A secure cryptoprocessor is a dedicated computer for carrying out cryptographic operations, embedded in a packaging with multiple physical security measures, which give it a degree of tamper resistance.
The purpose of a secure cryptoprocessor is to act as the keystone of a security sub-system, eliminating the need to protect the rest of the sub-system with physical security measures.
Smartcards are probably the most widely deployed form of secure cryptoprocessor, although more complex and versatile secure cryptoprocessors are widely deployed in systems such as ATMss. Some secure cryptoprocessors can even run general-purpose operating systems such as Linux inside their security boundary.
Security measures used in secure cryptoprocessors:
- tamper-detecting and tamper-evident containment
- automatic zeroization of secrets in the event of tampering
- internal battery backup
- chain of trust boot-loader which authenticates the operating system before loading it
- chain of trust operating system which authenticates application software before loading it
- hardware-based capability registers, implementing a one-way privilege separation model
The most famous secure cryptoprocessor is the IBM 4758. A team at the University of Cambridge reported the sucessful extraction of secret information from an IBM 4758, using a combination of guile, trickery, mathematics, and special-purpose codebreaking hardware.
Whilst the vulnerability they exploited was a flaw in the software loaded on the 4758, and not the architecture of the 4758 itself, their attack serves as a reminder that a security system is only as secure as its weakest link: the strong link of the 4758 hardware was rendered useless by flaws in the design and specification of the software loaded on it.
The software flaws reported by the Cambridge team have now been fixed, making the system more secure: a good example of the advantages of full disclosure.
Smartcards are significantly more vulnerable, as they are more open to physical attack.
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