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Versions: (draft-ietf-usefor-cancel-lock) 00 01 02 03 04 05 06

Independent Submission                                       M. Baeuerle
Internet-Draft                                            STZ Elektronik
Updates: 5537 (if approved)                           September 12, 2017
Intended status: Standards Track
Expires: March 16, 2018


                    Cancel-Locks in Netnews articles
                 draft-baeuerle-netnews-cancel-lock-06

Abstract

   This document defines an extension to the Netnews Article Format that
   may be used to authenticate the cancelling and superseding of
   existing articles.  If approved, this document updates RFC5537.

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 March 16, 2018.

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
   (https://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.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   3
     1.2.  Author's Note . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Header Fields . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Cancel-Lock . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Cancel-Key  . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Adding an initial Cancel-Lock header field to a proto-
           article . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Extending the Cancel-Lock header field of a proto-article   6
     3.3.  Adding a Cancel-Key header field to a proto-article . . .   6
     3.4.  Extending the Cancel-Key header field of a proto-article    7
     3.5.  Check a Cancel-Key header field . . . . . . . . . . . . .   7
   4.  Calculating the key data  . . . . . . . . . . . . . . . . . .   8
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Without UID . . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  With UID  . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.3.  Other examples  . . . . . . . . . . . . . . . . . . . . .  11
     5.4.  Manual checks . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Obsolete Syntax . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Algorithm Name Registration Procedure . . . . . . . . . .  15
     8.2.  Change control  . . . . . . . . . . . . . . . . . . . . .  16
     8.3.  Registration of the Netnews Cancel-Lock hash algorithms .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  20
   Appendix B.  Document History (to be removed by RFC Editor before
                publication) . . . . . . . . . . . . . . . . . . . .  20
     B.1.  Changes since -05 . . . . . . . . . . . . . . . . . . . .  20
     B.2.  Changes since -04 . . . . . . . . . . . . . . . . . . . .  21
     B.3.  Changes since -03 . . . . . . . . . . . . . . . . . . . .  22
     B.4.  Changes since -02 . . . . . . . . . . . . . . . . . . . .  22
     B.5.  Changes since -01 . . . . . . . . . . . . . . . . . . . .  23
     B.6.  Changes since -00 . . . . . . . . . . . . . . . . . . . .  24
     B.7.  Changes since draft-ietf-usefor-cancel-lock-01  . . . . .  25
     B.8.  Changes since draft-ietf-usefor-cancel-lock-00  . . . . .  25
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   The authentication system defined in this document is intended to be
   used as a simple method to verify that the withdrawal of an article
   is valid, that is to say either the poster, posting agent, moderator



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   or injecting agent that processed the original article has required
   to withdraw it via the use of a cancel control article ([RFC5537]
   Section 5.3) or a Supersedes header field ([RFC5537] Section 5.4).

   One property of this system is that it prevents tracking of
   individual users.

   There are other authentication systems available with different
   properties.  When everybody should be able to verify who the
   originator is, e.g. for control articles to add or remove newsgroups
   ([RFC5537] Section 5.2), an OpenPGP [RFC4880] signature is suited.

1.1.  Conventions Used in This Document

   Any term not defined in this document has the same meaning as it does
   in [RFC5536] or [RFC5537].

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

1.2.  Author's Note

   Please write the letters "ae" in "Baeuerle" as an a-umlaut (U+00E4,
   "ä" in XML), the first letter in "Elie" with an acute accent
   (U+00C9, "É" in XML), the letters "ss" in Janssen as an eszett
   (U+00DF, "ß" in XML) and the letters "ue" in Baden-Wuerttemberg
   as an u-umlaut (U+00FC, "ü" in XML) wherever this is possible.

2.  Header Fields

   This section describes the formal syntax of the new header fields
   using ABNF [RFC5234].  It extends the syntax in Section 3 of
   [RFC5536] and non-terminals not defined in this document are defined
   there.  The [RFC5536] ABNF should be imported first before attempting
   to validate these rules.

   The new header fields Cancel-Lock and Cancel-Key are defined by this
   document, they follow the rules described in [RFC5536] Section 2.2:

      fields =/ *( cancel-lock / cancel-key )

   Each of these header fields MUST NOT occur more than once in an
   article.

   Both new header field bodies contain lists of encoded values.  Every
   entry is based on a <scheme>:



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      scheme       = "sha256" / "sha512" / 1*scheme-char / obs-scheme
      scheme-char  = ALPHA / DIGIT / "-" / "/"

   The hash algorithms for <scheme> are defined in [RFC6234], see also
   [RFC1321] and [RFC6151] for MD5, [RFC3174] for SHA1 and [SHA] for the
   SHA2 family.  The Base64 encoding used is defined in Section 4 of
   [RFC4648].

   This document defines two values for <scheme>: "sha256" and "sha512".
   The hash algorithm "sha256" is mandatory to implement.

