IANA Registration of Trustword Lists: Guide, Template and IANA Considerations
draft-birk-pep-trustwords-04

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Network Working Group                                       B. Hoeneisen
Internet-Draft                                                   Ucom.ch
Intended status: Standards Track                              H. Marques
Expires: January 9, 2020                                  pEp Foundation
                                                           July 08, 2019

     IANA Registration of Trustword Lists: Guide, Template and IANA
                             Considerations
                      draft-birk-pep-trustwords-04

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),
   which hexadecimal strings are mapped to.  Such a mapping makes
   verification processes like fingerprint comparisons 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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 9, 2020.

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
   publication of this document.  Please review these documents

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   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
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  The Concept of Trustword Mapping  . . . . . . . . . . . . . .   4
     2.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Previous work . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Number of Trustwords for a language . . . . . . . . . . .   5
     2.4.  Language  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.5.  The nature of the words . . . . . . . . . . . . . . . . .   6
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Registration Template (XML chunk) . . . . . . . . . . . .   6
     5.2.  IANA Registration . . . . . . . . . . . . . . . . . . . .   8
       5.2.1.  Language Code (<languagecode>)  . . . . . . . . . . .   8
       5.2.2.  Bit Size (<bitsize>)  . . . . . . . . . . . . . . . .   8
       5.2.3.  Number Of Unique Words (<numberofuniquewords>)  . . .   8
       5.2.4.  Bijectivity (<bijective>) . . . . . . . . . . . . . .   8
       5.2.5.  Version (<version>) . . . . . . . . . . . . . . . . .   8
       5.2.6.  Registration Document(s) (<registrationdocs>) . . . .   9
       5.2.7.  Requesters (<requesters>) . . . . . . . . . . . . . .   9
       5.2.8.  Further Information (<additionalinfo>)  . . . . . . .   9
       5.2.9.  Wordlist (<wordlist>) . . . . . . . . . . . . . . . .  10
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  IANA XML Template Example  . . . . . . . . . . . . .  12
   Appendix B.  Document Changelog . . . . . . . . . . . . . . . . .  13
   Appendix C.  Open Issues  . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   In public-key cryptography, comparing the respective public key
   fingerprints for each of the communication partners involved is vital
   to ensure that there is no Man-in-the-Middle (MITM) attack on the
   communication channel.  These fingerprints normally consist of a
   chain of hexadecimal characters, which are often impractical,
   cumbersome, and prone to misunderstandings for end-users.

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   To mitigate these challenges, several systems offer Trustword
   comparison as an alternative to these hexadecimal strings.
   Trustwords are common words in a natural language (e.g., English),
   which these hexadecimal strings are mapped to.  Using Trustwords
   makes verification processes like fingerprint comparisons more
   natural for users.

   For example, in pEp's Privacy by Default proposition [I-D.birk-pep]
   Trustwords are used to facilitate easy contact verification for end-
   to-end encryption.  Trustword comparison is offered after the peers
   have opportunistically exchanged public keys.  Examples of Trustword
   lists used by current pEp implementations can be found here in CSV
   format: 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, deriving private keys in
   blockchain applications for cryptocurrencies, and to import or
   synchronize secret keys across multiple devices owned by a single
   user [I-D.hoeneisen-pep-keysync].  Further ideas include the use of
   Trustwords for private key recovery in case of loss, contact
   verification in Extensible Messaging and Presence Protocol (XMPP)
   [RFC6120], or for X.509 certificate verification in browsers
   [RFC3647].

1.1.  Requirements Language

   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].

1.2.  Terms

   The following terms are defined for the scope of this document:

   o  pEp Handshake: The process of one user contacting another over an
      independent channel in order to verify Trustwords (or by fallback:
      fingerprints).  This can be done in-person or through established
      verbal communication channels, like a phone call.
      [I-D.marques-pep-handshake]

   o  Man-in-the-middle (MITM) attack: cf. [RFC4949], which states: "A
      form of active wiretapping attack in which the attacker intercepts
      and selectively modifies communicated data to masquerade as one or
      more of the entities involved in a communication association."

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2.  The Concept of Trustword Mapping

2.1.  Example

   As already discussed, fingerprints normally consist of a string of
   hexadecimal characters.  A typical fingerprint looks like this:

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

   Instead of the hexadecimal string, Trustwords allow users to compare
   ten common words of a language of their choosing.  For example, the
   above fingerprint, mapped to English Trustwords, might appear as:

      dog house brother town fat bath school banana kite task

   The same fingerprint might appear in German Trustwords as:

      klima gelb lappen weg trinken alles kaputt rasen rucksack durch

   Note: These examples are for illustration purposes only, and are not
   derived from any published Trustword list.

2.2.  Previous work

   The basic concept of Trustword mapping - also known as a biometric
   word list - for fingerprint comparison is well-documented.  Examples
   of this concept are used with One-Time Passwords (OTP) [RFC1751]
   [RFC1760] [RFC2289], as well as the PGP Word List ("Pretty Good
   Privacy word list" [PGP.wl].  Furthermore, cryptocurrencies use a
   similar concept for deriving private keys [bitcoin.wl].

