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Network Working Group                                            V. Birk
Internet-Draft                                                H. Marques
Intended status: Standards Track                          pEp Foundation
Expires: September 12, 2019                                 B. Hoeneisen
                                                                 Ucom.ch
                                                          March 11, 2019


     IANA Registration of Trustword Lists: Guide, Template and IANA
                             Considerations
                      draft-birk-pep-trustwords-03

Abstract

   This document specifies the IANA Registration Guidelines for
   Trustwords, describes corresponding registration procedures, and
   provides a guideline for creating Trustword list specifications.

   Trustwords are common words in a natural language (e.g., English) to
   which the hexadecimal strings are mapped to.  This makes verification
   processes (e.g., comparison of fingerprints), more practical and less
   prone to misunderstandings.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 12, 2019.

Copyright Notice

   Copyright (c) 2019 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
   (https://trustee.ietf.org/license-info) in effect on the date of



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  The Concept of Trustword Mapping  . . . . . . . . . . . . . .   3
     3.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Previous work . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Number of Trustwords for a language . . . . . . . . . . .   4
     3.4.  Language  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.5.  The nature of the words . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Registration Template (XML chunk) . . . . . . . . . . . .   6
     4.2.  IANA Registration . . . . . . . . . . . . . . . . . . . .   7
       4.2.1.  Language Code (<languagecode>)  . . . . . . . . . . .   7
       4.2.2.  Bit Size (<bitsize>)  . . . . . . . . . . . . . . . .   7
       4.2.3.  Number Of Unique Words (<numberofuniquewords>)  . . .   7
       4.2.4.  Bijectivity (<bijective>) . . . . . . . . . . . . . .   8
       4.2.5.  Version (<version>) . . . . . . . . . . . . . . . . .   8
       4.2.6.  Registration Document(s) (<registrationdocs>) . . . .   8
       4.2.7.  Requesters (<requesters>) . . . . . . . . . . . . . .   8
       4.2.8.  Further Information (<additionalinfo>)  . . . . . . .   9
       4.2.9.  Wordlist (<wordlist>) . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  IANA XML Template Example  . . . . . . . . . . . . .  12
   Appendix B.  Document Changelog . . . . . . . . . . . . . . . . .  12
   Appendix C.  Open Issues  . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   In public-key cryptography comparing the public keys' fingerprints of
   the communication partners involved is vital to ensure that there is
   no man-in-the-middle (MITM) attack on the communication channel.
   Fingerprints normally consist of a chain of hexadecimal chars.
   However, comparing hexadecimal strings is often impractical for
   regular human users and prone to misunderstandings.




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   To mitigate these challenges, several systems offer the comparison of
   Trustwords as an alternative to hexadecimal strings.  Trustwords are
   common words in a natural language (e.g., English) to which the
   hexadecimal strings are mapped to.  This makes the verification
   process more natural for human users.

   For example, in pEp's proposition of Privacy by Default
   [I-D.birk-pep] Trustwords are used to achieve easy contact
   verification for end-to-end encryption.  Trustword comparison is
   offered after the peers have exchanged public keys opportunistically.
   Examples for Trustword lists used by current pEp implementations can
   be found in CSV format, here:
   https://pep.foundation/dev/repos/pEpEngine/file/tip/db.

   In addition to contact verification, Trustwords are also used for
   other purposes, such as Human-Readable 128-bit Keys [RFC1751], One
   Time Passwords (OTP) [RFC1760] [RFC2289], SSH host-key verification,
   VPN Server certificate verification, and to import or synchronize
   secret key across different devices of the same user
   [E-D.birk-pep-keysync].  Further ideas include to use Trustwords for
   contact verification in Extensible Messaging and Presence Protocol
   (XMPP) [RFC6120], for X.509 [RFC3647] certificate verification in
   browsers or in block chain applications for crypto currencies.