   Because the hash algorithm for <scheme> cannot be negotiated,
   unnecessary proliferation of hash algorithms should be avoided.  The
   hash algorithms "sha224" and "sha384" are only added to the Netnews
   Cancel-Lock hash algorithm registry (Section 8.3) because
   implementations exist that supports them.  Implementations SHOULD NOT
   use the hash algorithms "sha224" and "sha384" to generate <scheme>.

2.1.  Cancel-Lock

      cancel-lock     = "Cancel-Lock:" SP c-lock-list CRLF
      c-lock-list     = [CFWS] c-lock *(CFWS c-lock) [CFWS]
      c-lock          = scheme ":" c-lock-string
      c-lock-string   = *(4base64-char) [base64-terminal]
      base64-char     = ALPHA / DIGIT / "+" / "/"
      base64-terminal = 2base64-char "==" / 3base64-char "="

   Comments in CFWS can cause interoperability problems, so comments
   SHOULD NOT be generated but MUST be accepted.

   If <scheme> is not supported by an implementation, the corresponding
   <c-lock> element MUST be skipped and potential following <c-lock>
   elements MUST NOT be ignored.

   <c-lock-string> is the Base64 encoded output of a hash operation
   (defined by <scheme>) of the Base64 encoded key "K" that is intended
   to authenticate the person or agent that created or processed
   respectively the proto-article up to injection (inclusively):

      Base64(hash(Base64(K)))

   Because of the one-way nature of the hash operation the key "K" is
   not revealed.

   If multiple <c-lock> elements are present in a <c-lock-list>, each
   <c-lock> element MUST use a unique key K.





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2.2.  Cancel-Key

      cancel-key   = "Cancel-Key:" SP c-key-list CRLF
      c-key-list   = [CFWS] c-key *(CFWS c-lock) [CFWS]
      c-key        = scheme ":" c-key-string
      c-key-string = c-lock-string / obs-c-key-string

   Comments in CFWS can cause interoperability problems, so comments
   SHOULD NOT be generated but MUST be accepted.

   If <scheme> is not supported by an implementation, the corresponding
   <c-key> element MUST be skipped and potential following <c-key>
   elements MUST NOT be ignored.

   <c-key-string> is the Base64 encoded key "K" that was used to create
   the <c-lock> element in the Cancel-Lock header field body (as defined
   in Section 2.1 of this document) of the original article:

      Base64(K)

   The relaxed syntax definition of <c-key-string> above is required for
   backward compatibility with implementations that are not compliant
   with this specification.  Compliant implementations SHOULD generate
   valid Base64 (that is to say the syntax of <c-lock-string> as defined
   in Section 2.1 of this document) and MUST accept strings of
   <base64-octet> characters (that is to say the syntax of <obs-c-key-
   string> as defined in Section 6 of this document).

3.  Use

   The Cancel-Lock header field contains hashes of secret strings.  This
   secret strings can later be used to authenticate a cancel or
   supersede request.

   Use cases:

   o  The poster of an article wants to cancel or supersede existing
      articles.

   o  A moderator wants the ability to cancel articles after approving
      them.

   o  An injecting agent acts representitive for posting agents without
      support for the autentication system described in this document.

   o  A news administrator wants the ability to cancel articles that
      were injected by its system (because they e.g. violate its abuse
      policy).



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3.1.  Adding an initial Cancel-Lock header field to a proto-article

   A Cancel-Lock header field MAY be added to a proto-article by the
   poster or posting agent which will include one or more <c-lock>
   elements.

   If the poster or posting agent doesn't add a Cancel-Lock header field
   to a proto-article, then an injecting agent (or moderator) MAY add
   one, including one or more <c-lock> elements.

   If an injecting agent (or moderator) wants to act representitive for
   a posting agent without support for the authentication system
   described in this document, then it MUST be able to positively
   authenticate the poster and it MUST be able to automatically add a
   working Cancel-Key header field for all proto-articles with
   cancelling or superseding attempts from that poster.

   Other agents MUST NOT add this header field to articles or proto-
   articles that they process.

3.2.  Extending the Cancel-Lock header field of a proto-article

   If a Cancel-Lock header field has already been added to a proto-
   article then any agent further processing the proto-article up to the
   injecting agent (inclusively) MAY append additional <c-lock> elements
   to those already in the header field body.

   If an injecting agent (or moderator) wants to act representitive for
   a posting agent without support for the authentication system
   described in this document, then the same requirements apply as
   mentioned in Section 3.1.

   Once an article is injected then this header field MUST NOT be
   altered.  In particular, relaying agents beyond the injecting agent
   MUST NOT alter it.

3.3.  Adding a Cancel-Key header field to a proto-article

   The Cancel-Key header field contains one or more of the secret
   strings that were used to create the Cancel-Lock header field of the
   original article.  Knowledge of at least one of the secret strings is
   required to create a match for successful authentication.

   A Cancel-Key header field MAY be added to a proto-article containing
   a Control or Supersedes header field by the poster or posting agent
   which will include one or more <c-key> elements.  They will
   correspond to some or all of the <c-lock> elements in the article




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   referenced by the Control (with a "cancel" command as defined in
   [RFC5537]) or Supersedes header field.

