Secure cryptoprocessor

A secure cryptoprocessor is a dedicated computer on a chip or microprocessor 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.

Examples

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 Automated teller machines, TV set-top boxes, military applications,^[1] and high-security portable communication equipment. Some secure cryptoprocessors can even run general-purpose operating systems such as Linux inside their security boundary. Cryptoprocessors input program instructions in encrypted form, decrypt the instructions to plain instructions which are then executed within the same cryptoprocessor chip where the decrypted instructions are inaccessibly stored. By never revealing the decrypted program instructions, the cryptoprocessor prevents tampering of programs by technicians who may have legitimate access to the sub-system data bus. This is known as bus encryption. Data processed by a cryptoprocessor is also frequently encrypted.

The Trusted Platform Module is an implementation of a secure cryptoprocessor that brings the notion of trusted computing to ordinary PCs by enabling a secure environment. While envisioned by some as being a method to make it much harder to illegally copy copyrighted software, present implementations tend to focus more on providing a tamper-proof boot environment.

Hardware Security Modules contain one or more cryptoprocessors. These devices are high grade secure cryptoprocessors used with Enterprise servers. A hardware security module can have multiple levels of physical security with a single-chip cryptoprocessor as its most secure component. The cryptoprocessor does not reveal keys or executable instructions on a bus, except in encrypted form, and zeros keys by attempts at probing or scanning. The crypto chip(s) may also be potted in the hardware security module with other processors and memory chips that store and process encrypted data. Any attempt to remove the potting will cause the keys in the crypto chip to be zeroed. A hardware security module may also be part of a computer (for example an ATM) that operates inside a locked safe to deter theft, substitution, and tampering.

Features

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.

Degree of security

Secure cryptoprocessors, while useful, are not invulnerable to attack, particularly for well-equipped and determined opponents (e.g. a government intelligence agency) who are willing to expend massive resources on the project.

The most famous attack on a secure cryptoprocessor targeted the IBM 4758.^[2] A team at the University of Cambridge reported the successful extraction of secret information from an IBM 4758, using a combination of mathematics, and special-purpose codebreaking hardware.

While 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.

Smartcards are significantly more vulnerable, as they are more open to physical attack.

In the case of full disk encryption applications, especially when implemented without a boot PIN, a cryptoprocessor does not offer any protection against a cold boot attack.^[3] In this scenario, data remanence is exploited to dump memory contents after the operating system has retrieved the cryptographic keys from its TPM.

Some secure cryptoprocessors contain dual processor cores and generate inaccessible encryption keys when needed so that even if the circuitry is reverse engineered, it will not reveal any keys that are necessary to securely decrypt software booted from encrypted flash memory or communicated between cores.^[4]

The first single-chip cryptoprocessor design was for copy protection of personal computer software (see US Patent 4,168,396, Sept 18, 1979) and was inspired by Bill Gates' Open Letter to Hobbyists.

See also

• Computer insecurity
• Secure computing
• Security engineering
• Hardware Security Modules

References