The Dark Side of “Black-Box” Cryptography or: Should We Trust Capstone?

Abstract

The use of cryptographic devices as “black boxes”, namely trusting their internal designs, has been suggested and in fact Capstone technology is offered as a next generation hardware-protected escrow encryption technology. Software cryptographic servers and programs are being offered as well, for use as library functions, as cryptography gets more and more prevalent in computing environments. The question we address in this paper is how the usage of cryptography as a black box exposes users to various threats and attacks that are undetectable in a black-box environment. We present the SETUP (Secretly Embedded Trapdoor with Universal Protection) mechanism, which can be embedded in a cryptographic black-box device. It enables an attacker (the manufacturer) to get the user’s secret (from some stage of the output process of the device) in an unnoticeable fashion, yet protects against attacks by others and against reverse engineering (thus, maintaining the relative advantage of the actual attacker). We also show how the SETUP can, in fact, be employed for the design of “auto-escrowing key” systems. We present embeddings of SETUPs in RSA, El-Gamal, DSA, and private key systems (Kerberos). We implemented an RSA key-generation based SETUP that performs favorably when compared to PGP, a readily available RSA implementation. We also relate message-based SETUPs and subliminal channel attacks. Finally, we reflect on the potential implications of “trust management” in the context of the design and production of cryptosystems.

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