   If, as mentioned in Section 3.1, an injecting agent (or moderator)
   has added a Cancel-Lock header field to an article listed in the
   Control (with "cancel" command as defined in [RFC5537]) or Supersedes
   header field - representitive for the posting agent - then (given
   that it authenticates the poster as being the same as the poster of
   the original article) it MUST add the Cancel-Key header field with at
   least one <c-key> element that corresponds to that article.

   Other agents MUST NOT alter this header field.

3.4.  Extending the Cancel-Key header field of a proto-article

   If a Cancel-Key header field has already been added to a proto-
   article then any agent further processing the proto-article up to the
   injecting agent (inclusively) MAY append additional <c-key> elements
   to those already in the header field body.

   If, as mentioned in Section 3.2 an injecting agent (or moderator) has
   extended the Cancel-Lock header field in an article listed in the
   Control (with "cancel" command as defined in [RFC5537]) or Supersedes
   header field - representitive for the posting agent - then (given
   that it authenticates the poster as being the same as the poster of
   the original article) it MUST extend the Cancel-Key header field body
   with at least one <c-key> element that corresponds to that article.

   Once an article is injected then this header field MUST NOT be
   altered.  In particular, relaying agents beyond the injecting agent
   MUST NOT alter it.

3.5.  Check a Cancel-Key header field

   When a serving agent receives an article that attempts to cancel or
   supersede a previous article via Control (with a "cancel" command as
   defined in [RFC5537]) or Supersedes header field, the system defined
   in this document can be used for authentication.  The general
   handling of articles containing such attempts as defined in [RFC5537]
   is not changed by this document.

   To process the authentication, the received article must contain a
   Cancel-Key header field and the original article a Cancel-Lock header
   field.  If this is not the case, the authentication is not possible
   (failed).

   For the authentication check, every supported <c-key> element from
   the received article is processed as follows:



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   1.  The <c-key-string> part of the <c-key> element is hashed using
       the algorithm defined by its <scheme> part.

   2.  For all <c-lock> elements with the same <scheme> in the original
       article their <c-lock-string> part is compared to the calculated
       hash.

   3.  If one is equal, the authentication is passed and the processing
       of further elements can be aborted.

   4.  If no match was found and there are no more <c-key> elements to
       process, the authentication failed.

4.  Calculating the key data

   The following algorithm is RECOMMENDED to calculate the key "K" based
   on a local secret <sec>.

   The result of the function:

      K = HMAC(uid+mid, sec)

   is the key "K" for an article with Message-ID <mid> that belongs to
   the User-ID <uid> (e.g. the login name of the user).  HMAC is
   outlined in [RFC2104].  HMAC is computed over the data <uid+mid>
   (with '+' representing the concatenation operation), using <sec> as a
   secret key held locally that can be used for multiple articles.  This
   method removes the need for a per-article database containing the
   keys used for every article.

   A posting agent must add the Message-ID header field to the proto-
   article itself and use the content of the header field body as <mid>
   (including literal angle brackets).

   The User-ID <uid> must not contain angle brackets (to ensure that
   concatenation of different <uid> and <mid> elements cannot give the
   same results).

   A posting agent, that uses a dedicated local secret <sec> for every
   user, should use an empty string for the <uid> part.

   In general every agent must not use the same secret <sec> if multiple
   <c-lock> elements are added.

   The local secret <sec> should have a length of at least the output
   size of the hash function that is used by HMAC (256 bit / 32 octets
   for SHA256) and must be a random value.




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   Note that the hash algorithm used as base for the HMAC operation is
   not required to be the same as specified by <scheme>.  An agent that
   verifies a Cancel-Key header field body simply checks whether one of
   its <c-key> elements matches one of the <c-lock> elements with the
   same <scheme> in the Cancel-Lock header field body of the original
   article.

   Common libraries like OpenSSL can be used for the cryptographic
   operations.

5.  Examples

5.1.  Without UID

   Example data for creation of a <c-lock> element with HMAC-SHA256 and
   empty string as <uid> (as suggested in Section 4 for posting agents):

      Message-ID: <12345@mid.example>

      mid: <12345@mid.example>
      sec: ExampleSecret
      K  : HMAC-SHA256(mid, sec) ;mid used as data, sec as secret key

   Calculation of Base64(K) using the OpenSSL command line tools in a
   POSIX shell:

      $ printf "%s" "<12345@mid.example>" \
        | openssl dgst -sha256 -hmac "ExampleSecret" -binary \
        | openssl enc -base64
      qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=

   This can be used as <c-key-string> for cancelling or superseding the
   article <12345@mid.example>.

