EAP-pwd Salted: Adding Support for Salted Password Databases to EAP-pwd

Internet Engineering Task Force (IETF)                        D. Harkins
Request for Comments: 8146                                 HP Enterprise
Updates: 5931                                                 April 2017
Category: Informational
ISSN: 2070-1721


        Adding Support for Salted Password Databases to EAP-pwd

Abstract

   EAP-pwd is an Extensible Authentication Protocol (EAP) method that
   utilizes a shared password for authentication using a technique that
   is resistant to dictionary attacks.  It includes support for raw keys
   and double hashing of a password in the style of Microsoft Challenge
   Handshake Authentication Protocol version 2 (MSCHAPv2), but it does
   not include support for salted passwords.  There are many existing
   databases of salted passwords, and it is desirable to allow their use
   with EAP-pwd.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8146.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Background .................................................3
      1.2. Keyword Definition .........................................3
   2. Salted Passwords in EAP-pwd .....................................3
      2.1. Password Preprocessing .....................................3
      2.2. The Salting of a Password ..................................5
      2.3. Using UNIX crypt ...........................................5
      2.4. Using scrypt ...............................................6
      2.5. Using PBKDF2 ...............................................6
      2.6. Protocol Modifications .....................................7
      2.7. Payload Modifications ......................................8
   3. IANA Considerations .............................................8
   4. Security Considerations .........................................9
   5. References ......................................................9
      5.1. Normative References .......................................9
      5.2. Informative References ....................................10
   Acknowledgements ..................................................11
   Author's Address ..................................................11

1.  Introduction

1.1.  Background

   Databases of stored passwords present an attractive target for attack
   -- get access to the database, learn the passwords.  To confound such
   attacks, a random "salt" was hashed with the password and the
   resulting digest stored, along with the salt, instead of the raw
   password.  This has the effect of randomizing the password; even if
   two, distinct users have chosen the same password, the stored, and
   salted, password will be different.  It also requires an adversary
   who has compromised the security of the stored database to launch a
   dictionary attack per entry to recover passwords.

   Dictionary attacks, especially using custom hardware, represent real-
   world attacks and merely salting a password is insufficient to
   protect a password database.  To address these attacks, a sequential
   memory hard function, such as described in [RFC7914], is used.

   While salting a password database is not sufficient to deal with many
   real-world attacks, the historic popularity of password salting means
   there are a large number of such databases deployed, and EAP-pwd
   needs to be able to support them.  In addition, EAP-pwd needs to be
   able to support databases using more modern sequential memory hard
   functions for protection.

   EAP-pwd imposes an additional security requirement on a database of
   salted passwords that otherwise would not exist, see Section 4.

1.2.  Keyword Definition

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Salted Passwords in EAP-pwd

2.1.  Password Preprocessing

   EAP-pwd is based on the "dragonfly" Password-Authenticated Key
   Exchange (PAKE) -- see [RFC7664].  This is a balanced PAKE and
   requires that each party to the protocol obtain an identical
   representation of a processed password (see Section 4).  Therefore,
   salting of a password is treated as an additional password
   preprocessing technique of EAP-pwd.  The salt and digest to use are
   conveyed to the peer by the server, and the password is processed
   prior to fixing the password element (see Section 2.8.3 of
   [RFC5931]).

   This memo defines eight (8) new password preprocessing techniques for
   EAP-pwd:

   o  0x03: a random salt with SHA-1

   o  0x04: a random salt with SHA-256

   o  0x05: a random salt with SHA-512

   o  0x06: UNIX crypt()

   o  0x07: scrypt

   o  0x08: PBKDF2 with SHA-256

   o  0x09: PBKDF2 with SHA-512

   o  0x0A: SASLprep then a random salt with SHA-1

   o  0x0B: SASLprep then a random salt with SHA-256

   o  0x0C: SASLprep then a random salt with SHA-512

   o  0x0D: SASLprep then UNIX crypt()

