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The Low-Call Diet: Authenticated Encryption for Call Counting HSM Users

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Executive Summary

The authors present a new mode of operation for obtaining authenticated encryption suited for use in banking and government environments where cryptographic services are only available via a Hardware Security Module (HSM) which protects the keys but offers a limited API. The practical problem is that despite the existence of better modes of operation, modern HSMs still provide nothing but a basic (unauthenticated) CBC mode of encryption, and since they mediate all access to the key, solutions must work around this. Their mode of operation makes only a single call to the HSM, yet provides a secure authenticated encryption scheme; authentication is obtained by manipulation of the plaintext being passed to the HSM via a call to an unkeyed hash function.

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