2.  Terms

   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].

   o  pEp Handshake: The process when Alice - e.g., in-person or via
      phone - contacts Bob to verify Trustwords (or by fallback:
      fingerprints) is called pEp Handshake.
      [I-D.marques-pep-handshake]

   o  Man-in-the-middle attack (MITM): cf. [RFC4949]

3.  The Concept of Trustword Mapping

3.1.  Example

   A fingerprint typically looks like:

      F482 E952 2F48 618B 01BC 31DC 5428 D7FA ACDC 3F13

   Its mapping to English Trustwords could look like:

      dog house brother town fat bath school banana kite task



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   Or its mapping to German Trustwords could like like:

      klima gelb lappen weg trinken alles kaputt rasen rucksack durch

   Instead of the former hexadecimal string, users can compare ten
   common words of their language.

   Note: This examples are for illustration purposes only and do not
   make use any any published Trustword list.

3.2.  Previous work

   The basic concept of Trustwords mapping has been already documented
   in the past, e.g. for use in One-Time Passwords (OTP) [RFC1751]
   [RFC1760] [RFC2289] or the PGP Word List ("Pretty Good Privacy word
   list" [PGP.wl], also called a biometric word list, to compare
   fingerprints.

   Regarding today's needs, previous proposals have the following
   shortcomings:

   o  Limited number of Trustwords (small Trustword dictionaries), which
      generally results in more Trustwords to be compared

   o  Usually only available in English language, which does not
      normally allow its usage by non-English speakers in a secure
      manner

   Furthermore, there are differences in the basic concept:

   o  This work allows for better tailoring the target audience to
      ordinary human users, i.e. not technical stuff (or IT geeks) only.

   o  As in many usage scenarios the Trustwords are only read (and
      compared), but not written down nor typed in by humans, there is a
      less strong need to keep the Trustwords themselves short.  One
      such scenario is to use a side channel (e.g. phone) to compare the
      Trustwords.  In fact longer Trustwords increases increase the
      entropy, as the dictionary is larger and the likelihood for
      phonetic collision can be decreased.

3.3.  Number of Trustwords for a language

   If the number of Trustwords is low, a lot of Trustwords need to be
   compared, which make a comparison somewhat cumbersome for users.
   This may lead to degraded usability.





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   To reduce the number of Trustwords to compare, in pEp's proposition
   of Privacy by Default [I-D.birk-pep] 16-bit scalars are mapped to
   natural language words.  Therefore, the size (by number of key -
   value pairs) of any key - value pair structure is 65536.  However,
   the number of unique values to be used in a language may be less than
   65536.  This can be addressed e.g. by using the same value
   (Trustword) for more than one key.  In these cases, the entropy of
   the representation is slightly reduced.  (Example: A Trustwords list
   of just 42000 words still allows for an entropy of log_2(42000) ~=
   15.36 bits in 16-bit mappings.)

   On the other hand, small sized Trustword lists allow for Trustwords
   with shorter strings, which are easier to use in scenarios where
   Trustwords have to be typed in e.g.  OTP applications.

   The specification allows for different dictionary sizes.

3.4.  Language

   Although English is rather widespread around the world, the vast
   majority of the worlds' population does not speak English.  For an
   application to be useful for ordinary people, localization is a must.
   Thus, Trustword lists in different languages can be registered.

   For applications where two human establish communication it is very
   likely that they share a common language.  So far no real use case
   for translations between Trustword lists in different languages has
   been identified.  As translations also drastically increases the
   complexity for IANA registrations, translations of Trustwords beyond
   the scope of this document.

3.5.  The nature of the words

   Every Trustwords list SHOULD be cleared from swearwords in order to
   not offense users.  This is a task to be carried out by speakers of
   the respective natural language (i.e., by native language speakers).

4.  IANA Considerations

   Each natural language requires a different set of Trustwords.  To
   allow implementers for identical Trustword lists, a IANA registry is
   to be established.  The IANA registration policy according to
   [RFC8126] is "Expert Review" and "Specification Required".

