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.

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How good is flask? posted April 2014

I've used Django for my last project and I found the documentation unclear and the list of things I had to do to code simple things and deploy were... a bit too much for a simple project.

I've glanced at the Flask documentation and have found it über-clear. The syntax seems to be pretty straight-forward as well. I'm really thinking about learning Flask for my next project and putting Django on hold. What do you guys think?

There's also a talk on web2py in the current PyCon. I don't know if it's for me but I really need something I can do quick prototypes on.

Sometimes I wonder if I should go back to PHP and try the new Laravel that really looks super cool :)

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NAT with iptables : super fast tutorial posted April 2014

So I know how to use iptables, I know what a NAT is, but I don't want to learn how to exactly do it. Misery... I have to learn how to do it because I have an exam that will probably ask me how to do it in a few days. So I've been looking for a super simple tutorial, a 1 minute tutorial, on how to setup a NAT configuration with iptables in 1 minute. Couldn't really find it so here it is, if this is somewhat useful for someone, you're welcome.

First Step

For NAT to work, you have to allow forwarding on your server. Easy peasy:

$echo 1 > /proc/sys/net/ipv4/ip_forward  Also, before adding new iptables rules, be sure to check what rules you already have $ iptables -L

you should allow some forwarding for it to work (if the policy is default to DROP). But this not a tutorial about iptables.

Static

I have a server with:

• eth0 connected to the network

• eth1 connected to internet

Let's modify the PREROUTING part. Traffic coming from internet on our public address (@pub) and trying to reach our machine:

$iptables -t nat -A PREROUTING -d @pub -i eth0 -j DNAT --to-destination @priv Let's modify the table nat, append a rule to the pretrouting section : something is trying to reach @pub ? Let's put it in our input interface eth0, jump to the Destination Nat protocol, which tells us to send the packet to @priv. Now Let's modify the POSTROUTING part. Traffic coming from inside our network and trying to reach something, somewhere on internet: $ iptables -t nat -A POSTROUTING -s @priv -o eth1 -j SNAT --to-source @pub

If the packet is coming from @priv, let's put it on our output interface eth1 and jump to the Source Nat Protocol that will modify the packet so it has the public address (@pub) as source.

Here! You did it. One private IP address mapped to one public IP address.

Dynamic

Same kind of configuration but now we have several private addresses and only one public address.

$iptables -t nat -A POSTROUTING -s @priv/mask -j MASQUERADE We can modify every packets coming from the subnetwork @priv to get masqueraded. $ iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE

Or we can just tell all the network to get masqueraded.

And this is it. No PREROUTING Needed.

Again, you're welcome ;)

So... The Heartbleed Challenge has been completed posted April 2014

A few hours after the start of the Heartbleed challenge, actually, just 3 hours after the start of the Heartbleed challenge. Fedor Indutny seems to have cracked it.

So now, chaos begins. If you own a certificate, you not only have to change it, but you also have to revoke it. I wonder how many will change, and how many will revoke.

You can check that he indeed did it by doing this:

Just to confirm it: put this into your /etc/hosts “165.225.128.15 http://www.cloudflarechallenge.com ” and visit “https://www.cloudflarechallenge.com/ “.

here why it works:

Putting that mapping in /etc/hosts lets your machine skip DNS lookup for that hostname, and just use his IP for that domain name.
Then, your browser checks the received certificate against the authenticated TLS connection, and sees that all is well, allowing you to connect without a warning.
Since the browser does not warn of a certificate mismatch, he must have a valid certificate for 'cloudflarechallenge.com'. QED.

The Cloudflare team reviewing the attack:

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NSA was not aware of the Heartbleed bug posted April 2014

NSA is not happy. NSA is tweeting, tumblring (is this a verb?) and shouting loud and for all of who wants to hear it : they didn't know about the Heartbleed bug.

by the way they're talking about a "zero day" vulnerability, and now is a good time to learn what it is:

a so-called “Zero day” vulnerability because the developers of the vulnerable software have had zero days to fix it

I'm akin to trust them since... well. So many US websites were using OpenSSL and... it's not really nice if someone else eavesdrop on american citizen...

Anyway, this shows that the NSA has a long way to build trust again.

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Bordeaux, one more list posted April 2014

I don't write enough in the "Life in Bordeaux" section of this blog. So, here I am, trying to write something.

What can I tell you about Bordeaux ?

I ***** love Bordeaux. I love its student life, I love how practical it is for me to see my friends, I really like my campus especially that building with all those free-access computers with double screens and everything already setup on them.

I don't really like the weather though, it's raining quite often, I'd say at least once a week, but when it shines, it shines.

The public transport is the worse part of the city. There is no subway and trams are always PACKED. And when I say packed I mean "you will miss 5 trams in a row because you can't get in" packed. And I have to commute, every day... but I'll survive, it's not Paris and its awful subway :) far from that.

The streets are dirty, my apartment is crappy, really, but it's okay, I'm moving in June, hopefully to a better place. Still have to find a new place though, and looking for a place in Bordeaux is HARSH. I shiver just thinking about it.

Girls are pretty :o) and there are many girls. Bad thing is that my part of the campus is full of guys (and sometimes its hard to tell).

I don't know what else I could say. I like it better than Lyon, way better than Hamilton in Ontario, way way better than Paris. It lacks the feeling of "full of opportunities" that Beijing has though. But the air is breathable at least =) so yeah. Later in my life, I will consider settling in Bordeaux. Why not?

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Exams posted April 2014

We've been a group of 4-5 students spending each nights at the Crémi these few last days, this building of three floors where each floor has around 10 rooms full of computers.

We work, we eat, we play, and we crash each other computers.

There are a bunch of games installed on every computers but we mostly play SauerBraten, a quake-like.

My 15-year-old self would have spent most of his days here playing, if only he knew that his future campus would have such a sacred place :)

How do we crash each other computer? We just ssh into their machine and launch a fork bomb:

 :(){ :|:& };:

It operates by defining a function called ':', which calls itself twice, once in the foreground and once in the background.

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The Heartbleed Challenge posted April 2014

Cloudflare's engineers have set up a server vulnerable to Heartbleed, if you find the secret SSL keys and publish your solution you'll get 10,000\$. The challenge is here and there's a blog post here.

an attacker can get up to 64kB of the server’s working memory. This is the result of a classic implementation bug known as a Buffer over-read

Apparently it is not known if it is possible or not to find those keys. If it appears to be possible the results would be catastrophic as every single website that has used OpenSSL would have to revoke and ask for a new certificate. And as Cloudflare says:

the certificate revocation process is far from perfect and was never built for revocation at mass scale.

So it would then be very easy for any server to pretend they're someone else.

A heartbeat is a message that is sent to the server just so the server can send it back. This lets a client know that the server is still connected and listening. The heartbleed bug was a mistake in the implementation of the response to a heartbeat message.

This is the code in question:


p = &s->s3->rrec.data[0]

[...]

hbtype = *p++;
pl = p;

[...]

bp = buffer;

[...]


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How we got read access on Google’s production servers posted April 2014

The team at Detectify found a way to access files on one of google's production server. Thanks to an old google product (google toolbar) that was using a poorly secured XML parser.

They just used a simple XXE attack where they uploaded a poisoned xml files and saw what the application printed back

a xxe looks like this:

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "file:///etc/passwd" >]><foo>&xxe;</foo>

More on their blog

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