Paper 2003/050

Concealment and its Applications to Authenticated Encryption

Yevgeniy Dodis and Jee Hea An

Abstract

We introduce a new cryptographic primitive we call **concealment**, which is related, but quite different from the notion of commitment. A concealment is a publicly known randomized transformation, which, on input m, outputs a *hider* h and a *binder* b. Together, h and b allow one to recover m, but separately, (1) the hider h reveals "no information" about m, while (2) the binder b can be "meaningfully opened" by at most one hider h. While setting b=m, h=empty is a trivial concealment, the challenge is to make |b|<<|m|, which we call a "non-trivial" concealment. We show that non-trivial concealments are equivalent to the existence of collision-resistant hash functions. Moreover, our construction of concealments is extremely simple, optimal, and yet very general, giving rise to a multitude of efficient implementations. We show that concealments have natural and important applications in the area of **authenticated encryption**. Specifically, let AE be an authenticated encryption scheme (either public- or symmetric-key) designed to work on short messages. We show that concealments are **exactly** the right abstraction allowing one to use AE for encrypting long messages. Namely, to encrypt long m, one uses a concealment scheme to get h and b, and outputs authenticated ciphertext (AE(b),h). More surprisingly, the above paradigm leads to a very simple and general solution to the problem of **remotely keyed (authenticated) encryption** (RKAE). In this problem, one wishes to split the task of high-bandwidth authenticated encryption between a secure, but low-bandwidth/computationally limited device, and an insecure, but computationally powerful host. We give formal definitions for RKAE, which we believe are simpler and more natural than all the previous definitions. We then show that our composition paradigm satisfies our (very strong) definition. Namely, for authenticated encryption, the host simply sends a short value b to the device (which stores the actual secret key for AE), gets back AE(b), and outputs (AE(b),h) (authenticated decryption is similar). Finally, we also observe that several previous RKAE proposals are all special examples of our general paradigm.