The OPAQUE Asymmetric PAKE Protocol
draft-krawczyk-cfrg-opaque-03

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Last updated 2019-10-19
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Crypto Forum Research Group                                  H. Krawczyk
Internet-Draft                                       Algorand Foundation
Intended status: Informational                          October 21, 2019
Expires: April 23, 2020

                  The OPAQUE Asymmetric PAKE Protocol
                     draft-krawczyk-cfrg-opaque-03

Abstract

   This draft describes the OPAQUE protocol, a secure asymmetric
   password authenticated key exchange (aPAKE) that supports mutual
   authentication in a client-server setting without reliance on PKI and
   with security against pre-computation attacks upon server compromise.
   Prior aPAKE protocols did not use salt and if they did, the salt was
   transmitted in the clear from server to user allowing for the
   building of targeted pre-computed dictionaries.  OPAQUE security has
   been proven by Jarecki et al.  (Eurocrypt 2018) in a strong and
   universally composable formal model of aPAKE security.  In addition,
   the protocol provides forward secrecy and the ability to hide the
   password from the server even during password registration.

   Strong security, versatility through modularity, good performance,
   and an array of additional features make OPAQUE a natural candidate
   for practical use and for adoption as a standard.  To this end, this
   draft presents several optimized instantiations of OPAQUE and ways of
   integrating OPAQUE with TLS.

   This draft presents a high-level description of OPAQUE highlighting
   its components and modular design.  A detailed unambiguous
   specification for standardization will be presented in future
   revisions of this document, or separately.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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Krawczyk                 Expires April 23, 2020                 [Page 1]
Internet-Draft                   OPAQUE                     October 2019

   This Internet-Draft will expire on April 23, 2020.

Copyright Notice

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1.  Introduction

   Password authentication is the prevalent form of authentication in
   the web and in most other applications.  In the most common
   implementation, a user authenticates to a server by entering its user
   id and password where both values are transmitted to the server under
   the protection of TLS.  This makes the password vulnerable to TLS
   failures, including many forms of PKI attacks, certificate
   mishandling, termination outside the security perimeter, visibility
   to middle boxes, and more.  Moreover, even under normal operation,
   passwords are always visible in plaintext form at the server upon TLS
   decryption (in particular, storage of plaintext passwords is not an
   uncommon security incident, even among security-conscious companies).

   Asymmetric (or augmented) Password Authenticated Key Exchange (aPAKE)
   protocols are designed to provide password authentication and
   mutually authenticated key exchange without relying on PKI (except
   during user/password registration) and without disclosing passwords
   to servers or other entities other than the client machine.  A secure
   aPAKE should provide the best possible security for a password
   protocol, namely, it should only be open to inevitable attacks:
   online impersonation attempts with guessed user passwords and offline
   dictionary attacks upon the compromise of a server and leakage of its
   password file.  In the latter case, the attacker learns a mapping of
   a user's password under a one-way function and uses such a mapping to
   validate potential guesses for the password.  Crucially important is
   for the password protocol to use an unpredictable one-way mapping or
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