In computing, a trusted client is a device or program controlled by the user of a service, but with restrictions designed to prevent its use in ways not authorised by the provider of the service. Examples include video games played over a computer network or the content-scrambling system (CSS) to enforce regions in DVDs.

Trusted client software is considered fundamentally insecure: once the security is broken by one user, the break is trivially copyable and available to others. (As Bruce Schneier states, "Against the average user, anything works; there's no need for complex security software. Against the skilled attacker, on the other hand, nothing works.") Trusted client hardware is somewhat less insecure, but not a complete solution (Grand, 2000).

Trusted clients are attractive to business as a form of vendor lock-in: sell the trusted client at a loss and charge more than would be otherwise economically viable for the associated service. One early example was radio receivers that were subsidised by broadcasters, but restricted to receiving only their radio station. Modern examples include video recorders being forced by law to include Macrovision copy protection, the DVD region system and region-coded video game consoles.

Technically knowledgeable consumers and other manufacturers frequently bypass the limiting features of trusted clients — from the simple replacement of the fixed tuning potentiometer in the early locked radios to the successful DeCSS cryptographic attack on CSS in 1999. Manufacturers have resorted to legal threats via the Digital Millennium Copyright Act and similar laws to prevent their circumvention, with varying degrees of success.

Trusted computing aims to create a general-purpose computer immune to tampering or circumvention.

See also

• Dongle

References

• Bruce Schneier: The Fallacy of Trusted Client Software, Information Security Magazine, August 2000
• Attacks on and Countermeasures for USB Hardware Token Devices (PDF) (Joe Grand, Grand Ideas Studio, Proceedings of the Fifth Nordic Workshop on Secure IT Systems Encouraging Co-operation, Reykjavik, Iceland, October 12-13, 2000, pp 35-57, ISBN 99799483-0-2