   [[ Note: Further details of the IANA registry and requirements for
   the expert to assess the specification are for further study.  A
   similar approach as used in [RFC6117] is likely followed. ]]




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4.1.  Registration Template (XML chunk)

     <record>
       <languagecode>
         <!--  ISO 639-3 (e.g. eng, deu, ...) -->
       </languagecode>
       <bitsize>
         <!-- How many bits can be mapped with this list
              (e.g. 8, 16, ...) -->
       </bitsize>
       <numberofuniquewords>
         <!-- number of unique words registered
              (e.g. 256, 65536, ...) -->
       </numberofuniquewords>
       <bijective>
         <!-- whether or not the list allows for a two-way-mapping
              (e.g. yes, no) -->
       </bijective>
       <version>
         <!-- version number within language
              (e.g. b.1.2, n.0.1, ...)  -->
       </version>
       <registrationdocs>
         <!-- Change accordingly -->
         <xref type="rfc" data="rfc2551"/>
       </registrationdocs>
       <requesters>
         <!-- Change accordingly -->
         <xref type="person" data="John_Doe"/>
         <xref type="person" data="Jane_Dale"/>
       </requesters>
       <additionalinfo>
         <paragraph>
           <!-- Text with additional information about
                the Wordlist to be registered -->
         </paragraph>
         <artwork>
           <!-- There can be artwork sections, too -->
         </artwork>
       </additionalinfo>
       <wordlist>
         <!-- Change accordingly -->
         <w0>first</w0>
         <w1>second</w1>
         [...]
         <w65535>last<w65535>
       </wordlist>
     </record>



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     <people>
       <person id="John_Doe">
         <name> <!-- Firstname Lastname --> </name>
         <org> <!-- Organization Name --> </org>
         <uri> <!-- mailto: or http: URI --> </uri>
         <updated> <!-- date format YYYY-MM-DD --> </updated>
       </person>
       <!-- repeat person section for each person -->
     </people>

   Authors of a Wordlist are encouraged to use these XML chunks as a
   template to create the IANA Registration Template.

4.2.  IANA Registration

   An IANA registration will contain the fallowing elements:

4.2.1.  Language Code (<languagecode>)

   The language code follows the ISO 639-3 specification [ISO693], e.g.,
   eng, deu.

   [[ Note: It is for further study, which of the ISO 639 Specifications
   is most suitable to address the Trustwords' challenge. ]]

   Example usage for German:

   e.g.  <languagecode>deu</languagecode>

4.2.2.  Bit Size (<bitsize>)

   The bit size is the number of bits that can be mapped with the
   Wordlist.  The number of registered words in a word list MUST be 2 ^
   "(<bitsize>)".

   Example usage for 16-bit Wordlist:

   e.g.  <bitsize>16</bitsize>

4.2.3.  Number Of Unique Words (<numberofuniquewords>)

   The number of unique words that are registered.

   e.g.  <numberofuniquewords>65536</numberofuniquewords>







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4.2.4.  Bijectivity (<bijective>)

   Whether the registered Wordlist has a one-to-one mapping, meaning the
   number of unique words registered equals 2 ^ "(<bitsize>)".

   Valid content: ( yes | no )

   e.g.  <bijective>yes</bijective>

4.2.5.  Version (<version>)

   The version of the Wordlist MUST be unique within a language code.

   [[ Note: Requirements to a "smart" composition of the version number
   are for further study ]]

   e.g.  <version>b.1.2</version>

4.2.6.  Registration Document(s) (<registrationdocs>)

   Reference(s) to the Document(s) containing the Wordlist

   e.g.  <registrationdocs>
           <xref type="rfc" data="rfc4979"/>
         </registrationdocs>

   e.g.  <registrationdocs>
           <xref type="rfc" data="rfc8888"/> (obsoleted by RFC 9999)
           <xref type="rfc" data="rfc9999"/>
         </registrationdocs>

   e.g.  <registrationdocs>
           [International Telecommunications Union,
           "Wordlist for Foobar application",
           ITU-F Recommendation B.193, Release 73, Mar 2009.]
         </registrationdocs>

4.2.7.  Requesters (<requesters>)

   The persons requesting the registration of the Wordlist.  Usually
   these are the authors of the Wordlist.