   o  0x0E: OpaqueString then scrypt

   o  0x0F: OpaqueString then PBKDF2 with SHA-256

   o  0x10: OpaqueString then PBKDF2 with SHA-512

   When passing salt, the size of the salt SHOULD be at least as long as
   the message digest of the hash algorithm used.  There is no guarantee
   that deployed salted databases have followed this rule, and in the
   interest of interoperability, an EAP peer SHOULD NOT abort an EAP-pwd
   exchange if the length of the salt conveyed during the exchange is
   less than the message digest of the indicated hash algorithm.

   UNIX crypt() ([CRY]), scrypt ([RFC7914]), and PBKDF2 ([RFC8018])
   impose additional formatting requirements on the passed salt.  See
   below.

   Plain salting techniques using [SHS] are included for support of
   existing databases. scrypt and PBKDF2 techniques are RECOMMENDED for
   new password database deployments.

   SASLprep has been deprecated, but databases treated with SASLprep
   exist; it is necessary to provide code points for them.  When using
   SASLprep, a password SHALL be considered a "stored string" per
   [RFC3454]; therefore, unassigned code points are prohibited.  The
   output of SASLprep SHALL be the binary representation of the
   processed UTF-8 character string.  Prohibited output and unassigned
   code points encountered in SASLprep preprocessing SHALL cause a
   failure of preprocessing, and the output SHALL NOT be used with EAP-
   pwd.

   When performing one of the preprocessing techniques (0x0E-0x10), the
   password SHALL be a UTF-8 string and SHALL be preprocessed by
   applying the Preparation and Enforcement steps of the OpaqueString
   profile in [RFC7613] to the password.  The output of OpaqueString,
   also a UTF-8 string, becomes the EAP-pwd password and SHALL be hashed
   with the indicated algorithm.

   There is a large number of deployed password databases that use
   salting and hashing in the style of [RFC7616], but these deployments
   require a nonce contribution by the client (as well as the server),
   and EAP-pwd does not have the capability to provide that information.

2.2.  The Salting of a Password

   For both parties to derive the same salted password, there needs to
   be a canonical method of salting a password.  When using EAP-pwd, a
   password SHALL be salted by hashing the password followed by the salt
   using the designated hash function:

      salted-password = Hash(password | salt)

   The server stores the salted-password, and the salt, in its database
   and the client derives the salted password on the fly.

2.3.  Using UNIX crypt

   Different algorithms are supported with the UNIX crypt() function.
   The particular algorithm used is indicated by prepending an encoding
   of "setting" to the passed salt.  The specific algorithm used is
   opaque to EAP-pwd as the entire salt, including the encoded
   "setting", is passed as an opaque string for interpretation by
   crypt().  The salted password used for EAP-pwd SHALL be the output of
   crypt():

      salted-password = crypt(password, salt)

   The server stores the salted-password, and the encoded algorithm plus
   salt, in its database and the client derives the salted-password on-
   the-fly.

   If the server indicates a crypt() algorithm that is unsupported by
   the client, the exchange fails and the client MUST terminate the
   connection.

2.4.  Using scrypt

   The scrypt function takes several parameters:

   o  N, the cost parameter

   o  r, the block size

   o  p, the parallelization parameter

   o  dkLen, the length of the output

   These parameters are encoded into the "salt" field of the modified
   EAP-pwd message.  Parameters r and dkLen SHALL be 16-bit integers in
   network order.  The other parameters SHALL each be 32-bit integers in
   network order.  The "salt" field that gets transmitted in EAP-pwd
   SHALL therefore be:

      N || r || p || dkLen || salt

   where || represents concatenation.

   The value of N represents the exponent taken to the power of two in
   order to determine the CPU/Memory cost of scrypt -- i.e., the value
   is 2^N.  Per [RFC7914], the resulting CPU/Memory cost value SHALL be
   less than 2^(128 * r / 8), and the value p SHALL be less than or
   equal to ((2^32 - 1) * 32) / (128 * r).

