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Yesterday, a group from INRIA, Microsoft Research, Johns Hopkins, the University of Michigan, and the University of Pennsylvania published a deep analysis of the Diffie-Hellman algorithm as used in TLS and other protocols. This analysis included a novel downgrade attack against the TLS protocol itself called Logjam, which exploits EXPORT cryptography (just like FREAK).
First, let me start by saying that CloudFlare customers are not and were never affected. We don’t support non-EC Diffie-Hellman ciphersuites on either the client or origin side. We also won't touch EXPORT-grade cryptography with a 20ft stick.
But why are CloudFlare customers safe, and how does Logjam work anyway?
The image is "Logjam" as interpreted by @0xabad1dea.
Diffie-Hellman and TLS
This is a detailed technical introduction to how DH works and how it’s used in TLS—if you already know this and want to read about the attack, skip to “Enter export crypto, enter Logjam” below. If, instead, you are not interested in the nuts and bolts and want to know who’s at risk, skip to “So, what’s affected?”
To start a TLS connection, the two sides—client (the browser) and server (CloudFlare)—need to agree securely on a secret key. This process is called Continue reading