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Versions: 00 01                                                         
Network Working Group                                        S. Bellovin
Internet-Draft                                       Columbia University
Intended status: Standards Track                          March 11, 2012
Expires: September 12, 2012

                        Hashed Password Exchange


   Many systems (e.g., cryptographic protocols relying on symmetric
   cryptography) require that plaintext passwords be stored.  Given how
   often people reuse passwords on different systems, this poses a very
   serious risk if a single machine is compromised.  We propose a scheme
   to derive passwords limited to a single machine from a typed
   password, and explain how a protocol definition can specify this

Status of This Memo

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   This Internet-Draft will expire on September 12, 2012.

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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   Today, despite the lessons of more than 30 years [[cite Morris and
   Thomson]], many systems store plaintext passwords.  This is often
   done for good reasons, such as authenticating some cryptographic
   exchanges or as a convenience to users with many passwords; see, for
   example, the password store in many browsers or the Keychain in
   MacOS.  That said, this practice does pose a security risk to users,
   since their passwords are in danger if the system is compromised.

   The big problem is not compromise of the actual password used on that
   system; while regrettable, it is inherent in the service definition.
   Rather, the problem is that users tend to reuse passwords on
   different systems.  If a password is compromised on one machine, the
   user is at risk on many different systems.  Accordingly, we describe
   a scheme for storing a single-site-only password, derived from the
   user's typed password; a compromise of a service thus affects just
   that service.

   To accomplish this, we specify a "Hashed Password Exchange" standard,
   or rather, a metastandard.  Rather than specifying a precise way to
   store and use hashed passwords, we give rules for specifying hashed
   passwords for use in a given protocol or application.  We take
   advantage of the fact that unlike 1979, when users used very dumb
   terminals to transmit passwords directly to the receiving
   applications, most passwords these days are entered into user-
   controlled software; these programs in turn transmit the passwords to
   the verifying applications.  There is thus intelligence on the user's
   side; we will use this to irreversibly transform the entered password
   into some other string.  By the same token, the receiving system must
   apply the same transform to the authenticator supplied at user
   enrollment time or password change time.  Because two independent
   pieces of software must apply the same transformation, the algorithm
   must be precisely specified in standards documents.

   Note that defeating guessing attacks on a captured password file is
   not the primary goal of this work.  That goal, though laudable,
   ignores changes in technology and environment since the Morris and
   Thompson paper; today, far more passwords are lost to keystroke
   loggers, phishing attacks, direct compromise of the server itself, or
   (as was a problem even 30+ years ago) online guessing attacks.  Our
   scheme helps against this last attack, in that generation of the
   guesses becomes more expensive; against the other threats, password
   strength is completely irrelevant.  We also note that today, people
   have very many different passwords.  It is impossible to remember

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   large numbers of strong passwords; absent use of a password generator
   and manager, there *will* be reuse across different services.

1.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Definitions and Goals

   We use the following definitions:

   Username  An arbitrary string, the syntax of which is application-
      dependent, employed by both the user and the verifying system to
      uniquely identify a given user.

   Entered Password  The authenticator typed by the user to his or her
      own software.  The usual quality rules (length, special
      characters, etc.) can be applied; that is out of the scope of this

   Effective Password  The actual, over-the-wire, string transmitted by
      the user's software.

   Service  A particular application on a particular machine or cluster
      of machines appearing as a single machine

   Hostname  The hostname as supplied by the user.

   Service URI  A URI [RFC3986] for which this effective password should
      be valid.  Only the scheme name, userinfo, and host name portions
      are discussed here; use of path information is protocol-dependent.
      In the userinfo field, only the username is used.  An example is
      given below.

   Our scheme has the following goals:

   1.  No two users of a given service should have the same effecive
       password, even if the entered passwords are the same.

   2.  No two effective passwords for the same user should be the same
       for different services, even if the entered passwords are the

   3.  It should be infeasible to invert the hashing function to
       retrieve the entered password from an effective password and
       service URI.

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   4.  It should be computationally expensive to mount dictionary
       attacks on compromised effective passwords.

3.  The Hashed Password Scheme

   Fundamentally, we calculate the effective password by iterating HMAC
   [RFC2104], using the entered password as the key and the service URI
   as the data.  This meets all four of our goals:

   1.  Since the username is part of the service URI, different users
       will have different URIs, and hence different effective

   2.  Since the hostname is part of the URI, different services for any
       given user will have different URIs, and hence different
       effective passwords.

