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.

Funny matasano job post March 2015

I stumbled on this funny job post from jeff jarmoc:

This thread will, no doubt, be dominated by posts with laundry lists of requirements. Many employers will introduce themselves by describing what they want from you. At Matasano, we're a little different. We like to start by telling you about us. This month, I want to try to do that by drawing analogy to Mission Impossible.
What made the original show so great is exactly what was lost in the 'Tom Cruise takes on the world' reboot. The original 1960's and 70's Mission Impossible was defined primarily by a team working together against all odds to achieve their objective. It acknowledged that what they were doing was improbable, and more so for a solo James Bond or Tom Cruise character. As a team though, each character an expert in their particular focus area, the incredible became credible -- the impossible, possible.

the rest is here: https://news.ycombinator.com/item?id=9127813

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Freak Attack March 2015

If you're up to date on crypto news you will tell me I'm slow. But here it is, my favorite explanation of the recent Freak Attack is the one from Matthew Green here

TLS uses a cipher suite during the handshake so that old machines can still chat with new machines that use new protocols. In this list of ciphers there is one called "export suite" that is a 512bits RSA public key. It was made by the government back then to spy on foreigners since 512bits is "easy" to factor. The vulnerability comes from the fact that you can still ask a server to use that 512bits public key (even though it should have been removed a long time ago). This allows you to make a man in the middle attack where you don't have to possess a spoofed certificate. You can just change the cipher request of the client during the handshake so that he would ask for that 512bits key. 36% of the servers out there would accept that and reply with such a key. From here if we are in the middle we can just factor the key and use that to generate our own private key and see all the following exchange in clear.

More info here

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How to memorise a 20 item (and upto 1000 item) grocery list. March 2015

someone asked on Quora: What can I learn/know right now in 10 minutes that will be useful for the rest of my life?

And someone delivered! It's called the peg method, and it allows you to remember words in the long term really quickly. I knew about other techniques like creating a story where each words is like a double linked list of event or using each words as obstacles in a mental path. But this one seems way more useful and practical. But contrary to the other techniques, you have to memorize a few things before being able to use it:


I know it's not cryptography :) but from the header:

This is my blog about cryptography and security and other related topics that I find interesting.

Thanks @Loïs!

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Kasumi March 2015


KASUMI is a block cipher used in UMTS, GSM, and GPRS mobile communications systems. In UMTS, KASUMI is used in the confidentiality (f8) and integrity algorithms (f9) with names UEA1 and UIA1, respectively. In GSM, KASUMI is used in the A5/3 key stream generator and in GPRS in the GEA3 key stream generator.


(and Katsumi/Katsuni is a very famous french porn actress)

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Keep in touch with crypto February 2015

A long time ago, I think around 2007, I got violently addicted to RSS. I was subscribed to hundreds of different blogs about design, tech, web... One new story would pop in my feed every 5 minutes. I had to read everything and I felt stressed all the time. Clicking, reading, clicking, reading... If I wasn't in front of my computer I felt like I was missing out. I then decided to remove my RSS reader software and never touched a feed again. And for the past years my browsing habits have mostly narrowed down to hackernews and a reddit without any default subs. But now that I am studying crypto, I wanted to get more immersed in this world and I had the idea of using my tendency to get addicted for a good purpose. So I tried the latest recommended RSS readers (since google reader doesn't exist anymore) and I subscribed to every crypto/security blog I could find and I started reading. And since, I've been reading a lot. So I guess it works! I've been using Digg Reader mostly because of the ios app that is really good and also because when I have nothing to read I can dig into what's on my twitter.

I have collected a list of 60 blogs about cryptography and security. If you feel like one is missing or one shouldn't be here please tell me! The list is here

Here's the list if you hate RSS:

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The ghetto way of extracting the private key of superfish February 2015

A realy entertaining piece by Errata Security where Robert Graham ghetto reverse the current controversial superfish of Lenovo.

(if you have been living under a rock

The goal is to set the right break point before it actually infects your machine -- reversers have been known to infect themselves this way.

his ghetto way of reversing is first to infect himself with the "virus" and then using procdump to dump the process memory. Then dumping all the strings that the memory contains with the tool strings and voila. You have have the private certificate in the clear.

But the private certificate is protected by a passphrase. But apparently not, it was just protected by a password contained in the memory in clear as well...

I advise you to read the article, it comes with screenshots and nice commands that use text processing tools:

grep "^[a-z]*$" super.txt | sort | uniq > super.dict

spoiler alert, the password to protect the certificate is komodia the name of the company who created this mitm adware.

Note that if they would have used an RSA whitebox this would not have happened... so quickly.

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Big Numbers February 2015

An amazing story by Scott Aaronson on Big Numbers.

Keywords: exponential growth, NP-complete problems, Ackermann sequence, Turing Machine, Halting Problem, Busy Beaver,

some quotes :)

In an old joke, two noblemen vie to name the bigger number. The first, after ruminating for hours, triumphantly announces "Eighty-three!" The second, mightily impressed, replies "You win."

Consider, for example, the oft-repeated legend of the Grand Vizier in Persia who invented chess. The King, so the legend goes, was delighted with the new game, and invited the Vizier to name his own reward. The Vizier replied that, being a modest man, he desired only one grain of wheat on the first square of a chessboard, two grains on the second, four on the third, and so on, with twice as many grains on each square as on the last. The innumerate King agreed, not realizing that the total number of grains on all 64 squares would be 264-1, or 18.6 quintillion—equivalent to the world’s present wheat production for 150 years.

Rado called this maximum the Nth "Busy Beaver" number. (Ah yes, the early 1960’s were a more innocent age.)

