Independent Submission M. Bäuerle
Internet-Draft STZ Elektronik
Intended status: Standards Track March 10, 2017
Expires: September 11, 2017

Cancel-Locks in Netnews articles


This document defines an extension to the Netnews Article Format that may be used to authenticate the cancelling and superseding of existing articles.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on September 11, 2017.

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Table of Contents

1. Introduction

The authentication system defined in this document is intended to be used as a simple method to verify that the author of an article which cancels ([RFC5537] Section 5.3) or supersedes ([RFC5537] Section 5.4) another one is either the poster, posting agent, moderator or injecting agent that processed the original article when it was in its proto-article form.

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 messages 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][RFC7405]. 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 fields contain lists of encoded values. Every entry is based on a <scheme>:

   scheme       = %s"sha-256" / 1*scheme-char / obs-scheme
   scheme-char  = LOWER / DIGIT / "-"
   LOWER        = %x61-7A  ; lowercase characters [a-z]

The hash algorithms for <scheme> are defined in [SHA], see also [RFC1321] and [RFC6151] for MD5, [RFC3174] for SHA1 and [RFC6234] for the SHA2 family. The Base64 encoding used is defined in Section 6.8 of [RFC2045].

This document defines one value for <scheme>: "sha-256". This scheme is mandatory to implement.

Note that the obsolete syntax <obs-scheme> was defined case-insensitive. This is changed in this document and the scheme MUST now be generated with lowercase letters.

The case sensitivity of <scheme> is defined to simplify the checks.

2.1. Cancel-Lock

   cancel-lock     = "Cancel-Lock:" SP c-lock-list CRLF
   c-lock-list     = 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 "="

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 article up to injection (inclusively):


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

2.2. Cancel-Key

   cancel-key   = "Cancel-Key:" SP c-key-list CRLF
   c-key-list   = c-lock *(CFWS c-lock) [CFWS]
   c-key        = scheme ":" c-key-string
   c-key-string = 1*base64-octet
   base64-octet = ALPHA / DIGIT / "+" / "/" / "="

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 Cancel-Lock header field (as defined in Section 2.1 of this document) of the original article:


The relaxed syntax definition of <c-key-string> above is required for backward compatibility. New implementations always SHOULD generate valid Base64.

3. Use

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 an article, then an injecting agent (or moderator) MAY add one or more provided that it positively authenticates the author. The injecting agent (or moderator) MUST NOT add this header field to an article unless it is able to authenticate all cancelling or superseding attempts from the poster and automatically add working Cancel-Key header fields for such articles.

Other agents MUST NOT add this header 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.

Use cases for extending the Cancel-Lock header field:

Once an article is injected then this header 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

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

If, as mentioned in Section 3.2 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 then (given that it authenticates the poster as being the same as the poster of the original article) it MUST add (or extend, if already present) the Cancel-Key header field with at least one <c-key> element that correspond to that article.

Other agents MUST NOT alter this header.

3.4. 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:

  1. The <code-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 <code-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

This section is informative, not normative.

It is suggested to use the function K = HMAC(mid+sec) to create the key "K" for an article with Message-ID <mid>, where HMAC is outlined in [RFC2104].  <sec> is a secret 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.

The local secret <sec> should have a length of at least the output size of the hash function that is used by HMAC (32 octets for SHA-256). If the secret is not a random value, but e.g. some sort of human readable password, it should be much longer. In any case it is important that this secret can not be guessed.

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 simply checks whether it matches one of the <c-lock> elements with the same <scheme> in the Cancel-Lock header field of the original article.