   3.  For any reasonable underlying hash function, it is believed to be
       infeasible to invert HMAC; see [RFC2104] for details.  (Arguably,
       HMAC is overkill.  Nevertheless, it is a well-studied, well-
       understood mechanism for combining known plaintext with a secret
       key.  We see little benefit to concocting some other scheme.)

   4.  By iterating a sufficient number of times, dictionary attacks can
       be made arbitrarily expensive.  (Although guessing attacks can be
       made arbitrarily cheap today by use of cloud services or botnets,
       we prefer to look at it somewhat differently.  Whatever the
       resources the attacker has, his or her effective guessing rate is
       cut by a factor of the iteration count.)

   We do not use a salt in this scheme.  The primary purposes of a salt
   are to achieve our first and second goals, which we achieve in other
   ways.  A salt also protects against precomputation of possible
   passwords of known users in anticipation of a later password file
   compromise.  Our use of service-, host-, and user-specific hashed
   passwords provides the same protection against untargeted guessing
   attacks; furthermore, and as noted, guessing attacks are not the
   primary threat today.  Since the salt must be used in calculating the
   effective password, it would have to be known to the user as well as
   the server, and users typically have multiple devices on which they
   enter passwords.  Using a salt would require that users know it and
   reenter it, which we regard as of limited benefit and highly user-
   hostile: people will *not* tolerate copying random strings or numbers
   onto multiple platforms, especially phones and the like.

   Usernames and the hostname portions of service URIs must be
   canonicalized before applying HMAC.  Legal characters in a username
   are upper and lower case US-ASCII letters, period, hyphen,

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   underscore, and digits.  All other characters MUST be percent-
   encoded, per section 2.1 of [RFC3986].  Hostnames MUST be
   canonicalized per [RFC5890][RFC5891] and converted to lower case.
   How usernames and hostnames are entered is application- and
   implementation-dependent, and not part of this specification.  The
   hostname used is either the string users type or unambiguously
   derivable from it per specified rules.

   The URI scheme name is given by the protocol specification and MUST
   NOT be entered directly by the user.

   The iteration count is protocol- and use-dependent, and given in the
   protocol specification.

   The effective password, then, is calculated by iterating HMAC some
   number of times over the message


   with the entered password as the key.

3.1.  Examples


   Note that although someuser can specify the same entered password for
   both 'imap' and 'submission' on mail.example.com, the effective
   passwords will be different.

4.  Specifying Hashed Password Exchange

   The following elements must be in any protocol specification that
   uses Hashed Password Exchange.

   o  The scheme name MUST be specified.  Generally, this will be taken
      from the IANA name assigned to the port, but this is not required.
      Thus, a mail submission URI (TCP port 587) might use the scheme
      name "submission".

   o  The rules for deriving the hostname from what users enter MUST be
      specified.  They may be as simple as "use the name the user
      specifies, e.g., imap.example.com", or they may account for common
      alternatives: "If the specified host name does not begin with
      'www.', prepend it; thus, both 'example.com' and 'www.example.com'
      would use the hostname 'www.example.com' in forming the URI.

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   o  The iteration count MUST be specified.  The value -- typically in
      the hundreds of thousands with today's technology -- SHOULD be
      different for different services, and MAY be adjusted based on the
      platforms on which the calculations are typically done.  Note that
      the iteration is done at password change time rather than run-
      time, so expense is not a major concern.  (Just how long the
      iterations should take will depend on the protocol designers'
      understanding of likely platforms and usage patterns.  Something
      that will be run exclusively on fast devices and with stored
      hashed passwords should use a higher count; something where run-
      time user password entry on a slow device is considered likely
      should use a lower count.)

   o  To support internationalized, non-ASCII passwords, we adopt the
      specification text from [RFC6124].  The input password string
      SHOULD be processed according to the rules of the [RFC4103]
      profile of [RFC3454] A password SHOULD be considered a "stored
      string" per [RFC3454] and unassigned code points are therefore
      prohibited.  The output is the binary representation of the
      processed UTF-8 [RFC3629] character string.  Prohibited output and
      unassigned code points encountered in SASLprep preprocessing
      SHOULD cause a preprocessing failure and the output SHOULD NOT be

   o  The hash function to be used with HMAC MUST be specified.  MD5
      [RFC1321] is more than sufficient; however, the tradeoff is likely
      to be between what code is likely to be available in
      implenetations versus the iteration count.  SHA-512 [RFC6234] is
      much slower than MD5, but since the goal is constant time, this
      matters very little; thus, MD5 would have a higher iteration count
      than SHA-512 would for the same protocol.