To solve the Halting Problem for super machines, we’d need an even more powerful machine: a ‘super duper machine.’ And to solve the Halting Problem for super duper machines, we’d need a ‘super duper pooper machine.’

If we could run at 280,000,000 meters per second, there’d be no need for a special theory of relativity: it’d be obvious to everyone that the faster we go, the heavier and squatter we get, and the faster time elapses in the rest of the world. If we could live for 70,000,000 years, there’d be no theory of evolution, and certainly no creationism: we could watch speciation and adaptation with our eyes, instead of painstakingly reconstructing events from fossils and DNA. If we could bake bread at 20,000,000 degrees Kelvin, nuclear fusion would be not the esoteric domain of physicists but ordinary household knowledge.

But do people fear big numbers? Certainly they do. I’ve met people who don’t know the difference between a million and a billion, and don’t care. We play a lottery with ‘six ways to win!,’ overlooking the twenty million ways to lose. We yawn at six billion tons of carbon dioxide released into the atmosphere each year, and speak of ‘sustainable development’ in the jaws of exponential growth.

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Using crypto to replace database access. February 2015

A pretty fresh article on how you could use crypto to replace a lot of complicated schemes you might use on your website like password reset or mail confirmation:


tl;dr: instead of creating a table for tokens, you could create the password reset url like this:

http://www.example.com/password_reset/?user=username&expires=15649848949849&[email protected]&token=

and at the place of the token you would put the output of a MAC. Checking the MAC again after receiving the url would confirm that YOU created that url and it has not been modified. Remember, MAC provides integrity and authentication. The author also provides a way to only render this usable once: use the original hashed password as a nonce.

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Babun, Cmder and Tmux February 2015

I've used Cmder for a while on Windows. Which is a pretty terminal that brings a lot of tools and shortcuts from the linux world. I also have Chocolatey as packet manager. And all in all it works pretty great except Cmder is pretty slow.

I've ran into Babun yesterday, that seems to be kind of the same thing, but with zsh, oh-my-zsh and another packet manager: pact. The first thing I did was downloading tmux and learning how to use it. It works pretty well and I think I have found a replacement for Cmder =)

Here is a video of what is tmux:

and an awesome cheatsheet

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Implementation of Coppersmith attack (RSA attack using lattice reductions) February 2015

I've implemented the work of Coppersmith (to be correct the reformulation of his attack by Howgrave-Graham) in Sage.

You can see the code here on github.

I won't go too much into the details because this is for a later post, but you can use such an attack on several relaxed RSA models (meaning you have partial information, you are not totally in the dark).

I've used it in two examples in the above code:

Stereotyped messages

For example if you know the most significant bits of the message. You can find the rest of the message with this method.

The usual RSA model is this one: you have a ciphertext c a modulus N and a public exponent e. Find m such that m^e = c mod N.

Now, this is the relaxed model we can solve: you have c = (m + x)^e, you know a part of the message, m, but you don't know x. For example the message is always something like "the password today is: [password]". Coppersmith says that if you are looking for N^1/e of the message it is then a small root and you should be able to find it pretty quickly.

let our polynomial be f(x) = (m + x)^e - c which has a root we want to find modulo N. Here's how to do it with my implementation:

dd = f.degree()
beta = 1
epsilon = beta / 7
mm = ceil(beta**2 / (dd * epsilon))
tt = floor(dd * mm * ((1/beta) - 1))
XX = ceil(N**((beta**2/dd) - epsilon))
roots = coppersmith_howgrave_univariate(f, N, beta, mm, tt, XX)

You can play with the values until it finds the root. The default values should be a good start. If you want to tweak:

  • beta is always 1 in this case.
  • XX is your upper bound on the root. The bigger is the unknown, the bigger XX should be. And the bigger it is... the more time it takes.

Factoring with high bits known

Another case is factoring N knowing high bits of q.

The Factorization problem normally is: give N = pq, find q. In our relaxed model we know an approximation q' of q.

Here's how to do it with my implementation:

let f(x) = x - q' which has a root modulo q.
This is because x - q' = x - ( q + diff ) = x - diff mod q with the difference being diff = | q - q' |.

beta = 0.5
dd = f.degree()
epsilon = beta / 7
mm = ceil(beta**2 / (dd * epsilon))
tt = floor(dd * mm * ((1/beta) - 1))
XX = ceil(N**((beta**2/dd) - epsilon)) + 1000000000000000000000000000000000
roots = coppersmith_howgrave_univariate(f, N, beta, mm, tt, XX)

What is important here if you want to find a solution:

  • we should have q >= N^beta
  • as usual XX is the upper bound of the root, so the difference should be: |diff| < XX
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Nothing up my sleeve numbers February 2015

Why does the round constants in sha1 have those specific values?


Kt is the round constant of round t;

from SH1-1 - Wikipedia:

The constant values used are chosen to be nothing up my sleeve numbers: the four round constants k are 230 times the square roots of 2, 3, 5 and 10. The first four starting values for h0 through h3 are the same with the MD5 algorithm, and the fifth (for h4) is similar.

from Nothing up my sleeve number:

In cryptography, nothing up my sleeve numbers are any numbers which, by their construction, are above suspicion of hidden properties. They are used in creating cryptographic functions such as hashes and ciphers. These algorithms often need randomized constants for mixing or initialization purposes. The cryptographer may wish to pick these values in a way that demonstrates the constants were not selected for a nefarious purpose, for example, to create a backdoor to the algorithm. These fears can be allayed by using numbers created in a way that leaves little room for adjustment. An example would be the use of initial digits from the number π as the constants. Using digits of π millions of places into its definition would not be considered as trustworthy because the algorithm designer might have selected that starting point because it created a secret weakness the designer could later exploit.

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