The OPAQUE Asymmetric PAKE Protocol

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Last updated 2018-09-28
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Crypto Forum Research Group                                  H. Krawczyk
Internet-Draft                                              IBM Research
Intended status: Informational                          October 01, 2018
Expires: April 4, 2019

                  The OPAQUE Asymmetric PAKE Protocol


   This draft describes the OPAQUE protocol, a secure asymmetric
   password authenticated key exchange (aPAKE) that supports mutual
   authentication in a client-server setting without any reliance on
   PKI.  OPAQUE is the first PKI-free aPAKE to accommodate secret salt
   and therefore is the first to be secure against pre-computation
   attacks upon server compromise.  In contrast, 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, 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.

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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   This Internet-Draft will expire on April 4, 2019.

Krawczyk                  Expires April 4, 2019                 [Page 1]
Internet-Draft                     I-D                      October 2018

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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

   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 otherwise the attacker can pre-compute a deterministic list of
   mapped passwords leading to almost instantaneous leakage of passwords
   upon server compromise.

   Quite surprisingly, in spite of the existence of multiple designs for
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