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.

Toilet paper and MIT students

posted November 2015

It is possible to repeatedly fold a standard letter-sized sheet of paper at the midway point about six to seven times. In 2012, some MIT students were able to fold an 1.2 kilometer long toilet paper 13 times. And every time the paper was folded, the number of layers on top of each other doubled. Therefore, the MIT students ended up with 2^13 = 8192 layers of paper on top of each other. And poor Eve's job was to manually count all layers one by one.

From Solving the Discrete Logarithm of a 113-bit Koblitz Curve with an FPGA Cluster

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/r/crypto wiki

posted November 2015

There's a subreddit for crypto and it had an empty wiki.

So I filled it.

Below is a copy.

/r/crypto wiki

Please refer to this Wiki before asking questions that might have been asked before (use your common sense).

Cryptography is also usually more interesting than decrypting your random ciphertext, so consider that when posting, read what we are sharing first to see if it's really pertinent.

How to learn about Cryptography?

The first thing you should do, is to sign-up for Dan Boneh's course Crypto I on Coursera.

You do not have to finish it. Watching a few videos will already give you an idea of what is crypto and how easy/hard it is for you. If you can finish the course, and enjoy it at the same time, then you're in for a lot of fun.

Don't try signing-up for Crypto II though.

Alright, that was interesting, MOAR

There are many ways to learn more about crypto. Here's a non-exhaustive list:

But my favorite way: read whitepapers. Look at the ePrint archives and check what papers interest you. Often papers will come with an introduction section that explains the basics.

What about studying cryptography in a real school?

First, it's important to cultivate your new passion. Don't let your course get in the way of reading or building and doing side projects involving crypto.

Second, You need either a bachelor in mathematics or computer science. Depending on which part you find the more interesting in crypto. Usually Math => Theorical Cryptography, CS => Applied Cryptography.

Now either you don't have a master, and you could choose to do a cryptography master. There are a few: Rennes, Bordeaux, Limoges in France.Stanford, etc...

Or you can do a Computer Science or Number Theory oriented master and pick a crypto subject for a phd. Note that a phd will often lead you into theorical research in university, although some phd can be done within a company and might involve applied crypto. But companies around the world might find that relatively relevant (in France a phd will get you to some places).

It's important to know what you want to do, theorical or applied or in the middle? Usually finding an internship in a applied crypto company helps to get out of academia for a while. A good way to see what please you the most. And good news, in cryptography internships are pretty easy to find (at least at the moment).

Alright I'm studying crypto now, how to get more involved?

Cryptography is a big world, many things are happening and it's sometimes hard to follow everything. Especially some mediums give you a high noise-to-quality ratio. So here they are:

  • Check some mailing lists in your field of interest (metzdown, ietf, cfrg, curves, modern crypto, etc...)

  • Follow conventions (listed here on the IACR website). Look out for ECC, Crypto, Asiacrypt, Eurocrypt, Real World Crypto, CHES, etc...

  • Blogs

  • Twitter. Look for cryptographers and check out for interesting discussions and/or links they share.

  • CTFs (and their write-ups)

Where to work in crypto?

Now is your time to find an internship or a job? Apply everywhere, in the world. But where? Where other cryptographers are working, or have worked. You can find that on their published papers (usually written in the header), on Twitter, on Linkedin, etc...

Another good way is to check for "who's hiring" posts on hackernews or reddit

Iacr also list a number of positions here.

Finally, universities around the world usually have room for internships as long as you don't mind not being paid.


About the easiness of SVP

posted November 2015

First of all, we stress that SVP and its variants should all be considered easy when the lattice dimension is less than 70. Indeed, we will see in Section 4 that exhaustive search techniques can solve SVP within an hour up to dimension 60. But because such techniques have exponential running time, even a 100-dimensional lattice is out of reach. When the lattice dimension is beyond 100, only approximation algorithms like LLL, DEEP and BKZ can be run.

I can't remember from what article I got that from. Must have been something Phong Nguyen wrote.

It states that a lattice of dimension 60 could be easily solved, in an hour, by an exhaustive search (or similar techniques (enumeration?)). Something to dig into.

