).

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Notes on ECC (Elliptic Curve Cryptography) & Internship progress posted May 2014

One last exam, ECC, and then I'm free to do whatever I want (no I still haven't found an internship, but I talked with TrueVault, Cloudflare, MatterMark, Spotify and maybe Matasano so this has been a good experience nonetheless).

I stumbled upon the notes of Ben Lynn an ex Stanford's student that took an ECC class there. They're pretty awesome and I kinda want to do something like that on this blog. Maybe next year it's a bit late for that :)

The notes are here

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Toom-Cook multiplication for dummies posted April 2014

We're learning a lot of algorithm in my algebre et calcul formel class. One of them is the Toom-Cook algorithm used for multiplication of large integers.

I found a super simple explanation of it on a forum, it helps:

Say, we want to multiply 23 times 35.
We write,
p(x) = 2x + 3,
q(x) = 3x + 5.
We are using our realization that any integer can be written as a polynomial.
Here, p(x), represents 23, and q(x), represents 35, when x equals 10.
We write,
p(x)q(x) = r(x).
That is, p(x) times q(x), equals r(x).
So,
(2x + 3)(3x + 5) = ax^2 + bx + c = r(x).
Now,
p(0)q(0) = r(0).
So,
(20 + 3)(30 + 5) = a0 + b0 + c.
Therefore,
c = 15.
Now,
p(1)q(1) = r(1).
Therefore, when we do the substitutions (for x and c),
a + b = 25.
Now,
p(-1)q(-1) = r(-1).
Therefore, when we do the substitutions (for x and c),
a - b = -13.
Now, we already know c, and we just need to find a and b.
We have two linear equations and two unknowns,
a + b = *25,
a - b = -13.
We just add the two equations and we get,
2a = 12.
Therefore,
a = 6.
Now, we can substitute 6 for a in,
a + b = 25,
and we get,
b = 19.
So,
r(x) = 6x^2 + 19x + 15.
Now, we substitute 10 for x in r(x), and we are done,
r(10) = 600 + 190 + 15 = 805.
Believe it or not!

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Berlekamp & GO posted April 2014

I knew that my principal cryptography professor Gilles Zémor was a GO player.

Which is pretty amazing in itself :)

But this keeps going on.

I have an algebra class this semester, and I'm trying to understand Berlekamp's algorithm. Trying to find videos on youtube about him I discover that he is as well a go player! And doing researches about the game at that! So cool :D

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Slides with LaTeX posted April 2014

If you read this blog, you know that recently I gave a talk on bitcoins.

I also gave a talk on whitebox cryptography last week.

One part of giving a talk that a lot of people tend to overlook is making good slides. I've always used Powerpoint for that, but for my last talk on whitebox cryptography I had two other persons on my team. Powerpoint was not an option if we were all working on the same file. LaTeX is the solution.

It's a real text file so you can use a revision control system like git, it's constant in its layout. You configure it at the beginning of the file and then you don't have to worry about it later.

We also had a fight (we were tired) on what theme to used. I went for no theme at all. Because everything else is visual noise.

Here's a great article from Zach Holman on the subject. If you ask me, and I'm not saying my slides are perfect, there are way too many crappy slides out there!

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Procrastination posted April 2014

Procrastination

I've read a LOT of stuff about procrastination. I have techniques:

I think those are all my techniques, I can't really think of any others.

2 Ideas

A few months ago I started counting my calories intake, and I lost weight! It's magic. As soon as you start counting the bad stuff, you realize how much you're doing of it. I've had this idea for quite a long time, a slack counter, that times how long you slack per day. It compares that to the average. I don't know how to implement that, but a cellphone app would be the best suited I think. We always carry it, so we just have to launch the app and start a timer when we procrastinate. This + a firefox/chrome app that recognizes websites that are "time consumer" to add to your statistics. I'm sure that would help me work more!

When I'm really in trouble, and I can't seem to get motivated, I always take my iPod out and start a 30 minute timer. I do that, and then I find something to work on, anything, and I don't have to be efficient or know right away how I'm gonna work on it. I just have to do it, non-stop, to focus for just 30 minutes. And when the timer stops, I'm usually motivated enough to either keep going, or take a small break and start a bigger timer. I had the idea of implementing a 30 minute timer + a chain system, everyday you can start this 30 minute timer once and if you do work non-stop for 30 minutes while it runs, you will validate a day. Try validating the most days in a row!

Other techniques ?

This article brings 3 good points:

There are two ways to look at any task. You can do something because you see it as a way to end up better off than you are now – as an achievement or accomplishment. As in, if I complete this project successfully I will impress my boss, or if I work out regularly I will look amazing. Psychologists call this a promotion focus

when we say things like “I just can’t get out of bed early in the morning, “ or “I just can’t get myself to exercise,” what we really mean is that we can’t get ourselves to feel like doing these things.

Making an if-then plan is more than just deciding what specific steps you need to take to complete a project – it’s also deciding where and when you will take them.
If it is 2pm, then I will stop what I’m doing and start work on the report Bob asked for.

You have to let go

This article helped/helps me a lot. Every time I fall into a procrastination period (because it's a disease you'll have to live with for the rest of your life :D), I read that article.

I hadn’t figured out the skill that would save me from the procrastination.
Until I learned about letting go.

Learn from the pro

Katia Verresen is a "Mental Energy coach" for CEOs and founders. Here's a nice article about her and interesting stuff she has to say about being efficient, in the zone.

Verresen is a big fan of a tactic called “calendar blocking,” and she encourages her clients to identify the chunks of time on their calendars when they have the most physical energy for work.

The mood you wake up in is critical. Consider it to be the default font for your entire day.

For a marathon runner, people say that the day before and the day before that are the most important. If you mess up with your sleep one day, you'll feel the consequences until two days after.

Verresen actually tells her clients that if they haven’t gotten enough sleep, they shouldn’t take their emotions seriously the next day.

“Mental energy is the ability to separate yourself from your thoughts,” Verresen says. “Everyday, we have millions of thoughts that cause stress, anxiety, depression — that can stall you out in a million different ways — but you don’t have to believe them.”

“The best way to reset your mental energy is to get it up and out into the physical world,” Verresen says. “Screens don’t count. You should put whatever it is that’s bothering you or that you need to get done up on a white board. Write it down and stick it up on a wall.

I sometime stop what I'm doing to do the dishes. It clears my mind.

Verresen advises her clients to block off two slots ranging between 90 minutes to 2 hours every week as mental white space. They need to literally put it on their calendar and make sure they aren’t interrupted or disturbed during this time — ideally they shouldn’t have meetings right afterwards either.

I do this by doing my laundry. I take a paper and a pen with me and I go do my laundry for an hour.

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