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   e.g.  <requesters>
           <xref type="person" data="John_Doe"/>
         </requesters>

         <people>
           <person id="John_Doe">
             <name>John Doe</name>
             <org>Example Inc.</org>
             <uri>mailto:john.doe@example.com</uri>
             <updated>2018-06-20</updated>
           </person>
         </people>

   Note: If there is more than one requester, there must be one <xref>
   element per requester in the <requesters> element, and one <person>
   chunk per requester in the <people> element.

4.2.8.  Further Information (<additionalinfo>)

   Any other information the authors deem interesting.

   e.g.  <additionalinfo>
           <paragraph>more info goes here</paragraph>
         </additionalinfo>

   Note: If there is no such additional information, then the
   <additionalinfo> element is omitted.

4.2.9.  Wordlist (<wordlist>)

   The full Wordlist to be registered.  The number of words MUST be a
   power of 2 as specified above.  The element names serve as key used
   for enumeration of the Trustwords (starting at 0) and the elements
   contains the values being individual natural language words in the
   respective language.

   e.g.  <wordlist>
           <w0>first</w0>
           <w1>second</w1>
           [...]
           <w65535>last<w65535>
         </wordlist>

   ] ]>

   [[ Note: The exact representation of the Wordlist is for further
   study.  ]]




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5.  Security Considerations

   There are no special security considerations.

6.  Acknowledgements

   The authors would like to thank the following people who have
   provided feedback or significant contributions to the development of
   this document: Andrew Sullivan, Claudio Luck, Daniel Kahn Gilmore,
   Michael Richardson, Rich Salz, and Yoav Nir.

   This work was initially created by pEp Foundation, and then reviewed
   and extended with funding by the Internet Society's Beyond the Net
   Programme on standardizing pEp.  [ISOC.bnet]

7.  References

7.1.  Normative References

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

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

7.2.  Informative References

   [E-D.birk-pep-keysync]
              Birk, V. and H. Marques, "pretty Easy privacy (pEp): Key
              Synchronization Protocol", June 2018,
              <https://pep.foundation/dev/repos/internet-
              drafts/file/tip/pep-keysync/
              draft-birk-pep-keysync-NN.txt>.

              Early draft

   [I-D.birk-pep]
              Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
              Privacy by Default", draft-birk-pep-03 (work in progress),
              March 2019.



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   [I-D.marques-pep-handshake]
              Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
              Contact and Channel Authentication through Handshake",
              draft-marques-pep-handshake-01 (work in progress), October
              2018.

   [ISO693]   "Language codes - ISO 639", n.d.,
              <https://www.iso.org/iso-639-language-codes.html>.

   [ISOC.bnet]
              Simao, I., "Beyond the Net. 12 Innovative Projects
              Selected for Beyond the Net Funding. Implementing Privacy
              via Mass Encryption: Standardizing pretty Easy privacy's
              protocols", June 2017, <https://www.internetsociety.org/
              blog/2017/06/12-innovative-projects-selected-for-beyond-
              the-net-funding/>.

   [PGP.wl]   "PGP word list", November 2017,
              <https://en.wikipedia.org/w/
              index.php?title=PGP_word_list&oldid=749481933>.

   [RFC1751]  McDonald, D., "A Convention for Human-Readable 128-bit
              Keys", RFC 1751, DOI 10.17487/RFC1751, December 1994,
              <https://www.rfc-editor.org/info/rfc1751>.

   [RFC1760]  Haller, N., "The S/KEY One-Time Password System",
              RFC 1760, DOI 10.17487/RFC1760, February 1995,
              <https://www.rfc-editor.org/info/rfc1760>.

   [RFC2289]  Haller, N., Metz, C., Nesser, P., and M. Straw, "A One-
              Time Password System", STD 61, RFC 2289,
              DOI 10.17487/RFC2289, February 1998,
              <https://www.rfc-editor.org/info/rfc2289>.

   [RFC3647]  Chokhani, S., Ford, W., Sabett, R., Merrill, C., and S.
              Wu, "Internet X.509 Public Key Infrastructure Certificate
              Policy and Certification Practices Framework", RFC 3647,
              DOI 10.17487/RFC3647, November 2003,
              <https://www.rfc-editor.org/info/rfc3647>.

