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. Unlike cryptographic processors that output decrypted data onto a bus in a secure environment, a secure cryptoprocessor does not output decrypted data or decrypted program instructions in an environment where security cannot always be maintained. The purpose of a secure cryptoprocessor is to act as the keystone of a security subsystem, eliminating the need to protect the rest of the subsystem with physical security measures.
Examples A hardware security module (HSM) contains one or more secure cryptoprocessor chips. 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. Modern 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, 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 and 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 (TPM) is an implementation of a secure cryptoprocessor that brings the notion of trusted computing to ordinary PCs by enabling a secure environment. Present TPM implementations focus on providing a tamper-proof boot environment, and persistent and volatile storage encryption. Security chips for embedded systems are also available that provide the same level of physical protection for keys and other secret material as a smartcard processor or TPM but in a smaller, less complex and less expensive package. They are often referred to as cryptographic authentication devices and are used to authenticate peripherals, accessories and/or consumables. Like TPMs, they are usually turnkey integrated circuits intended to be embedded in a system, usually soldered to a PC board.
Features Security measures used in secure cryptoprocessors:
Tamper-detecting and tamper-evident containment. Conductive shield layers in the chip that prevent reading of internal signals. Controlled execution to prevent timing delays from revealing any secret information. Automatic zeroization of secrets in the event of tampering. 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.
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