 Hey! I'm David, the author of the Real-World Cryptography book. I was previously a crypto architect at O(1) Labs (working on the Mina cryptocurrency), before that 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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# I got my first dogecoins today posted May 2014

Time to celebrate!

and thanks r/dogecoin for tipping me!

If you want some of my dogecoins just comment :D

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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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# Do you know what Elliptic Curve Cryptography is? posted May 2014

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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!

# Why can't I copy PS3 games and play them on another console? posted April 2014

I've always wondered how it is that we can't easily copy the entire content of a CD/DVD/Bluray on another one and play it with a PS1/PS2/PS3 and I guess PS4 and its competition.

Here's part of an answer on psx-scene's forum:

Whenever you insert a disc (bluray one that is) the ps3 drive will look at a special area of the disc called the Pic Zone (the BD ROM Mark is actually used in movie discs but not in game unlike what I first thought).This area cannot easily be dumped (you'd pretty much need a bluray drive with a hacked firmware) and of course that specific area cannot be burned on any kind of discs or with any kind of burners commercially available.

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# Weblang posted April 2014

I've been writing html, xhtml, and now html5 for ages. I think I started in 2001 (13 years ago).

I had to go through <br> becoming <br /> becoming <br> again.

I had to go through different doctypes

I had to go through new divs like <header> and <footer>

But I never had to go through a syntax change. Why is that? I don't understand why HTML is a language based on tags. It is unnecessary and it just adds time and confusion to typing in html. I haven't ran into any project directed at changing that syntax. And I thought, why not doing it myself? (and if there is already such a project please tell me!)

So I thought about a new language to write static web pages called web or weblang. No tags. Indentation. Simple doctype.

A simple index.web would looks like that:

\web:1 // this is a doctype

$title: 'Weblang example';$css: 'css/app.css';

\body

$header .monheader{$h1 "Weblang";
}

$section #introduction{$h2 "What is Weblang?";
$p "Weblang is an elegant way of writing static webpages" "HTML is annoying to write." // there will be a breakline here$p{
what about just writing text like this,
it's kinda easier
}
}

// what about just writing text
This is a text block, it will just render as text
in this text I want a list here : $ul{$li "with text in it";
}

$ul .links{$li{
$a "more info" href: 'https://github.com/mimoo/weblang'; }$li $a{ tags can be chained } }$javascript 'js/jquery.js';
$script 'js/script.js' type: 'javascript'; This is just a first draft. The biggest problem is that plain text and code is mixed. The trick I used here is to use$ to tell the render engine that it is not plaintext. Might not be super clever. I need to brainstorm a bit more about this.

Also I need to look at sass' code to see how a compiler works. Seems to be a bunch of regex.