   [RFC6117]  Hoeneisen, B., Mayrhofer, A., and J. Livingood, "IANA
              Registration of Enumservices: Guide, Template, and IANA
              Considerations", RFC 6117, DOI 10.17487/RFC6117, March
              2011, <https://www.rfc-editor.org/info/rfc6117>.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://www.rfc-editor.org/info/rfc6120>.



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Appendix A.  IANA XML Template Example

   This section contains a non-normative example of the IANA
   Registration Template XML chunk.

     <record>
       <languagecode>lat</languagecode>
       <bitsize>16</bitsize>
       <numberofuniquewords>57337</numberofuniquewords>
       <bijective>no</bijective>
       <version>n.0.1</version>
       <registrationdocs>
         <xref type="rfc" data="rfc2551"/>
       </registrationdocs>
       <requesters>
         <xref type="person" data="Julius_Caesar"/>
       </requesters>
       <additionalinfo>
         <paragraph>
           This Wordlist has been optimized for
           the Roman Standards Process.
         </paragraph>
       </additionalinfo>
       <wordlist>
         <w0>errare</w0>
         <w1>humanum</w1>
         [...]
         <w65535>est<w65535>
       </wordlist>
     </record>

     <people>
       <person id="Julius_Caesar">
         <name>Julius Caesar</name>
         <org>Curia Romana</org>
         <uri>mailto:julius.cesar@example.com</uri>
         <updated>1999-12-31</updated>
       </person>
     </people>

Appendix B.  Document Changelog

   [[ RFC Editor: This section is to be removed before publication ]]

   o  draft-birk-pep-trustwords-02:

      *  Minor editorial changes and bug fixes




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      *  Added more items to Open Issues

      *  Add usage example

   o  draft-birk-pep-trustwords-01:

      *  Included feedback from mailing list and IETF-101 SECDISPATCH
         WG, e.g.

         +  Added more explanatory text / less focused on the main use
            case

         +  Bit size as parameter

      *  Explicitly stated translations are out-of-scope for this
         document

      *  Added draft IANA XML Registration template, considerations,
         explanation and examples

      *  Added Changelog to Appendix

      *  Added Open Issue section to Appendix

Appendix C.  Open Issues

   [[ RFC Editor: This section should be empty and is to be removed
   before publication ]]

   o  More explanatory text for Trustword use cases, properties and
      requirements

   o  Further details of the IANA registry and requirements for the
      expert to assess the specification

   o  Decide which ISO language code either 639-1 or 639-3 to use, i.e.,
      ISO-639-1 (e.g., ca, de, en, ...) as currently used in pEp
      implementations (running code) or ISO-693-3 (eng, deu, ita, ...)

   o  Adjust exact representation of wordlists

      *  e.g.  XML, CSV, ...

      *  Syntax for non-ASCII letters or language symbols (UTF-8) in
         Wordlists

   o  Need for optional entropy value assigned to words, to account for
      similar phonetics among words in the same wordlist?



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   o  Need for an additional field, to define what a wordlist is
      optimized for, e.g., "entropy", "minimize word lengths", ...?

   o  Work out (requirements for) "smart" composition of the version
      number

   o  Decide whether in non-bijective Wordlists the redundant words need
      to be repeated in the IANA Registration

   o  Register only a hash over the wordlist with IANA?

   o  Does it make sense to open registrations for other patterns than
      just words, e.g., images?

   o  Create terms section by file inclusion - cf. other drafts

Authors' Addresses

   Volker Birk
   pEp Foundation
   Oberer Graben 4
   CH-8400 Winterthur
   Switzerland

   Email: volker.birk@pep.foundation
   URI:   https://pep.foundation/


   Hernani Marques
   pEp Foundation
   Oberer Graben 4
   CH-8400 Winterthur
   Switzerland

   Email: hernani.marques@pep.foundation
   URI:   https://pep.foundation/


   Bernie Hoeneisen
   Ucom Standards Track Solutions GmbH
   CH-8046 Zuerich
   Switzerland

   Phone: +41 44 500 52 44
   Email: bernie@ietf.hoeneisen.ch (bernhard.hoeneisen AT ucom.ch)
   URI:   https://ucom.ch/





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