   [[ TODO: Explain each previous usage a bit further and synchronize
   with section Section 1. ]]

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

   o  Small/limited word lists, which generally result in more words to
      compare

   o  Existing word lists are usually only available in English, which
      limits their usefulness for non-English speakers

   Furthermore, there are differences in the basic concept:

   o  The Trustword concept suggested herein intends to improve
      usability and security for all users, instead of only the
      technically-savvy.

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   o  In many use cases, Trustwords are only read (aloud) during the
      comparison process, rather than being written or typed.  For
      example, two users might compare their respective Trustwords
      during a phone call.  Verbal comparison reduces the need to keep
      the actual Trustwords short.  The use of longer Trustwords
      increases the entropy within the system, as it allows for a larger
      dictionary, and thus reduces the likelihood of phonetic
      collisions.

2.3.  Number of Trustwords for a language

   If the number of Trustwords in a dictionary is low, shorter parts of
   the original string (e.g., fingerprint) can be mapped to a single
   Trustword.  Thus, many Trustwords will need to be compared, which
   results in a potentially cumbersome process for users, and lead to
   reduced usability.

   To reduce the number of Trustwords that need to be compared, pEp's
   Privacy by Default proposition [I-D.birk-pep] calls for 16-bit
   scalars to be 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 smaller than this number.  This discrepancy can be addressed
   by using the same value, or Trustword, for more than one key.  In
   such cases, the entropy of the representation is slightly reduced.
   For example, a Trustword list of 42000 words still allows for an
   entropy of log_2(42000), which is roughly 15.36 bits in 16-bit
   mappings.  As a consequence such Trustword lists are not bijective.

   On the other hand, small Trustword lists allow for Trustwords
   consisting of words with shorter strings (number of short words per
   natural language is normally limited), which are easier to use in
   implementations where Trustwords have to be typed or written, such as
   in OTP applications.

   Note: This specification allows for registration of variable numbers
   of Trustwords per dictionary.

2.4.  Language

   Although English is used around the world, the vast majority of the
   global population is not English-speaking.  For an application to be
   useful to as wide of a user base as possible, localization is
   essential.  Therefore, this specification allows for registration of
   Trustword lists in different languages.

   In applications where two humans are attempting to establish secure
   communications, it is likely that they share a common language.  At

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   this time, no real-world use cases for Trustword list translation
   capability have been identified.  Because the translation process
   inherently - and drastically - increases complexity from an IANA
   registration standpoint, the topic of Trustword translation is beyond
   the scope of this document.

2.5.  The nature of the words

   Every Trustword list SHOULD be clear of offensive language (i.e.,
   swear/curse words, slurs, derogatory language, etc.).  This process
   SHOULD be performed by native speakers of each respective language.

3.  Security Considerations

   There are no specific security considerations.

4.  Privacy Considerations

   [[ TODO ]]

5.  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. ]]

5.1.  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) -->

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       </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>

     <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.

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5.2.  IANA Registration

   An IANA registration will contain the fallowing elements:

5.2.1.  Language Code (<languagecode>)

   The language code follows the ISO 639-3 specification [ISO639], 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>

5.2.2.  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>

5.2.3.  Number Of Unique Words (<numberofuniquewords>)

   The number of unique words that are registered.

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

5.2.4.  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>

5.2.5.  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 ]]

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

5.2.6.  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>

5.2.7.  Requesters (<requesters>)

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

   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.

5.2.8.  Further Information (<additionalinfo>)

   Any other information the authors deem interesting.

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

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

5.2.9.  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.  ]]

6.  Acknowledgments

   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,
   Kelly Bristol, Michael Richardson, Rich Salz, Volker Birk, 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>.

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   [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

   [bitcoin.wl]
              "Seed Phrase", June 2019, <https://en.bitcoin.it/w/
              index.php?title=Seed_phrase&oldid=66492#Word_Lists>.

   [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.

   [I-D.hoeneisen-pep-keysync]
              Hoeneisen, B. and H. Marques, "pretty Easy privacy (pEp):
              Key Synchronization Protocol", draft-hoeneisen-pep-
              keysync-00 (work in progress), July 2019.

   [I-D.marques-pep-handshake]
              Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
              Contact and Channel Authentication through Handshake",
              draft-marques-pep-handshake-03 (work in progress), July
              2019.

   [ISO639]   "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>.

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   [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>.

Appendix A.  IANA XML Template Example

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

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     <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-04:

      *  Add Privacy Considerations section

      *  Swapped Security and IANA Consideration Sections

      *  Corrected typo in ISO references

      *  Updated Introduction, Terms and concept Sections

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   o  draft-birk-pep-trustwords-03:

      *  Update references

      *  Minor edits

   o  draft-birk-pep-trustwords-02:

      *  Minor editorial changes and bug fixes

      *  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  Better explain previous work on Trustwords

   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

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   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-639-3 (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?

   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?

Authors' Addresses

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

   Phone: +41 44 500 52 40
   Email: bernie@ietf.hoeneisen.ch (bernhard.hoeneisen AT ucom.ch)
   URI:   https://ucom.ch/

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

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

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