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I'm back! Oh and also: my master defense

posted November 2015

So I'm now completely out of the loop, because I've been traveling a bunch. If you have any interesting crypto paper/blog post that was released in these last couple of months please post it here :)

Also I obtained my master. If you speak french or are just curious, you can check that here:

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ECC 2015 in Bordeaux

posted October 2015


After defending my master thesis in Labri's amphitheatrum I thought I would never have to go back there again. Little did I know, ECC 2015 took place in the exact same room. I was back in school.



It was a first for me, but for many people it was only one more ECC. Most people knew each other, a few were wandering alone, mostly students. The atmosphere was serious although relaxed. People were mostly in their late 30s and 40s, a good part was french, others came from all over the world, a good minority were government people. Rumor has it that NSA was somewhere hidden.


Nothing really groundbreaking was introduced, as everybody knows ECC is more about politics than math these days. The content was so rare that a few talks were not even talking about ECC. Like that talk about Logjam (was a good talk though) or a few about lattices.

Rump Session



We got warmed up by a one hour cocktail party organized by Microsoft, by 6pm most people were "canard" as the belgium crypto people were saying. We left Bordeaux's Magnificient sun and sat back into the hot room with our wine glasses. Then every 5 minutes a random person would show up on stage and present something, sometimes serious, sometimes ridiculous, sometimes funny.


The panel was introduced by Benjamin Smith and was composed of 7 figures. Dan Bernstein that needs no introduction, Bos from NXP, Flori from the french government agency ANSSI, Hamburg from Cryptography Research (who was surprised that his company let him assist to the panel), Lochter from BSI (German government) and Moody from NIST.


It was short and about standardization, here are the notes I took then. Please don't quote anything from here, it's inexact and redacted after the fact.

  • Presenter: you have very different people in front of you, you have exactly 7 white people in front of you, hopefuly it will be different next year.

  • The consensus is that standardisation in ECC is not working at all. Maybe it should be more like the AES one. Also, people are disapointed that not enough academics were involved... general sadness.

  • Lochter: it's not good to change too much, things are working for now and Post-Quantum will replace ECC. We should start standardizing PQ. Because everything is slow, mathematics takes years to get standardized, then implemented, etc... maybe the problem is not in standardization but keeping software up-to-date.

  • Hamburg: PQ is the end of every DLP-based cryptosystem.

  • Bos: I agree we shouldn't do this (ECC2015) too often. Also we should have a framework where we can plugin different parameters and it would work with any kind of curves.

  • Someone: why build new standards if the old/current one is working fine. This is distracting implementers. How many crypto standards do we already have? (someone else: a lot)

  • Bos: Peter's talk was good (about formal verification, other panelists echoed that after). It would be nice for implementers to have tools to test. Even a database with a huge amount of test vectors would be nice

  • Flori: people don't trust NIST curves anymore, surely for good reasons, so if we do new curves we should make them trustable. Did anyone here tried generating nist, dan, brainpool etc...? (3 people raised their hands).

  • Bernstein: you're writing a paper? Why don't you put the Sage script online? Like that people won't make mistakes or won't run into a typo in your paper, etc...

  • Lochter: people have to implement around patents all the time (ranting).

  • Presenter: NSA said, if you haven't moved to ECC yet, since there will be PQ, don't get into too much trouble trying to move to ECC. Isn't that weird?

  • Bernstein: we've known for years that PQ computers are coming. There is no doubt. When? It is not clear. NSA's message is nice. Details are weird though. We've talked to people at the NSA about that. Really weird. Everybody we've talked to has said "we didn't see that in advance" (the announcement). So who's behind that? No one knows. (someone in the audience says that maybe the NSA's website got hacked)

  • Flori: I agree it's hard to understand what the NSA is saying. So if someone in the audience wants to make some clarification... (waiting for some hidden NSA agent to speak. No one speaks. People laugh).

  • Hamburg: usually they say they do not deny, or they say they do not confirm. This time they said both (the NSA about Quantum computers).

  • Lochter: 30 years is the lifetime of secret data, could be 60 years if you double it (grace period?). We take the NSA's announcement seriously, satelites have stuff so we can upgrade them with curves (?)