   Calculation of Base64(SHA256(Base64(K))) required for <c-lock-string>
   using the OpenSSL command line tools in a POSIX shell:

      $ printf "%s" "qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=" \
        | openssl dgst -sha256 -binary \
        | openssl enc -base64
      s/pmK/3grrz++29ce2/mQydzJuc7iqHn1nqcJiQTPMc=

   Inserted into the Cancel-Lock header field body of article
   <12345@mid.example> it looks like this:

      Cancel-Lock: sha256:s/pmK/3grrz++29ce2/mQydzJuc7iqHn1nqcJiQTPMc=





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   Inserted into the Cancel-Key header field body of an article that
   should cancel or supersede article <12345@mid.example> it looks like
   this:

      Cancel-Key: sha256:qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=

5.2.  With UID

   Example data for creation of a <c-lock> element with HMAC-SHA256 and
   "JaneDoe" as <uid> (as suggested in Section 4):

      Message-ID: <12345@mid.example>

      uid: JaneDoe
      mid: <12345@mid.example>
      sec: AnotherSecret
      K  : HMAC-SHA256(uid+mid, sec) ;uid+mid as data, sec as secret key

   Calculation of Base64(K) using the OpenSSL command line tools in a
   POSIX shell:

      $ printf "%s" "JaneDoe<12345@mid.example>" \
        | openssl dgst -sha256 -hmac "AnotherSecret" -binary \
        | openssl enc -base64
      yM0ep490Fzt83CLYYAytm3S2HasHhYG4LAeAlmuSEys=

   This can be used as <c-key-string> for cancelling or superseding the
   article <12345@mid.example>.

   Calculation of Base64(SHA256(Base64(K))) required for <c-lock-string>
   using the OpenSSL command line tools in a POSIX shell:

      $ printf "%s" "yM0ep490Fzt83CLYYAytm3S2HasHhYG4LAeAlmuSEys=" \
        | openssl dgst -sha256 -binary \
        | openssl enc -base64
      NSBTz7BfcQFTCen+U4lQ0VS8VIlZao2b8mxD/xJaaeE=

   Inserted into the Cancel-Lock header field body of article
   <12345@mid.example> it looks like this:

      Cancel-Lock: sha256:NSBTz7BfcQFTCen+U4lQ0VS8VIlZao2b8mxD/xJaaeE=

   Inserted into the Cancel-Key header field body of an article that
   should cancel or supersede article <12345@mid.example> it looks like
   this:

      Cancel-Key: sha256:yM0ep490Fzt83CLYYAytm3S2HasHhYG4LAeAlmuSEys=




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5.3.  Other examples

   Other matching pair of Cancel-Lock and Cancel-Key header fields:

      Cancel-Lock: sha256:RrKLp7YCQc9T8HmgSbxwIDlnCDWsgy1awqtiDuhedRo=
      Cancel-Key: sha256:sSkDke97Dh78/d+Diu1i3dQ2Fp/EMK3xE2GfEqZlvK8=

   With obsolete syntax (uses a <c-key-string> with invalid/missing
   Base64 padding):

      Cancel-Lock: sha1:bNXHc6ohSmeHaRHHW56BIWZJt+4=
      Cancel-Key: ShA1:aaaBBBcccDDDeeeFFF

   Let's assume that all the examples above are associated to the same
   article (e.g. created by different agents):

      Cancel-Lock: sha256:s/pmK/3grrz++29ce2/mQydzJuc7iqHn1nqcJiQTPMc=
                   sha256:NSBTz7BfcQFTCen+U4lQ0VS8VIlZao2b8mxD/xJaaeE=
                   sha256:RrKLp7YCQc9T8HmgSbxwIDlnCDWsgy1awqtiDuhedRo=
                   sha1:bNXHc6ohSmeHaRHHW56BIWZJt+4=
      Cancel-Key: sha256:qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=
                  sha256:yM0ep490Fzt83CLYYAytm3S2HasHhYG4LAeAlmuSEys=
                  sha256:sSkDke97Dh78/d+Diu1i3dQ2Fp/EMK3xE2GfEqZlvK8=
                  ShA1:aaaBBBcccDDDeeeFFF

   Remember that <scheme> must be parsed case insensitive.

5.4.  Manual checks

   Manual checks using the OpenSSL command line tools in a POSIX shell:





















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      $ printf "%s" "qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=" \
        | openssl dgst -sha256 -binary \
        | openssl enc -base64
      s/pmK/3grrz++29ce2/mQydzJuc7iqHn1nqcJiQTPMc=

      $ printf "%s" "yM0ep490Fzt83CLYYAytm3S2HasHhYG4LAeAlmuSEys=" \
        | openssl dgst -sha256 -binary \
        | openssl enc -base64
      NSBTz7BfcQFTCen+U4lQ0VS8VIlZao2b8mxD/xJaaeE=

      $ printf "%s" "sSkDke97Dh78/d+Diu1i3dQ2Fp/EMK3xE2GfEqZlvK8=" \
        | openssl dgst -sha256 -binary \
        | openssl enc -base64
      RrKLp7YCQc9T8HmgSbxwIDlnCDWsgy1awqtiDuhedRo=

      $ printf "%s" "aaaBBBcccDDDeeeFFF" \
        | openssl dgst -sha1 -binary \
        | openssl enc -base64
      bNXHc6ohSmeHaRHHW56BIWZJt+4=

6.  Obsolete Syntax

   Implementations of earlier drafts of this specification defined a
   different value for <scheme> than this version.  The following value
   for <scheme> is now deprecated and SHOULD NOT be generated anymore.
   Serving agents SHOULD still accept it for a transition period as long
   as the corresponding hash function is not considered unsafe (see
   Section 7 for details), or already marked as OBSOLETE in the Netnews
   Cancel-Lock hash algorithm registry (Section 8.3).

      obs-scheme = "sha1"

   It is important for backward compatibility that the deprecated value
   for <scheme> is not phased out too early.  Security and compatibility
   concerns should be carefully weighed before choosing to remove <obs-
   scheme> from existing implementations (or not implementing it in new
   ones).