   o  The encoding rules for sending the effective password over the
      wire are not crucial but must be specified.  The output of HMAC is
      an arbitrary byte string.  Given the length of typical HMAC output
      and the infrequency with which they are sent, transmission
      efficiency is not a major concern, so a simple hexadecimal
      encoding is fine.  Implementations MAY specify truncation;
      however, they SHOULD NOT use effective passwords shorter than 16
      octets before encoding.

   o  If the password is not transmitted but is used internally (e.g.,
      as part of a cryptopgrahic exchange), how the effective password
      is used MUST be specified.  Some protocols will use it directly as
      a key; others will use the hexadecimal ASCII string in place of a

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   o  Some protocols, such as HTTP, permit multiple hosts to appear on a
      single IP address.  For such protocols, the desired hostname must
      be transmitted prior to or along with the hashed password, to
      allow the host to calculate the proper hashed password value.  How
      this is done MUST be specified.

   o  If the protocol permits negotiation of authentication methods, a
      separate code point MUST be assigned to this scheme.

   How passwords are changed -- that is, how new effective passwords are
   supplied to the verifying machine -- is beyond the scope of this
   specification.  If the entered password is sent directly at password
   change time, quality checks can be enforced; however, this exposes
   entered passwords to attacks who have compromised the verifying
   machine.  This is not a major risk, since the rate of password change
   is low.  Conversely, client-side code (e.g., Javascript) can make
   advisory recommendations on password strength; while the server
   cannot enforce this, since it will see only effective passwords, very
   few users will have the will and the skill to override this.

   If effective passwords are used only for the usual password
   verification and not for cryptographic purposes, they should be
   treated with the care used for ordinary password, i.e., read-
   protected, hashed, etc.  There is little need for extra iterations,
   though, since the iteration used in calculating them already provides
   strong protection against dictionary attacks, and it is unlikely that
   the extra server-side iterations will be significantly larger than
   the iterations already performed to comply with this specification.
   As before, there is no need for an additional salt.

5.  Related Work

   A number of papers have described schemes for browser-based password
   stores that simplify the process of having separate effective
   passwords for different web sites.  Many -- [[Abadi--pwdhash]]
   [[Halderman et al.]] -- use a cryptographic function of the domain
   name and a master password to calculate it.  [[Abadi-pwdhash]] has
   many pointers.

   This work differs in two important ways.  First, it applies to more
   services than just HTTP.  Second, it specifies how other protocol
   specification documents should handle the situation, independent of
   requirements for password strength.

6.  Acknowledgments

   A number of people made useful comments and suggestions, even if they
   didn't agree with all parts of this document.  They include Martin

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   Abadi, Uri Blumenthal, Dan Harkins, Mouse, Yaron Sheffer, Joe Touch,
   and Sujing Zhou.

7.  Security Considerations

   To be written.

8.  Normative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, June 2005.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

   [RFC6124]  Sheffer, Y., Zorn, G., Tschofenig, H., and S. Fluhrer, "An
              EAP Authentication Method Based on the Encrypted Key
              Exchange (EKE) Protocol", RFC 6124, February 2011.

   [RFC6234]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.

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Appendix A.  Change History

A.1.  Changes from -00 to -01

      Added more text explaining why salting isn't particularly helpful

      Add the requirement to transmit the hostname for some services

      Started a related work section

      Clarified the internationalization requirement

      Miscellaneous edits

Appendix B.  Open Issues

      How should related domains (e.g., www.amazon.com and
      www.amazon.co.uk) be handled, if the site wishes the same password
      to work on all of them.

      A particular case in point is the way the prefix "www." should be
      handled.  Should there be a general rule about the service name
      appearing in the hostname?

Author's Address

   S.M. Bellovin
   Columbia University
   1214 Amsterdam Avenue
   MC 0401
   New York, NY  10027

   Phone: +1 212 939 7149
   EMail: bellovin@acm.org

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