Safely Turn Authentication Credentials Into Entropy (STACIE)

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Network Working Group                                         L. Levison
Internet-Draft                                               Lavabit LLC
Intended status: Experimental                               May 10, 2018
Expires: November 11, 2018

      Safely Turn Authentication Credentials Into Entropy (STACIE)


   This document specifies a method for Safely Turning Authentication
   Credentials Into Entropy (STACIE) using an efficient Zero Knowledge
   Password Proof (ZKPP), and is provided as a standalone component
   suitable for use as a building block in other protocol development
   efforts.  The scheme was created to fill the emerging need for a
   standard which allows a single low entropy password to be used for
   user authentication and the derivation of strong encryption keys.
   The design is modular, and is conservative in its use of an arbitrary
   one-way cryptographic hash function.  The security of the scheme
   depends on the difficulty associated with reversing the hash function
   output back into the plain text input.  STACIE attempts to make
   discovering the plain text input through the use of brute force more
   difficult by correlating the amount of processing to the length of a
   user's plain text password.  The shorter the plain text password, the
   more processing is required, with the amount of additional,
   artificially required, work scaling exponentially for each character.

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Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Encodings
   4.  Derivation Process
     4.1.  Hash Rounds
     4.2.  Entropy Extraction
     4.3.  Key Derivation
     4.4.  Token Derivation
     4.5.  Realm Key Derivation
   5.  Encryption
     5.1.  Envelope
     5.2.  Payload
   6.  Password Changes
     6.1.  Shallow Password Change
     6.2.  Deep Password Change
     6.3.  Hybrid Password Change
   7.  Protocol
     7.1.  Login
       7.1.1.  Login Request
       7.1.2.  Login Response
     7.2.  Authenticate
       7.2.1.  Authenticate Request
       7.2.2.  Authenticate Response
     7.3.  Create
     7.4.  Password Changes
     7.5.  Fetch Realm Specific Shard Values
     7.6.  Add Realm Specific Shard Value
   8.  Security Considerations
   9.  IANA Considerations
     9.1.  Servers
     9.2.  Clients
     9.3.  Shared
   10. Feedback
   11. Acknowledgments
   12. Normative References
   Appendix A.  Test Vectors
     A.1.  Inputs
     A.2.  Outputs
   Author's Address

1.  Introduction

   A number of emerging client/server protocols are currently being
   developed which rely on endpoint encryption schemes for protection
   against server compromises and pervasive surveillance efforts.  All
   of these protocols share a common need for the ability to
   authenticate users based on their account password, without having to
   share a plain text password with the server.  While several proposals
   have emerged which rely on a Zero Knowledge Password Proof (ZKPP),
   none of them provide a standardized method for deriving a symmetric
   encryption key suitable for use with Authenticated Encryption with
   Associated Data (AEAD) ciphers using the same user password.

   This specification describes a standalone scheme which solves these
   problems by Safely Turning Authentication Credentials Into Entropy
   (STACIE).  Unlike previous efforts, STACIE can uniquely provide a
   configurable level of resistance against off-line brute force attacks
   aimed at recovering the original plain text password, or the derived
   encryption keys.  Client side key stretching ensures attackers
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