Welcome to the resource topic for 2025/1922
Title:
Guaranteeing a Dishonest Party’s Knowledge (Or: Setup Requirements for Deniable Authentication)
Authors: Nils Fleischhacker, Guilherme Rito
Abstract:Deniable authentication is a very desirable guarantee for
secure messaging — and in particular Off-The-Record messaging: it allows
a sender Alice to authenticate her messages while retaining plausible
deniability on whether she authenticated any of her messages.
While deniable authentication is not achievable in standard Public-Key
Infrastructures, Dodis et al. (TCC ’09) have shown that it is achievable
in so-called Key-Registration with Knowledge setups. These guarantee:
- dishonest parties’ key-pairs are honestly generated; and
- dishonest parties have access to their secret keys.
However, these are expensive assumptions, and it is unclear how securely
realize them in practice.
In this paper we study the weakest setup requirements that are sufficient
to enable deniable authentication. One of our main findings is that
deniable authentication does require an expensive — but not necessarily
impractical — Key-Registration procedure. Roughly, this procedure must
guarantee that if a user’s key registration is successful, then the user can
extract by themselves valid secret keys from their interaction with the
registration authority. We show this setup is sufficient by capturing it via
an appropriate security model and then proving the anonymity of a ring
signature based on it. On the other hand, we explain why this setup seems
inherently necessary by listing a series of attacks that void deniability as
soon as a user manages to successfully register a public key for which it
can convincingly claim it does not know a valid (corresponding) secret
key.
The requirements we identify for key-registration protocols are expensive,
but not necessarily impractical. Our second main contribution is showing
how to amortize the per-user Key-Registration cost, which brings deniable
authentication guarantees a step closer to practicality.
ePrint: https://eprint.iacr.org/2025/1922
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