   Earlier drafts of this specification allowed more liberal syntax for
   <c-key-string>:

      obs-c-key-string = 1*base64-octet
      base64-octet     = ALPHA / DIGIT / "+" / "/" / "="

   <obs-c-key-string> SHOULD NOT be generated but MUST be accepted.






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7.  Security Considerations

   The authentication system defined in this document provides no
   integrity checking properties.  Arbitrary modifications can be
   applied to an article on its way through the network, regardless of
   the presence of a Cancel-Key header field.  A serving agent, who
   receives an article that contains a Cancel-Key header field with a
   matching <c-key> element, only get the information that the
   withdrawal of the target article was approved by a legitimate person
   or agent.

   Example: A valid <c-key> element is extracted from a cancel control
   article and inserted into a forged supersede article.  All servers on
   the network that receive the forged supersede article before the
   cancel control article should accept the forged supersede.  But
   because everybody can post articles with forged identity information
   in the header (same as with spam e-mail), the same result can be
   achieved by sending a forged new article using no authentication
   system at all.

   For originator and integrity checks a signature based authentication
   system is required (normally OpenPGP [RFC4880] is used for this
   purpose).  Both systems can be combined.

   The important property of the hash function used for <scheme> is the
   preimage resistance.  A successful preimage attack either reveals the
   real Cancel-Key (that was used to create the Cancel-Lock of the
   original article) or gives a different Cancel-Key (that matches a
   Cancel-Lock too).  This would break the authentication system defined
   in this document.

   Collision resistance of the hash function used for <scheme> is less
   important.  Finding two <c-key> elements for the Cancel-Key header
   field that match to a <c-lock> element of an arbitrary Cancel-Lock
   header field is not helpful to break the authentication system
   defined in this document (if a specific article is defined as
   target).  Only collateral damage by arbitrary cancel or supersede is
   possible.

   Currently there is no known practicable preimage and second preimage
   attack against the hash function SHA1.  Therefore there is no hurry
   to replace it.  The reasons why this document specifies hash
   functions from the SHA2 family are:

   o  The last draft for the authentication system defined in this
      document is nearly two decades old.  The client side
      implementations are moving forward extremely slowly too
      (newsreaders from the last millennium are still in heavy use).



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      What is defined today should be strong enough for at least the
      next decades.

   o  The collision resistance of SHA1 is already broken, therefore it
      is now obsolete for digital signatures as used in TLS.  It is
      intended that an implementation of the authentication system
      defined in this document can share the same cryptographic library
      functions that are used for TLS.

   o  It is intended that the same hash function can be used for
      <scheme> and (as base) for the HMAC that is suggested in
      Section 4.  See notes below for HMAC-MD5 and HMAC-SHA1.

   o  The SHA2 family of hash algorithms is widely supported by
      cryptographic libraries.  In contrast, SHA3 is currently not
      supported by e.g.  OpenSSL.

   The operation HMAC(uid+mid, sec) as suggested in Section 4 must be
   able to protect the local secret <sec>.  The Message-ID <mid> is
   public (in the Message-ID header field body) and <uid> is optional.
   An attacker who wants to steal/use a local secret only need to break
   this algorithm (regardless of <scheme>), because Cancel-Key header
   fields are explicitly published for every request to cancel or
   supersede existing articles.

   Even if HMAC-MD5 and HMAC-SHA1 are not considered broken today, it is
   desired to have some more security margin here.  Breaking <scheme>
   only allows to authenticate a single forged cancel or supersede
   request.  With <sec> in hand it is possible to forge such requests
   for all articles that contain Cancel-Lock header field bodies with
   elements that are generated with this <sec> in the past.  Changing
   <sec> in regular intervals can be used to mitigate the potential
   damage.

   If an implementation chooses to not implement the key calculation
   algorithm recommended in Section 4, or to implement it with HMAC
   based on a different hash function than <scheme>, the key size used
   should be at least 128 bit with "sha256" for <scheme> and at least 80
   bit with "sha1" for <scheme>.  [[Q2: Security review: Should these
   recommendations remain in the document, or does an RFC exist to refer
   to with regards to security recommendations? ]]

8.  IANA Considerations

   IANA has registered the following header fields in the Permanent
   Message Header Field Repository, in accordance with the procedures
   set out in [RFC3864]:




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      Header field name: Cancel-Lock
      Applicable protocol: netnews
      Status: standard
      Author/change controller: IETF
      Specification document(s): This document

      Header field name: Cancel-Key
      Applicable protocol: netnews
      Status: standard
      Author/change controller: IETF
      Specification document(s): This document

   The Netnews Cancel-Lock hash algorithm registry will be maintained by
   IANA.