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

5. Examples

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

   Message-ID: <12345@mid.example>
   mid: <12345@mid.example>
   sec: ExampleSecret
   K  : HMAC-SHA256(mid+sec) ;"mid" used as HMAC message, "sec" used as HMAC 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

This can be used as <c-key-string> for canceling 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

Inserted into the header of article <12345@mid.example> it looks like this:

   Cancel-Lock: sha-256:s/pmK/3grrz++29ce2/mQydzJuc7iqHn1nqcJiQTPMc=

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

   Cancel-Key: sha-256:qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=

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

   Cancel-Lock: sha-256:RrKLp7YCQc9T8HmgSbxwIDlnCDWsgy1awqtiDuhedRo=
   Cancel-Key: sha-256:sSkDke97Dh78/d+Diu1i3dQ2Fp/EMK3xE2GfEqZlvK8=

With obsolete syntax (requires case-insensitive parsing of <scheme> and 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: sha-256:s/pmK/3grrz++29ce2/mQydzJuc7iqHn1nqcJiQTPMc=
   Cancel-Key: sha-256:qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=

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

   $ printf "%s" "qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=" \
     | openssl dgst -sha256 -binary \
     | openssl enc -base64

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

   $ printf "%s" "aaaBBBcccDDDeeeFFF" \
     | openssl dgst -sha1 -binary \
     | openssl enc -base64

6. Obsolete Syntax

Implementations of earlier drafts of this specification allowed more liberal (case insensitive) syntax and 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.

   obs-scheme   = "sha1"

<obs-scheme> MUST be parsed case-insensitive.

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).

7. Security Considerations

The important properties of the hash function used for <scheme> are the preimage and second preimage resistance. A successful preimage attack would reveal the real Cancel-Key that was used to create the Cancel-Lock of the original article. A successful second preimage attack would allow to create a new, different Cancel-Key that matches a Cancel-Lock too. Both cases would break the authentication system defined in this document.

Collision resistance of the hash function used for <scheme> is less important. Finding two Cancel-Keys that match an arbitary Cancel-Lock is not helpful to break the authentication system defined in this document (if a specific article is defined as target). Only collateral damage like arbitrary deletion or spam 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 SHA-256 (aka SHA2-256) are:

The operation HMAC(mid+sec) as suggested in Section 4 must be able to protect the local secret <sec>. The Message-ID <mid> is public (in the article header). An attacker who wants to steal/use a local secret only need to break this algorithm (regardless of <scheme>), because Cancel-Keys are explicitly published for every request to modify or delete 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 modify or delete request. With <sec> in hand it is possible to forge such requests for all articles that contain Cancel-Locks based on Cancel-Keys generated with this <sec> in the past.

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]:

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

The registry will be available at <>.

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 <>:

Authors may seek community review by posting a specification of their proposed algorithm as an Internet-Draft. Netnews Cancel-Lock hash algorithms intended for widespread use should be standardized through the normal IETF process, when appropriate.

8.2. 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.

9. References

9.1. Normative References

[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3864] Klyne, G., Nottingham, M. and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, September 2004.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC5536] Murchison, K., Lindsey, C. and D. Kohn, "Netnews Article Format", RFC 5536, DOI 10.17487/RFC5536, November 2009.
[RFC5537] Allbery, R. and C. Lindsey, "Netnews Architecture and Protocols", RFC 5537, DOI 10.17487/RFC5537, November 2009.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, December 2014.
[SHA] National Institute of Standards and Technology, "Secure Hash Standard (SHS)", FIPS 180-4, DOI 10.6028/FIPS.180-4, August 2015.

9.2. Informative References

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, DOI 10.17487/RFC1321, April 1992.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997.
[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D. and R. Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/RFC4880, November 2007.
[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.
[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.

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 <> 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: Julien Élie, Richard Kettlewell, Holger Marzen. And Peter Faust, Urs Janßen and Alfred Peters for providing statistic data about the algorithms currently in use.

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

B.1. Changes since -01

B.2. Changes since -00

B.3. Changes since draft-ietf-usefor-cancel-lock-01

B.4. Changes since draft-ietf-usefor-cancel-lock-00

Author's Address

Michael Bäuerle STZ Elektronik Hofener Weg 33C Remseck, Baden-Württemberg 71686 Germany Fax: +49 7146 999061 EMail: