Abstract
The algebraic-group model (AGM), which lies between the generic group model and the standard model of computation, provides a means by which to analyze the security of cryptosystems against so-called algebraic adversaries. We formalize the AGM within the framework of universal composability, providing formal definitions for this setting and proving an appropriate composition theorem. This extends the applicability of the AGM to more-complex protocols, and lays the foundations for analyzing algebraic adversaries in a composable fashion. Our results also clarify the meaning of composing proofs in the AGM with other proofs and they highlight a natural form of independence between idealized groups that seems inherent to the AGM and has not been made formal before—these insights also apply to the composition of game-based proofs in the AGM. We show the utility of our model by proving several important protocols universally composable for algebraic adversaries, specifically: (1) the Chou-Orlandi protocol for oblivious transfer, and (2) the SPAKE2 and CPace protocols for password-based authenticated key exchange.
J. Loss—Work done while at the University of Maryland.
J. Xu—Work done while at George Mason University.
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Notes
- 1.
One can consider formalizing the AGM within the UC framework by introducing a functionality \(\mathcal {F}_\mathsf{AGM}\) that “forces” arbitrary algorithms to behave algebraically by registering group elements and their representations in a central repository. This has a number of disadvantages that we discuss in the full version [3]. Our approach is closer to the spirit of the AGM, which idealizes groups by quantifying over restricted classes of adversaries.
- 2.
For a review of the security proofs available for both protocols at the time, see https://mailarchive.ietf.org/arch/msg/cfrg/47pnOSsrVS8uozXbAuM-alEk0-s.
- 3.
Formally, we assume an encoding of group elements that distinguishes them from arbitrary strings. This can be done by simply prefixing any group element with a 0 and any other string (not necessarily representing a group element) with a 1. Following prior work [19], we use bold capital letters to denote group elements (except for the generator g).
- 4.
Formally, we consider protocols having access to a \(\mathcal {F}_{\mathrm {CRS}}\) functionality, where \(\mathcal {F}_{\mathrm {CRS}}\) runs a group-generation algorithm to obtain \(\mathcal {G}\) (and possibly additional group elements), and then sends \(\mathcal {G}\) (and any other elements) to parties that request it. Note that the protocol may use other groups, but we only require the adversary to be algebraic with respect to \(\mathcal {G}\).
- 5.
Note that this \(\mathcal {S}_\pi \) is an imaginary machine supposed to run inside \(\mathcal {S}\), whereas the “actual” \(\mathcal {S}_\pi \) is the simulator in the execution of \(\phi \). Same with step 3 below.
- 6.
This step could be replaced with a search for a consistent entry using a DDH oracle to the fixed basis \(\mathbf {A}\), resulting in a tighter reduction to Strong SqDH where the \(q_{\mathrm {H}}\) factor disappears.
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Abdalla, M., Barbosa, M., Katz, J., Loss, J., Xu, J. (2021). Algebraic Adversaries in the Universal Composability Framework. In: Tibouchi, M., Wang, H. (eds) Advances in Cryptology – ASIACRYPT 2021. ASIACRYPT 2021. Lecture Notes in Computer Science(), vol 13092. Springer, Cham. https://doi.org/10.1007/978-3-030-92078-4_11
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