   Note: EAP-pwd uses the salted password directly as the authentication
   credential and will hash it with a counter in order to obtain a
   secret element in a finite field.  Therefore, it makes little sense
   to use dkLen greater than the length of the digest produced by the
   underlying hash function, but the capability is provided to do so
   anyway.

2.5.  Using PBKDF2

   The PBKDF2 function requires two parameters:

   o  c, the iteration count

   o  dkLen, the length of the output
   These parameters are encoded into the "salt" field of the modified
   EAP-pwd message.  The parameters SHALL be 16-bit integers in network
   order.  The "salt" field that gets transmitted in EAP-pwd SHALL
   therefore be:

      c || dkLen || salt

   where || represents concatenation.

   Note: EAP-pwd uses the salted password directly as the authentication
   credential and will hash it with a counter in order to obtain a
   secret element in a finite field.  Therefore, it makes little sense
   to use a dkLen greater than the length of the digest produced by the
   underlying hash function, but the capability is provided to do so
   anyway.

2.6.  Protocol Modifications

   Like all EAP methods, EAP-pwd is server initiated, and the initial
   identity supplied by the client is not useful for authentication
   purposes.  Because of this, the server is required to indicate its
   intentions, including the password preprocessing it wishes to use,
   before it knows the true identity of the client.  This prevents the
   server from supporting multiple salt digests simultaneously in a
   single password database.  To support multiple salt digests
   simultaneously, it is necessary to maintain multiple password
   databases and use the routable portion of the client identity to
   select one when initiating EAP-pwd.

   The server uses the EAP-pwd-ID/Request to indicate the password
   preprocessing technique.  The client indicates its acceptance of the
   password preprocessing technique and identifies itself in the EAP-
   pwd-ID/Response.  If the client does not accept any of the offered
   preprocessing techniques, it SHALL terminate the exchange.  Upon
   receipt of the EAP-pwd-ID/Response, the server knows the identity of
   the client and can look up the client's salted password and the salt
   from the database.  The server adds the length of the salt and the
   salt itself to the EAP-pwd-Commit/Request message (see Section 2.7).

   The server can fix the password element (Section 2.8.3 of [RFC5931])
   as soon as the salted password has been looked up in the database.
   The client, though, is required to wait until receipt of the server's
   EAP-pwd-Commit/Request before it begins fixing the password element.

2.7.  Payload Modifications

   When a salted password preprocessing technique is agreed upon during
   the EAP-pwd-ID exchange, the EAP-pwd-Commit payload is modified to
   include the salt and salt length (see Figure 1).  The server passes
   the salt and salt length in the EAP-pwd-Commit/Request; the client's
   EAP-pwd-Commit/Response is unchanged, and it MUST NOT echo the salt
   length and salt in its EAP-pwd-Commit/Response.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Salt-len    |                                               |
      +-+-+-+-+-+-+-+-+                                               ~
      ~                            Salt             +-+-+-+-+-+-+-+-+-+
      |                                             |                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                 ~
      |                                                               |
      ~                           Element                             ~
      |                                                               |
      ~                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
      |                                                               |
      ~                            Scalar             +-+-+-+-+-+-+-+-+
      |                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 1: Salted EAP-pwd-Commit/Request

   The "salt-len" SHALL be non-zero, and it indicates the length, in
   octets, of the salt that follows.  The "Salt" SHALL be a binary
   string.  The "Element" and "Scalar" are encoded according to
   Section 3.3 of [RFC5931].

   Note: when a non-salted password preprocessing method is used, for
   example, any of the methods from [RFC5931], the EAP-pwd-Commit
   payload MUST NOT be modified to include the salt and salt length.

3.  IANA Considerations

   IANA has allocated fourteen (14) values from the "password
   preprocessing method registry" established by [RFC5931].

