Welcome to the resource topic for
**2003/050**

**Title:**

Concealment and its Applications to Authenticated Encryption

**Authors:**
Yevgeniy Dodis, 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.

**ePrint:**
https://eprint.iacr.org/2003/050

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