   The registry will be available at <https://www.iana.org/assignments/
   netnews-parameters/ netnews-parameters.xhtml#cancel-lock-hash-
   algorithms>.

8.1.  Algorithm Name Registration Procedure

   IANA will register new Cancel-Lock hash algorithm names on a First
   Come First Served basis, as defined in BCP 26 [RFC5226].  IANA has
   the right to reject obviously bogus registration requests, but will
   perform no review of claims made in the registration form.

   Registration of a Netnews Cancel-Lock hash algorithm is requested by
   filling in the following template and sending it via electronic mail
   to IANA at <iana@iana.org>:

      Subject: Registration of Netnews Cancel-Lock hash algorithm X
      Netnews Cancel-Lock hash algorithm name:
      Security considerations:
      Published specification (recommended):
      Contact for further information:
      Intended usage: (One of COMMON, LIMITED USE, or OBSOLETE)
      Owner/Change controller:
      Note: (Any other information that the author deems relevant may be
            added here.)

   Any name that conforms to the syntax of a Netnews Cancel-Lock hash
   algorithm (see definition of <scheme> in Section 2) can be used.
   Especially, Netnews Cancel-Lock algorithms are named by strings
   consisting of letters, digits, hyphens and/or slashes.

   Authors may seek community review by posting a specification of their
   proposed algorithm as an Internet-Draft.  Netnews Cancel-Lock hash




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   algorithms intended for widespread use should be standardized through
   the normal IETF process, when appropriate.

   The IESG is considered to be the owner of all Netnews Cancel-Lock
   hash algorithms that are on the IETF Standards Track.

8.2.  Change control

   Once a Netnews Cancel-Lock hash algorithm registration has been
   published by IANA, the owner may request a change to its definition.
   The change request follows the same procedure as the initial
   registration request.

   The owner of a Netnews Cancel-Lock hash algorithm may pass
   responsibility for the algorithm to another person or agency by
   informing IANA; this can be done without discussion or review.

   The IESG may reassign responsibility for a Netnews Cancel-Lock hash
   algorithm.  The most common case of this will be to enable changes to
   be made to algorithms where the owner of the registration has died,
   has moved out of contact, or is otherwise unable to make changes that
   are important to the community.

   Netnews Cancel-Lock hash algorithm registrations MUST NOT be deleted;
   algorithms that are no longer believed appropriate for use can be
   declared OBSOLETE by a change to their "intended usage" field; such
   algorithms will be clearly marked in the registry published by IANA.

   The IESG is considered to be the owner of all Netnews Cancel-Lock
   hash algorithms that are on the IETF Standards Track.

8.3.  Registration of the Netnews Cancel-Lock hash algorithms

   This section gives a formal definition of the Netnews Cancel-Lock
   hash algorithms as required by Section 8.1 for the IANA registry.

     Netnews Cancel-Lock hash algorithm name: md5
     Security considerations: See corresponding section of this document
     Published specification: This document
     Contact for further information: Author of this document
     Intended usage: OBSOLETE
     Owner/Change controller: IESG <iesg@ietf.org>
     Note: Do not use this algorithm anymore








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     Netnews Cancel-Lock hash algorithm name: sha1
     Security considerations: See corresponding section of this document
     Published specification: This document
     Contact for further information: Author of this document
     Intended usage: LIMITED USE
     Owner/Change controller: IESG <iesg@ietf.org>
     Note: This algorithm is intended for backward compatibility

   Netnews Cancel-Lock hash algorithm name: sha224
   Security considerations: See corresponding section of this document
   Published specification: This document
   Contact for further information: Author of this document
   Intended usage: LIMITED USE
   Owner/Change controller: IESG <iesg@ietf.org>
   Note: sha256 should be used instead, this is a truncated variant of it

     Netnews Cancel-Lock hash algorithm name: sha256
     Security considerations: See corresponding section of this document
     Published specification: This document
     Contact for further information: Author of this document
     Intended usage: COMMON
     Owner/Change controller: IESG <iesg@ietf.org>
     Note: This algorithm is mandatory to implement

   Netnews Cancel-Lock hash algorithm name: sha384
   Security considerations: See corresponding section of this document
   Published specification: This document
   Contact for further information: Author of this document
   Intended usage: LIMITED USE
   Owner/Change controller: IESG <iesg@ietf.org>
   Note: sha512 should be used instead, this is a truncated variant of it

     Netnews Cancel-Lock hash algorithm name: sha512
     Security considerations: See corresponding section of this document
     Published specification: This document
     Contact for further information: Author of this document
     Intended usage: COMMON
     Owner/Change controller: IESG <iesg@ietf.org>
     Note: This algorithm is optional

9.  References

9.1.  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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   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/info/rfc3864>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5536]  Murchison, K., Ed., Lindsey, C., and D. Kohn, "Netnews
              Article Format", RFC 5536, DOI 10.17487/RFC5536, November
              2009, <https://www.rfc-editor.org/info/rfc5536>.

