The technique exploits web sessions protected by the Secure Sockets Layer and Transport Layer Security protocols when they use one of two data-compression schemes designed to reduce network congestion or the time it takes for webpages to load. Short for Compression Ratio Info-leak Made Easy, CRIME works only when both the browser and server support TLS compression or SPDY, an open networking protocol used by both Google and Twitter. Microsoft’s Internet Explorer, Google’s Chrome and Mozilla’s Firefox browsers are all believed to be immune to the attack, but at time of writing smartphone browsers and a myriad of other applications that rely on TLS are believed to remain vulnerable.
A side effect of compression, security experts have long known, is that it leaks clues about the encrypted contents. That means it provides a “side channel” to adversaries who have the ability to monitor the data. A research paper published in 2002 by John Kelsey looks eerily similar to CRIME, but only in retrospect.