Читать книгу Security Engineering - Ross Anderson - Страница 211

In the old days, anyone with physical access to a computer controlled all of it: you could load whatever software you liked, inspect everything in memory or on disk and change anything you wanted to. This is the model behind discretionary access control (DAC): you start your computer in supervisor mode and then, as the administrator, you can make less-privileged accounts available for less-trusted tasks – such as running apps written by companies you don't entirely trust, or giving remote logon access to others. But this can make things hard to manage at scale, and in the 1970s the US military started a huge computer-security research program whose goal was to protect classified information: to ensure that a file marked ‘Top Secret’ would never be made available to a user with only a ‘Secret’ clearance, regardless of the actions of any ordinary user or even of the supervisor. In such a multilevel secure (MLS) system, the sysadmin is no longer the boss: ultimate control rests with a remote government authority that sets security policy. The mechanisms started to be described as mandatory access control (MAC). The supervisor, or root access if you will, is under remote control. This drove development of technology for mandatory access control – a fascinating story, which I tell in Part 2 of the book.

From the 1980s, safety engineers also worked on the idea of safety integrity levels; roughly, that a more dependable system must not rely on a less dependable one. They started to realise they needed something similar to multilevel security, but for safety. Military system people also came to realise that the tamper-resistance of the protection mechanisms themselves was of central importance. In the 1990s, as computers and networks became fast enough to handle audio and video, the creative industries lobbied for digital rights management (DRM) in the hope of preventing people undermining their business models by sharing music and video. This is also a form of mandatory access control – stopping a subscriber sharing a song with a non-subscriber is in many ways like stopping a Top Secret user sharing an intelligence report with a Secret user.

In the early 2000s, these ideas came together as a number of operating-system vendors started to incorporate ideas and mechanisms from the MAC research programme into their products. The catalyst was an initiative by Microsoft and Intel to introduce cryptography into the PC platform to support DRM. Intel believed the business market for PCs was saturated, so growth would come from home sales where, they believed, DRM would be a requirement. Microsoft started with DRM and then realised that offering rights management for documents too might be a way of locking customers tightly into Windows and Office. They set up an industry alliance, now called the Trusted Computing Group, to introduce cryptography and MAC mechanisms into the PC platform. To do this, the operating system had to be made tamper-resistant, and this is achieved by means of a separate processor, the Trusted Platform Module (TPM), basically a smartcard chip mounted on the PC motherboard to support trusted boot and hard disk encryption. The TPM monitors the boot process, and at each stage a hash of everything loaded so far is needed to retrieve the key needed to decrypt the next stage. The real supervisor on the system is now no longer you, the machine owner – it's the operating-system vendor.

MAC, based on TPMs and trusted boot, was used in Windows 6 (Vista) from 2006 as a defence against persistent malware¹. The TPM standards and architecture were adapted by other operating-system vendors and device OEMs, and there is now even a project for an open-source TPM chip, OpenTitan, based on Google's product. However the main purpose of such a design, whether the design itself is open or closed, is to lock a hardware device to using specific software.