  • Presenter: maybe they (the NSA) are scared of all the curve standardization happening and that we might find a curve by accident that they can't break. (audience laughing)

  • Bos: we have to follow standards when we implement in smartcards...

  • Lochter: we can't blame the standard. Look at Openssl, they did this mess themselves.

  • Moody: standards give a false sense a security but we are better with them than without (lochter looks at him weirdly, Moody seems embarassed that he doesn't have anything else to say about it).

  • Bernstein: we can blame it on the standard!

  • Lochter: blame the process instead. Implementers should get involved in the standardization process.

  • Bernstein: I'll give you an example of implementers participating in standardization, Rivest sent a huge comment to the NIST ("implementers have enough rope to hang themselve"). It was one scientific involved in the standardization.

  • Presenter: we got 55 minutes of the panel done before the first disagreement happened. Good. (everybody laughs)

  • Bos: we don't want every app dev to be able to write crypto. It is not ideal. We can't blame the standards. We need cryptographers to implement crypto.
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Factoring RSA Keys With TLS Perfect Forward Secrecy

posted September 2015

Note: This is a blogpost I initialy wrote for the NCC Group blog here.

Here's the story:

Florian Weimer from the Red Hat Product Security team has just released a technical report entitled "Factoring RSA Keys With TLS Perfect Forward Secrecy"

Wait what happened?

A team of researchers ran an attack for nine months, and from 4.8 billion of ephemeral handshakes with different TLS servers they recovered hundreds of private keys.

The theory of the attack is actually pretty old, Lenstra's famous memo on the CRT optimization was written in 1996. Basicaly, when using the CRT optimization to compute a RSA signature, if a fault happens, a simple computation will allow the private key to be recovered. This kind of attacks are usually thought and fought in the realm of smartcards and other embedded devices, where faults can be induced with lasers and other magical weapons.

The research is novel in a way because they made use of Accidental Faults Attack, which is one of the rare kind of remote side-channel attacks.

This is interesting, the oldest passive form of Accidental Fault Attack I can think of is Bit Squatting that might go back to 2011 at that defcon talk.

But first, what is vulnerable?

Any library that uses the CRT optimization for RSA might be vulnerable. A cheap countermeasure would be to verify the signature after computing it, which is what most libraries do. The paper has a nice list of who is doing that.

cryptlib 3.4.2disabled by default
GNUTLSsee libgcrypt and Nettle
Go 1.4.1no
libgcrypt 1.6.2no
Nettle 3.0.0no
ocaml-nocrypto 0.5.1no
OpenJDK 8yes
OpenSSL 1.0.1lyes
OpenSwan 2.6.44no
PolarSSL 1.3.9no

But is it about what library you are using? Your server still has to be defective to produce a fault. The paper also have a nice table displaying what vendors, in their experiments, where most prone to have this vulnerability.

Hillstone Networks237nolow
Fortinet2novery low

If you're using one of these you might want to check with your vendor if a firmware update or other solutions were talked about after the discovery of this attack. You might also want to revoke your keys.

Since the tests were done on a broad scale and not on particular machines, it is obvious that more are vulnerable to this attack. Also only instances connected to internet that offered TLS on port 443 were tested. The vulnerability could potentially exist in any stack using this CRT optimization with RSA.

The first thing you should do is asses where in your stack the RSA algorithm is used to sign. Does it use CRT? If so, does it verifies the signature? Note that the blinding techniques we talked about in one of our cryptography bulletin (may first of this year) will not help.

What can cause your server to produce such erroneous signatures

They list 5 reasons in the paper:

  • old or vulnerable libraries that have broken operations on integer. For example CVE-2014-3570 the square operations of OpenSSL was not working properly for some inputs

  • race conditions, when applications are multithreaded

  • arithmetic unit of the CPU is broken by design or by fatigue

  • corruption of the private key

  • errors in the CPU cache, other caches or the main memory.

Note that at the end of the paper, they investigate if a special hardware might be the cause and end up with the conclusion that several devices leaking the private keys were using Cavium hardware, and in some cases their "custom" version of OpenSSL.