   [RFC5537]  Allbery, R., Ed. and C. Lindsey, "Netnews Architecture and
              Protocols", RFC 5537, DOI 10.17487/RFC5537, November 2009,
              <https://www.rfc-editor.org/info/rfc5537>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

9.2.  Informative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <https://www.rfc-editor.org/info/rfc1321>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
              <https://www.rfc-editor.org/info/rfc3174>.





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   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880,
              DOI 10.17487/RFC4880, November 2007,
              <https://www.rfc-editor.org/info/rfc4880>.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <https://www.rfc-editor.org/info/rfc6151>.

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

   [USEFOR-CANCEL-LOCK]
              Lyall, S., "Cancel-Locks in Usenet articles.", Work in
              Progress, November 1998.

































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Appendix A.  Acknowledgements

   The author acknowledges the original author of the Cancel-Lock
   authentication system as documented in draft-ietf-usefor-cancel-lock:
   Simon Lyall.  He has written the original draft and former version
   [USEFOR-CANCEL-LOCK] and approved the usage of his work for this
   document.  This document is mostly based on his work and was
   originally intended as revision 02.  It must be renamed because the
   USEFOR IETF WG is now closed.

   The author would like to thank the following individuals for
   contributing their ideas and reviewing this specification: Russ
   Allbery, Urs Janssen, Richard Kettlewell, Marcel Logen, Holger
   Marzen, Dennis Preiser, Emil Schuster.  And Peter Faust and Alfred
   Peters for providing statistic data about the algorithms currently in
   use.

   Special thanks to the Document Shepherd, Julien Elie and to the
   Responsible Area Director, Alexey Melnikov.

Appendix B.  Document History (to be removed by RFC Editor before
             publication)

B.1.  Changes since -05

   o  Modified text in Section 3.1 and Section 3.2 to make it clear that
      an injecting agent only must be able to authenticate the poster if
      it wants to act representitive for him.

   o  Added/moved general description text to Section 3 to make things
      easier to understand (suggested by GEN-Art Last Call review).

   o  Removed text for importance of second preimage resistance in
      Section 7 (suggested by Secdir review).

   o  Added note that local secret must be random in Section 4
      (suggested by Secdir review).

   o  Added note that <uid> is not allowed to contain angle brackets in
      Section 4 (suggested by Secdir review).

   o  Changed copyright notice (because Simon Lyall has licensed his
      work to the IETF Trust in the meantime).

   o  Fixed spelling in Section 3.3, Section 3.4, Section 7 and
      Section 8.1 (reported by Julien Elie).





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   o  Changed proposed location of IANA registry in Section 8.  Should
      be more consistent with existing registries now (suggested by
      Julien Elie)

   o  Added note to not use the same secret if multiple <c-lock>
      elements are added in Section 2.1 and Section 4 (suggested by
      Secdir review).

   o  Unified the term "cancel control article".

   o  Added notes for impersonation and content forging attacks in
      Section 7.

   o  Description text modified in Section 1.

B.2.  Changes since -04

   o  Added note that the IESG is the owner of all Netnews Cancel-Lock
      hash algorithms that are on the IETF Standards Track in
      Section 8.1.

   o  Changed the algorithm from informative to RECOMMENDED in
      Section 4.

   o  Replaced "code-string" with "c-lock-string" for Step 2 in
      Section 3.5.

   o  Replaced "code-string" with "c-key-string" for Step 1 in
      Section 3.5.

   o  Added a short explanation in Section 3.3.

   o  Added a short explanation in Section 3.1.

   o  Replaced link to RFC2045 with link to RFC4648 in Section 2.

   o  Replaced normative reference RFC2045 (for Base64 algorithm) with
      RFC4648.

   o  Added case insensitivity note in Section 5.3.

   o  RFC6234 (listed in the downref registry) is now a normative
      reference (formerly informative) as recommended by Shepherd Write-
      Up.

   o  NIST SHS standard is now an informative reference (formerly
      normative) as recommended by Shepherd Write-Up.




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   o  Added "sha224" and "sha384" schemes in Section 8.3 (because
      implementations exists that supports them).

   o  Refer to Section 8.3 instead of Section 8.1 for hash algorithm
      registry.

   o  Fixed some typos.

   o  Fixed line length in Section 5.1.

B.3.  Changes since -03

   o  Added note for change interval of <sec> in Section 7.

   o  Changed wording in Section 7.

   o  Splitted Section 5 into multiple subsections.

   o  Added example with UID in Section 5.

   o  Changed "SHOULD NOT" to uppercase in Section 6.

   o  Reformatted Section 8, Section 8.1 and Section 8.3.

   o  Fixed spelling in Section 4.