4.  Security Considerations

   EAP-pwd requires each side to produce an identical representation of
   the (processed) password before the password element can be fixed.
   This symmetry undercuts one of the benefits to salting a password
   database because the salted password from a compromised database can
   be used directly to impersonate the EAP-pwd client -- since the
   plaintext password need not be recovered, no dictionary attack is
   needed.  While the immediate effect of such a compromise would be
   compromise of the server, the per-user salt would still prevent the
   adversary from recovering the password, barring a successful
   dictionary attack, to use for other purposes.

   Salted password databases used with EAP-pwd MUST be afforded the same
   level of protection as databases of plaintext passwords.

   Hashing a password with a salt increases the work factor for an
   attacker to obtain the cleartext password, but dedicated hardware
   makes this increased work factor increasingly negligible in real-
   world scenarios.  Cleartext password databases SHOULD be protected
   with a scheme that uses a sequential memory hard function such as
   [RFC7914].

   EAP-pwd sends the salt in the clear.  If EAP-pwd is not tunneled in
   another, encrypting, EAP method, an adversary that can observe
   traffic from server to authenticator or from authenticator to client
   would learn the salt used for a particular user.  While knowledge of
   a salt by an adversary may be of a somewhat dubious nature (pre-image
   resistance of the hash function used will protect the client's
   password and, as noted above, the database of salted passwords must
   still be protected from disclosure), it does represent potential
   additional meta-data in the hands of a untrusted third party.

5.  References

5.1.  Normative References

   [CRY]      Linux Programmer's Manual, "CRYPT(3)", August 2015,
              <http://man7.org/linux/man-pages/man3/crypt.3.html>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              DOI 10.17487/RFC3454, December 2002,
              <http://www.rfc-editor.org/info/rfc3454>.

   [RFC5931]  Harkins, D. and G. Zorn, "Extensible Authentication
              Protocol (EAP) Authentication Using Only a Password",
              RFC 5931, DOI 10.17487/RFC5931, August 2010,
              <http://www.rfc-editor.org/info/rfc5931>.

   [RFC7613]  Saint-Andre, P. and A. Melnikov, "Preparation,
              Enforcement, and Comparison of Internationalized Strings
              Representing Usernames and Passwords", RFC 7613,
              DOI 10.17487/RFC7613, August 2015,
              <http://www.rfc-editor.org/info/rfc7613>.

   [RFC7914]  Percival, C. and S. Josefsson, "The scrypt Password-Based
              Key Derivation Function", RFC 7914, DOI 10.17487/RFC7914,
              August 2016, <http://www.rfc-editor.org/info/rfc7914>.

   [RFC8018]  Moriarty, K., Ed., Kaliski, B., and A. Rusch, "PKCS #5:
              Password-Based Cryptography Specification Version 2.1",
              RFC 8018, DOI 10.17487/RFC8018, January 2017,
              <http://www.rfc-editor.org/info/rfc8018>.

   [SHS]      National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015,
              <http://csrc.nist.gov/publications/fips/fips180-4/
              fips-180-4.pdf>.

5.2.  Informative References

   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616,
              DOI 10.17487/RFC7616, September 2015,
              <http://www.rfc-editor.org/info/rfc7616>.

   [RFC7664]  Harkins, D., Ed., "Dragonfly Key Exchange", RFC 7664,
              DOI 10.17487/RFC7664, November 2015,
              <http://www.rfc-editor.org/info/rfc7664>.

Acknowledgements

   Thanks to Stefan Winter and the eduroam project for its continued
   interest in using EAP-pwd.  Thanks to Simon Josefsson for his advice
   on support for scrypt and PBKDF2.

Author's Address

   Dan Harkins
   HP Enterprise
   3333 Scott Boulevard
   Santa Clara, CA  95054
   United States of America

   Email: dharkins@arubanetworks.com