Attacks on Cryptoprocessor Transaction Sets

By Mike Bond
Attacks are presented on the IBM 4758 CCA and the Visa Security Module. Two new attack principles are demonstrated. Related key attacks use known or chosen differences between two cryptographic keys. Data protected with one key can then be abused by manipulation using the other key. Meet in the middle attacks work by generating a large number of unknown keys of the same type, thus reducing the key space that must be searched to discover the value of one of the keys in the type. Design heuristics are presented to avoid these attacks and other common errors.
A cryptoprocessor is a tamper-resistant processor designed to manage cryptographic keys and data in high-risk situations. The concept of a cryptoprocessor arose because conventional operating systems are too bug-ridden and computers too physically insecure to be trusted with information of high value. A normal microprocessor is enclosed within a tamper-resistant environment, so that sensitive information can only be altered or released through a tightly defined software interface – a transaction set. In combination with access control, the transaction set should prevent abuse of the sensitive information. However, as the functionality and flexibility of transaction sets have been pushed up by manufacturers and clients, this extra complexity has made bugs in transaction sets inevitable.
Sections 2 and 3 of this paper give an overview of cryptoprocessors in the context of four important architectural principles, and then describe the new vulnerabilities in a generalised way. Sections 4 and 5 introduce attacks on two widely fielded cryptoprocessors – the IBM 4758, and the Visa Security Module. Finally, some straightforward design heuristics are suggested that, whilst not guaranteeing the security of a transaction set, will at least stop the same mistakes being made over again.

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