B.4.  Changes since -02

   o  Added Section 8.2.

   o  Added note about algorithm names in Section 8.1.

   o  Added "/" to scheme-char in Section 2.

   o  Removed case sensitivity of scheme and normative reference to
      RFC7405 in Section 2 again.

   o  Added "sha512" scheme in Section 2.

   o  Changed wording in Section 8.3.

   o  Fixed typo "canceling" in Section 5.

   o  Changed calculation formulas to use "Base64" in Section 2.1 and
      Section 2.2.

   o  Added obsolete algorithm "md5" in Section 8.3.




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   o  Added note that posting agents should add the Message-ID header
      field to proto-articles and use its content for <mid> in
      Section 4.

   o  Added <uid> part to key calculation in Section 4.

   o  Added note to generate CFWS without comments in Section 2.1 and
      Section 2.2.

   o  Changed ABNF to allow CFWS at the beginning of header fields in
      Section 2.1 and Section 2.2.

   o  Changed wording for "header"/"header field"/"header field body".

   o  Added Section 3.4.

   o  Changed wording in Section 3.1.

   o  Allowed additional whitespace at the beginning of header fields in
      Section 2.1 and Section 2.2.

   o  Changed definition of "c-key-string" in Section 2.2.

   o  Added "obs-c-key-string" to Section 6.

   o  Fixed typo in Section 2.2 ("c-lock" replaced by "c-key").

   o  Added key length recommendation in Section 7.

   o  Renamed "sha-256" scheme to "sha256".

   o  Modified header and abstract section to list RFC5537 as updated by
      this document again.

   o  Added "USEFOR-CANCEL-LOCK" as informative reference.

   o  Changed wording in Section 4.

B.5.  Changes since -01

   o  Changed wording in Section 7.

   o  Added example for HMAC calculation in Section 5.

   o  Changed wording in Section 4.

   o  Added use cases to Section 3.2.




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   o  Replaced wording "injecting-agent" by "injecting agent".

   o  Added Definition for "LOWER" in Section 2.

   o  Added Section 8.3.

   o  Added Section 8.1.

   o  Added new entries for header field registry in Section 8.

   o  Removed recommendation that moderators and injecting agents should
      add only one Cancel-Lock or Cancel-Key resprectively to the list
      in Section 3.1, Section 3.2 and Section 3.3.

   o  Added missing headerfield termination to Section 2.1 and
      Section 2.2.

   o  Removed definition for "code-string" from Section 2.  Added
      stricter definition "c-lock-string" to Section 2.1.  Added
      backward compatible definition "c-key-string" to Section 2.2.

   o  Use different wording in Section 2.2.

   o  Changed wording to reflect that an injecting agent is allowed to
      create Cancel-Lock headerfields in Section 2.1.

   o  Fixed wording and typo in Section 2.

   o  Added normative reference to RFC7405 because case-sensitivity is
      used in ABNF.

   o  Added reference to RFC5536 (Section 2.2) in Section 2.

   o  Added references to RFC4880 and RFC5537 in Section 1.

   o  Replaced the wordings "remove" by "cancel" and "replace" by
      "supersede".

   o  Modified header and abstract section to no longer list RFC5536 and
      RFC5537 as updated by this document.

B.6.  Changes since -00

   o  Added additional note that deprecated "scheme" values should be
      preserved for backward compatibility as long as reasonable.

   o  Removed deprectated scheme "md5" (not in use anymore).




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   o  Added descriptions how to generate "code-string" to Section 2.1
      and Section 2.2.

   o  Removed length limitiation in ABNF of "scheme".

   o  Changed copyright notice to use text from TLP section 6.c.iii.

   o  Removed references from "abstract" section.

   o  Changed "SHOULD NOT" to uppercase in Section 6.

   o  Added line wraps to CLI commands in Section 5.

B.7.  Changes since draft-ietf-usefor-cancel-lock-01

   o  Renamed document because the USEFOR IETF WG is now closed.

   o  Added more details how to check Cancel-Key header fields in
      Section 3.5.

   o  Added more details to Section 7.

   o  Added updated ABNF for Cancel-Lock and Cancel-Key header fields.

   o  Deprecated "md5" and "sha1" schemes.

   o  Added "sha-256" scheme.

   o  Reworded the abstract section and added references.

   o  Added note to other authentication systems to Section 1.

   o  Added command line check examples to Section 5.

B.8.  Changes since draft-ietf-usefor-cancel-lock-00

   o  References to SHA-160 changed to SHA1

   o  "scheme" is now a case insensitive token and the number "1" has
      been changed to "sha1".

   o  Added some examples and fixed the section numbering.

   o  Updated 2nd paragraph on section 2.2 to make clear what exactly is
      being hashed and how.

   o  Changed paragraph 2 of 3.1 to discourage injection agents from
      adding the header.



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   o  Removed the Clue-string as this complicated the scheme without
      adding realistic functionality

   o  Moderators can now add these headers under the same conditions as
      injection agents.

Author's Address

   Michael Baeuerle
   STZ Elektronik
   Hofener Weg 33C
   Remseck, Baden-Wuerttemberg  71686
   Germany

   Fax:   +49 7146 999061
   EMail: michael.baeuerle@stz-e.de



































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