9.1 Introduction

A Trusted Third Party (TTP) is an entity in a network that is trusted to provide a security service to the other users of that network. Examples of the use of TTPs in secure systems include key generation and distribution, electronic notarisation and digital archiving. Typically, a TTP service is offered by a single (or small number of) dedicated machine(s) that have strong security counter-measures installed to prevent the TTP's service from being compromised. This centralised approach is easy to manage, update and test.

The use of a TTP in a security system is almost always based on a business agreement. The users trust the TTP because they know that any attempt the TTP makes to subvert or make malicious use of the service that it offers will result in a violation of the business agreement. If the TTP service is being offered by a company then this could involve the contract to run the TTP service being cancelled and the business sued. If the TTP service is being offered by some internal entity within a company then the manager of that entity will have to explain the reason for the deviation from the security policy.

There are a couple of problems that are typically associated with traditional TTPs. The first is that, by their very nature, a TTP provides a static, centralised point of attack. For example, an attacker can compromise a whole Public Key Infrastructure (PKI) by compromising the CA that...