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{{short description|Set of all computer components critical to its security}}
{{textbook|date=February 2020}}▼
{{distinguish|Trusted Computing}}
▲{{textbook|date=February 2020}}
The '''trusted computing base''' ('''TCB''') of a [[computer system]] is the set of all [[Computer hardware|hardware]], [[firmware]], and/or [[software]] components that are critical to its [[computer security|security]], in the sense that [[Software bug|bugs]] or [[Vulnerability (computing)|vulnerabilities]] occurring inside the TCB might jeopardize the security properties of the entire system. By contrast, parts of a computer system that lie outside the TCB must not be able to misbehave in a way that would leak any more [[privilege (computer science)|privilege]]s than are granted to them in accordance to the system's [[security policy]].
The careful design and implementation of a system's trusted computing base is paramount to its overall security. Modern [[operating system]]s strive to reduce the size of the TCB{{Citation needed lead|date=February 2019}} so that an exhaustive examination of its code base (by means of manual or computer-assisted [[software audit review|software audit]] or [[program verification]]) becomes feasible.
==Definition and characterization==
The term
{{cite conference
| first = John
| last = Rushby
| title = Design and Verification of Secure Systems
|
| pages = 12–21
| year = 1981
| ___location = Pacific Grove, California, US
}}</ref> who defined it as the combination of [[
In the classic paper ''Authentication in Distributed Systems: Theory and Practice''<ref>B. Lampson, M. Abadi, M. Burrows and E. Wobber, [http://citeseer.ist.psu.edu/lampson92authentication.html Authentication in Distributed Systems: Theory and Practice], [[ACM Transactions on Computer Systems]] 1992, on page 6.</ref> [[Butler Lampson|Lampson]] et al. define the TCB of a [[computer system]] as simply
: ''a small amount of software and hardware that security depends on and that we distinguish from a much larger amount that can misbehave without affecting security.''
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: ''<nowiki>[t]</nowiki>he ability of a trusted computing base to enforce correctly a unified security policy depends on the correctness of the mechanisms within the trusted computing base, the protection of those mechanisms to ensure their correctness, and the correct input of parameters related to the security policy.''
In other words, a given piece of hardware or software is a part of the TCB if and only if it has been designed to be a part of the mechanism that provides its security to the computer system. In [[operating system]]s, this typically consists of the
==Properties==
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===Software parts of the TCB need to protect themselves===
As outlined by the aforementioned Orange Book, software portions of the trusted computing base need to protect themselves against tampering to be of any effect. This is due to the [[von Neumann architecture]] implemented by virtually all modern computers: since [[machine code]] can be processed as just another kind of data, it can be read and overwritten by any program.
In many modern [[CPU]]s, the protection of the memory that hosts the TCB is achieved by adding in a specialized piece of hardware called the [[memory management unit]] (MMU), which is programmable by the operating system to allow and deny a running program's access to specific ranges of the system memory
===Trusted vs. trustworthy===
As stated [[#A prerequisite to security|above]], [[Trusted system|trust]] in the trusted computing base is required to make any progress in ascertaining the security of the computer system. In other words, the trusted computing base is “trusted” first and foremost in the sense that it ''has'' to be trusted, and not necessarily that it is trustworthy. Real-world operating systems routinely have security-critical bugs discovered in them, which attests
The alternative is formal [[software verification]], which uses mathematical proof techniques to show the absence of bugs. Researchers at [[NICTA]] and its spinout [[Open Kernel Labs]] have recently performed such a formal verification of
{{ cite conference
| first = Gerwin
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| last13 = Winwood
| title = seL4: Formal verification of an OS kernel
|
| pages = 207–220
|date=October 2009
| ___location = Big Sky, Montana, US
| url = http://www.sigops.org/sosp/sosp09/papers/klein-sosp09.pdf
}}</ref>
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==Examples==
[[AIX operating system|AIX]] materializes the trusted computing base as an optional component in its install-time package management system.<ref>[
==See also==
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* [[Trust anchor]]
* [[Hardware security]]
==References==
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