I'm curious. How does that work?


Remember, RSA signature is basically \(y = x^d \pmod{n}\) with \(x\) the message, \(d\) the private key and \(n\) the public modulus. Also you might want to use a padding system but we won't cover that here. And then you can verify a signature by doing \(y^e \pmod{n}\) and verify if it is equal to \(x\) (with \(e\) the public exponent).

CRT is short for Chinese Remainder Theorem (I should have said that earlier). It's an optimization that allows to compute the signatures in \(\mathbb{Z}_p\) and \(\mathbb{Z}_q\) and then combine it into \(\mathbb{Z}_n\) (remember \(n = pq\)). It's way faster like that.

So basically what you do is:

$$ \begin{cases} y_p = x^d \pmod{p} \\ y_q = x^d \pmod{q} \end{cases} $$

and then combine these two values to get the signature:

$$ y = y_p q (q^{-1} \pmod{p}) + y_q p (p^{-1} \pmod{q}) \pmod{n} $$

And you can verify yourself, this value will be equals to \(y_p \pmod{p}\) and \(y_q \pmod{q}\).

The vulnerability

Let's say that an error occurs in only one of these two elements. For example, \(y_p\) is not correctly computed. We'll call it \(\widetilde{y_p}\) instead. It is then is combined with a correct \(y_q\) to produce a wrong signature that we'll call \(\widetilde{y}\) .

So you should have:

$$ \begin{cases} \widetilde{y} = \widetilde{y}_p \pmod{p}\\ \widetilde{y} = y_q \pmod{q} \end{cases} $$

Let's notice that if we raise that to the power \(e\) and remove \(x\) from it we get:

$$ \begin{cases} \widetilde{y}^e - x = \widetilde{y}^e_p - x = a \pmod{p}\\ \widetilde{y}^e - x = y_q^e - x = 0 \pmod{q} \end{cases} $$

This is it. We now know that \(q \mid \widetilde{y}^e - x\) while it also divides \(n\). Whereas \(p\) doesn't divide \(\widetilde{y}^e - x\) anymore. We just have to compute the Greatest Common Divisor of \(n\) and \(\widetilde{y}^e - x\) to recover \(q\).

The attack

The attack could potentially work on anything that display a RSA signature. But the paper focuses itself on TLS.

A normal TLS handshake is a two round trip protocol that looks like this:


The client (the first person who speaks) first sends a helloClient packet. A thing filled with bytes saying things like "this is a handshake", "this is TLS version 1.0", "I can use this algorithm for the handshake", "I can use this algorithm for encrypting our communications", etc...

Here's what it looks like in Wireshark:

client hello

The server (the second person who speaks) replies with 3 messages: a similar ServerHello, a message with his certificate (and that's how we authenticate the server) and a ServerHelloDone message only consisting of a few bytes saying "I'm done here!".

A second round trip is then done where the client encrypts a key with the server's public key and they later use it to compute the TLS shared key. We won't cover them.

Another kind of handshake can be performed if both the client and the server accepts ephemeral key exchange algorithms (Diffie-Hellman or Elliptic Curve Diffie-Hellman). This is to provide Perfect Forward Secrecy: if the conversations are recorded by a third party, and the private key of the server is later recovered, nothing will be compromised. Instead of using the server's public key to compute the shared key, the server will generate a ephemeral public key and use it to perform an ephemeral handshake. Usualy just for this session or for a limited number.


An extra packet called ServerKeyExchange is sent. It contains the server's ephemeral public key.

Interestingly the signature is not computed over the algorithm used for the ephemeral key exchange, that led to a long series of attack which recently ended with FREAK and Logjam.

By checking if the signature is correctly performed this is how they checked for the potential vulnerability.

I'm a researcher, what's in it for me?

Well what are you waiting for? Go read the paper!

But here are a list of what I found interesting:

  • instead of DDoSing one target, they broadcasted their attack.

