## Cryptology ePrint Archive: Report 2020/529

CHIP and CRISP: Compromise Resilient Identity-based Symmetric PAKEs

Moni Naor and Shahar Paz and Eyal Ronen

Abstract: Password Authenticated Key Exchange (PAKE) protocols allow parties to establish a shared key based only on the knowledge of a low entropy password. In this work, we propose a novel notion called Identity-based PAKE'' (iPAKE) -- providing resilience against compromise of one or more parties. iPAKE protocols protect all parties in the symmetric setting, whereas in Asymmetric PAKE (aPAKE) only one party (a server) is protected w.r.t compromise. Binding each party to its identity prevents impersonation between devices with different roles and allows the revocation of compromised parties. We achieve this by using ideas from Identity-Based Key-Agreement (IB-KA) while using only a low entropy password, without requiring a trusted center.

We further strengthen the notion by introducing Strong iPAKE'' (siPAKE) that is additionally immune to pre-computation (analogous to Strong aPAKE'' (saPAKE) strengthening of aPAKE). To mount an (inevitable) offline dictionary attack, an adversary must first compromise a device and only then start an exhaustive search over the entire password dictionary. Rather than storing its password in the clear, each party derives a password file using its identity and a secret random salt (salted hash''). The challenge is that although the random salts are independently selected, any pair of parties should be able to establish a cryptographically secure shared key from these files.

We formalize the iPAKE and siPAKE notions in the Universally Composable (UC) framework. We propose CHIP: a compiler from PAKE to iPAKE using IB-KA and prove its UC-security in the Random Oracle Model (ROM). We then present CRISP: a construction of siPAKE from any PAKE using bilinear groups with Hash2Curve''. We prove CRISP's UC-security in the Generic Group Model (GGM) and show that each offline password guess requires at least one pairing operation.

Category / Keywords: cryptographic protocols / Password authentication, Identity based key exchange, PAKE

Date: received 6 May 2020, last revised 15 Oct 2020

Contact author: eyal ronen at cs tau ac il

Available format(s): PDF | BibTeX Citation

Short URL: ia.cr/2020/529

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