LavaRand in Production: The Nitty-Gritty Technical Details

Introduction

Courtesy of @mahtin

As some of you may know, there's a wall of lava lamps in the lobby of our San Francisco office that we use for cryptography. In this post, we’re going to explore how that works in technical detail. This post assumes a technical background. For a higher-level discussion that requires no technical background, see Randomness 101: LavaRand in Production.

Background

As we’ve discussed in the past, cryptography relies on the ability to generate random numbers that are both unpredictable and kept secret from any adversary. In this post, we’re going to go into fairly deep technical detail, so there is some background that we’ll need to ensure that everybody is on the same page.

True Randomness vs Pseudorandomness

In cryptography, the term random means unpredictable. That is, a process for generating random bits is secure if an attacker is unable to predict the next bit with greater than 50% accuracy (in other words, no better than random chance).

We can obtain randomness that is unpredictable using one of two approaches. The first produces true randomness, while the second produces pseudorandomness.

True randomness is any information learned through the Continue reading

Randomness 101: LavaRand in Production

Introduction

Courtesy of @mahtin

As some of you may know, there's a wall of lava lamps in the lobby of our San Francisco office that we use for cryptography. In this post, we’re going to explore how that works. This post assumes no technical background. For a more in-depth look at the technical details, see LavaRand in Production: The Nitty-Gritty Technical Details.

Background

Randomness in Cryptography

As we’ve discussed in the past, cryptography relies on the ability to generate random numbers that are both unpredictable and kept secret from any adversary.

But “random” is a pretty tricky term; it’s used in many different fields to mean slightly different things. And like all of those fields, its use in cryptography is very precise. In some fields, a process is random simply if it has the right statistical properties. For example, the digits of pi are said to be random because all sequences of numbers appear with equal frequency (“15” appears as frequently as “38”, “426” appears as frequently as “297”, etc). But for cryptography, this isn’t enough - random numbers must be unpredictable.

To understand what unpredictable means, it helps to consider that all Continue reading

Understanding Our Cache and the Web Cache Deception Attack

About a month ago, security researcher Omer Gil published the details of an attack that he calls the Web Cache Deception attack. It works against sites that sit behind a reverse proxy (like Cloudflare) and are misconfigured in a particular way. Unfortunately, the definition of "misconfigured" for the purposes of this attack changes depending on how the cache works. In this post, we're going to explain the attack and then describe the algorithm that our cache uses to decide whether or not to cache a given piece of content so that customers can be sure that they are secure against this attack.

The Attack

First, we'll explain the basics of the Web Cache Deception attack. For those who want a more in-depth explanation, Omer's original post is a great resource.

Imagine that you run the social media website example.com, and that each of your users has a newsfeed at example.com/newsfeed. When a user navigates to their newsfeed, the HTTP request generated by their browser might look something like this:

GET /newsfeed HTTP/1.1  
Host: example.com  
...

If you use Cloudflare, you don't want us to cache this request because if Continue reading