The recent warnings that al Qaeda was planning attacks on specific targets in the United States was said to come from recent people, and information, captured in Pakistan. One of the two key al Qaeda people captured was Mohammed Naeem Noor Khan, an English speaking Pakistani computer engineer. Khan was running an al Qaeda communications network, using email and encryption to distribute messages that could not be read with out the decoding keys. What was left out of these reports was any mention of public key cryptography, PGP (a version of public key cryptography freely available to Internet users), the National Security Agency (NSA) and cracking (decoding) PGP. Theres more to Mister Khans capture than meets the eye.
During the 1990s, the NSA was frequently in court trying to keep PGP off the market. In the 1980s, NSA was trying to get the key length of commercial ciphers kept shorter than business wanted. NSA is in charge of developing systems to keep American government messages secret, and figuring out how to crack the codes other nations use. Although the NSA never admitted it, most cryptography experts believed NSA wanted to keep longer keys out of use, because NSA did not have powerful enough techniques, or computers, to crack longer keys.
OK, all this talk of cipers and keys length doesnt make sense to most people, so lets offer a brief explanation. It starts with the appearance of inexpensive computers, when it became possible to use new methods to turn messages into apparent gibberish with coding systems. This was done using a key, which was a string of letters and numbers. Think of it a kind of password. If the person receiving the message had the right key, the message could be decoded (using a mathematical routine similar to the one that garbled the message in the first place.) The most popular of these techniques was eventually released as an inexpensive commercial product called PGP (Pretty Good Privacy). Users could post a public key that other users of the PGP program could use to scramble an email message or data file. When you received a message scrambled with your public key, you entered your private key into PGP and it descrambled the email or file.
The trouble with PGP was that, as far as NSA was concerned, it was too good. NSA got the U.S. government to declare programs like PGP to be military equipment, and subject to export controls. Trying to stop the spread of PGP was absurd, however, and the government eventually backed off. But NSAs problem with PGP encoded messages remained. Or did it? NSA, obviously, is not going to admit that it can, or cannot, crack PGP encoded messages.
The U.S. and Pakistan may have found a way to read months, or years, worth of secret al Qaeda messages. No one is saying anything about that, but it works like this.
The most straightforward to read an encoded PGP message, without the decoding key is to use a computer to run through every possible combination of the key, to find the ones that work. A 516 digit key can be cracked using this computerized brute force method. For example, if you have a computer that can perform a million instructions per second going at it for 30,000 years, you can read the message encoded with a 516 digit key. Thats a little misleading, as a thousand dollar PC in 2004 can generate several thousand MIPS. So it would only take you ten years to crack that code. Of course, if you got several hundred of these PCs working together, you could get the job done in a few weeks. You can see where this is going. NSA has billions of dollars a year to play with, and building supercomputers out of cheap PCs has become quite popular.
However, increase the key to 768 characters, and it takes about 6,600 times longer to crack it. Go to key size of 1024, and it takes 1,500 times longer than the 768 character key. Go to a 2048 key size and it takes a billion times longer than a 1024 character long key. PGP can use a 1024 character key, and many users go for the larger key for obvious reasons.
Down at the NSA, all is not lost. Cracking a PGP code can be done with clever math as well as powerful computers. While we know there are a lot of cheap, powerful computers available to the NSA, we also know the NSA has a lot of clever people who specialize in figuring out better ways to crack codes. What we dont know is if NSA is able to crack PGP messages scrambled using 1024 character codes.
If al Qaedas computer savvy Mohammed Naeem Noor Khan was in charge of an al Qaeda communications network that used the Internet, then he was probably using PGP. Al Qaeda use of PGP has been reported many times, as have complaints about the potential for that providing al Qaeda with an invulnerable encryption system. NSA has remained silent. But if Kahn was the man in charge, and he had many of the decryption keys with him when he was captured, that enables the NSA to read many previous messages. How many? Depends on how many keys were captured. Could be months worth. Its also possible that months worth of actual messages was captured. This kind of information is invaluable in figuring out what al Qaeda has been doing, and is currently planning. Did Kahn have the keys? This is something you dont want to discuss, one way or the other. You want the al Qaeda users of Kahns network to sweat a bit, even if the keys were not grabbed. And if the keys were obtained, there are tough times ahead for terrorist users of "invulnerable" encryption.