Читать книгу Security Engineering - Ross Anderson - Страница 139

Passwords are just one example of a more general concept, the security protocol. Protocols specify the steps that principals use to establish trust relationships in a system, such as authenticating a claim to identity, demonstrating ownership of a credential, or establishing a claim on a resource. Cryptographic authentication protocols are used for a wide range of purposes, from basic entity authentication to providing infrastructure for distributed systems that allows trust to be taken from where it exists to where it is needed. Security protocols are fielded in all sorts of systems from remote car door locks through military IFF systems to authentication in distributed computer systems.

Protocols are surprisingly difficult to get right. They can suffer from a number of problems, including middleperson attacks, modification attacks, reflection attacks, and replay attacks. These threats can interact with implementation vulnerabilities and poor cryptography. Using mathematical techniques to verify the correctness of protocols can help, but it won't catch all the bugs. Some of the most pernicious failures are caused by creeping changes in the environment for which a protocol was designed, so that the protection it gives is no longer relevant. The upshot is that attacks are still found frequently on protocols that we've been using for years, and sometimes even on protocols for which we thought we had a security proof. Failures have real consequences, including the rise in car crime worldwide since car makers started adopting passive keyless entry systems without stopping to think about relay attacks. Please don't design your own protocols; get a specialist to help, and ensure that your design is published for thorough peer review by the research community. Even specialists get the first versions of a protocol wrong (I have, more than once). It's a lot cheaper to fix the bugs before the protocol is actually deployed, both in terms of cash and in terms of reputation.