We implemented a crawler which performs TLS handshakes and looks for miscomputed RSA signatures. We ran this crawler for several months. The intention behind this configuration is to spread the load as widely as possible. We did not want to target particular servers because that might have been viewed as a denial-of-service attack by individual server operators. We assumed that if a vulnerable implementation is out in the wild and it is somewhat widespread, this experimental setup still ensures the collection of a fair number of handshake samples to show its existence. We believe this approach—probing many installations across the Internet, as opposed to stressing a few in a lab—is a novel way to discover side-channel vulnerabilities which has not been attempted before.

  • they used public information to choose what to target, like scans.io, tlslandscape and certificate-transparency.

  • Some TLS servers need a valid Server Name Indication to complete a handshake, so connecting on port 443 of random IPs should not be very efficient. But they found that it was actually not a problem and most key found like that were from weird certificates that wouldn't even be trusted by your browser.

  • To avoid too many DNS resolutions they bypassed the TTL values and cached everything (they used PowerDNS for that)

  • They guess what devices were used to perform the TLS handshakes from what was written in the x509 certificates in the subject distinguished name field or Common Name field

  • They used SSL_set_msg_callback() (see doc) to avoid modifying OpenSSL.
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Timing and Lattice Attacks on a Remote ECDSA OpenSSL Server: How Practical Are They Really?

posted August 2015

Alright! My master thesis is done. Here's a download link

It's a timing attack on an vulnerable version of OpenSSL. In particular its ECDSA signature with binary curves.

There was an optimization right before the constant-time scalar multiplication of the nonce with the public point. That leads to a timing attack that leaks the length of the ephemeral keys of an Openssl server's signatures.

In this paper I explain how to setup such an attack, how to use lattices to recover the private key out of just knowing the lengths of the nonces of a bunch of signatures taken during an ephemeral handshake.

If this doesn't make sense to you just read the paper :D

Also everything is on this github repo. You can reproduce my setup for a vulnerable server and an attacker. Patch and tools are there. If you end up getting better results than the ones in the paper, well tell me!

Also here's a demo:

EDIT: it's on the ePrint archive as well now.


Key Ceremony

posted August 2015

There was a DNSSEC KSK (Key Signing Key) Ceremony. I guess it most not be too far away from what is a Key Ceremony

In public-key cryptography and computer security, a root key ceremony is a procedure where a unique pair of public and private root keys is generated. Depending on the certificate policy, the generation of the root Keys may require notarization, legal representation, witnesses and ‘key holders’ to be present, as the information on the system is a responsibility of the parties. The 'best practice' is to follow the SAS 70 standard for root key ceremonies.

The actual Root Key-Pair generation is normally conducted in a secure vault that has no communication or contact with the outside world other than a single telephone line or intercom. Once the vault is secured, all personnel present must prove their identity using at least two legally recognized forms of identification. Every person present, every transaction and every event is logged by the lawyer in a Root Key Ceremony Log Book and each page is notarized by the notary. From the moment the vault door is closed until it is re-opened, everything is also video recorded. The lawyer and the organization’s two signatories must sign the recording and it too is then notarized.

Finally, as part of the above process, the Root Key is broken into as many as twenty-one parts and each individual part is secured in its own safe for which there is a key and a numerical lock. The keys are distributed to as many as twenty-one people and the numerical code is distributed to another twenty-one people.

It's not that interesting, but I was just curious so I watched the footage of what it is here: http://data.iana.org/ksk-ceremony/21/KSK21-CAM1.mp4

It's boring.

But if you have nothing better to do, or you are curious like me, well here you go

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I got a job!

posted August 2015

Hey you! So mmm, I don't know if you've been reading my blog for long, but it all started when I got accepted at the University of Bordeaux' Cryptography Master. At first it was just a place where I would talk about my (then) new life in Bordeaux and what I was doing in class.

2 years and 287 blog posts later, here I am, still blogging and still in school. But not for long! Well not in school for long, I'm still gonna blog don't worry.

So yeah, the big news is, I'll be starting full time as a security consultant for the Cryptography Services team of NCC Group in November!

woop woop

Woop woop!

Pardon? You are here for the crypto? ah umm, wait, I have this:


It's from this paper: Practical realisation and elimination of an ECC-related software bug attack by B.B.Brumley, Barbosa, Page and Vercauteren.