Security through simplicity

If you're trying to estimate the security provided by a particular key management technique, you can essentially ignore the chances of any cryptographic algorithm being defeated. The chances of a peer-reviewed algorithm being weak, particularly when it has a rigorous proof of security is extremely small. On the other hand, the chances of a weakness in some component of a key management system is significant. It's very hard to implement and keep a secure configuration, and even if you can do this, you'll still exposed to bugs in the software and hardware that implements the key management system. You can formalize this if you really want to by writing an expression for the security of a system in terms of the security of each of its components, but even without explicitly writing that down, it should be fairly clear that simpler systems are inherently more secure than more complex ones.

Using this principle, let's look at two architectures and see which is more secure: one that implements X.509-based PKI or one that implements identity-based encryption. Here's a diagram that shows the components of a PKI system and how they interact when Alice sends an encrypted message to Bob. For this system to operate securely, each of its components needs to operate securely.

  

Now here's a diagram that shows the components of an IBE system and how they interact. Alice, who wants to send a message to Bob, just needs to get a set of public parameters from a secure key server that's usually called the private key generator, or PKG. Once she has these, she can encrypt messages to Bob. She can actually encrypt them to anyone else also, so she actually only needs to do that once. Once Bob receives Alice's message, he gets the key needed to decrypt it from the PKG. You don't need to do key look-up with IBE, so there's no component that acts like the LDAP directory does for a PKI, and because IBE keys are calculated as needed, there's also no need for a component that acts like a key archive server.

 

This system has only one-fourth the number of components of a system that implements PKI, which means that it has fewer ways to fail, which means that's it's more secure. Simpler is definitely better.