What they are likely referring it is software like "Mujahideen Secrets", which terrorists have been using for about a decade to encrypt messages. It includes a unique fingerprint/signature that can easily be searched for, as shown below.The text string in these sorts of things is not hard to identify at all. The string is used by programs to know where to start decrypting (all sorts of crypto programs do this).
The obvious "highly unique signature" the FBI should be looking for, to catch this software, is the string:It's speculation that this is what Yahoo was looking for, but as Mythbusters would say, "Plausible."
### Begin ASRAR El Mojahedeen v2.0 Encrypted Message ###Indeed, if this is the program the NSA/FBI was looking for, they've now caught this message in their dragnet of incoming Yahoo! mail.
I posted about this a couple years ago, in a post titled How do you say "Hey, NSA! Look over here!" on the Internet? In it, I said:
I've been very critical about how NSA is spying on citizens unsuspected of any crime, but this seems to be precisely what they should be doing. I'm even OK with secret FISA court warrants allowing automated monitoring (and even attacking) anyone using al Qaeda code. Seems like that falls under "probable cause" to me.In most jurisdictions possession of lock picking tools is presumptive evidence of wrongdoing (if you're not a locksmith). It seems plausible that possession of custom Al Qaeda encryption software is also presumptive of wrongdoing. Sure, there's a First Amendment argument that can be made here, cryptographic research, yadda yadda - but this seems quite narrowly tailored to me. If this is what Yahoo was looking for, it seems reasonable to me.