There once was a very good article about user interface issues in PGP 5.0, called “Why Johnny Can’t Encrypt“. In this year’s Usenix Security Symposium (Simply called “Security” by those in the know), there was an article called “Why (Special Agent) Johnny (Still) Can’t Encrypt“, and I, being a fan of the former paper, immediately downloaded and read the latter, especially because it had won an outstanding paper award.
This paper is an excellent example of a system security analysis, in this case for so-called tactical radio, which is radio used by US law enforcement agents of all kinds when they are in the field. Of course, such radio communication has some security requirements:
- It should be possible to encrypt voice messages so that eavesdroppers can’t hear what is being said.
- It should not be possible to inject communication into an otherwise encrypted conversation.
- It should be difficult to jam.
And many more. The authors very systematically look at these issues and find problem after problem, many of them minor when taken in isolation, but devastating when they come together:
- Unencrypted voice messages can be heard by all team members, even if the rest of the team is using encryption.
- Encryption is easy to turn off while turning an unrelated knob.
- The “encrypted” indicator is usually outside the radio operator’s line of sight.
- The radio can be jammed by selectively jamming parts of important data frames, using nothing more than a $15 toy instant messenger device.
- in practice, a lot of the radio traffic is unencrypted anyway, containing much interesting (and probably secret) information.
The paper explicitly focuses on how the interaction of minor flaws conspire to render the whole system essentially broken. The paper thus is an excellent example of system security analysis, and I recommend it muchly, no less because of the tongue-in-cheek tone in which it is written. Examples:
We implemented a complete receiver and exciter for an effective P25 jammer by installing custom firmware in a $15 toy “instant messenger” device marketed to pre-teen children.
Or, in an analysis of the keying procedure, after concluding that encryption keys are often out of sync:
This scenario is a sharp counterexample of the oft-repeated cryptographic folk wisdom (apparently believed as an article of faith by many end users) that frequently changing one’s key yields more security.
Or, on the “encryption” indicator symbol:
On Motorola radios, this symbol is a circle with a line through it, unaccompanied by any explanatory label. This is […] also the symbol used in many automobiles to indicate whether the air conditioning vents are open or closed.
So go ahead and grab a copy. It’s good reading, and good science, too!