Independent Submission M. Baeuerle
Internet-Draft STZ Elektronik
Updates: 5536,5537 (if approved) March 6, 2017
Intended status: Standards Track
Expires: September 7, 2017
Cancel-Locks in Netnews articles
draft-baeuerle-netnews-cancel-lock-00
Abstract
This document defines an extension to the Netnews Article Format that
may be used to authenticate the removal or replacement of existing
articles. If approved, this document updates [RFC5536] and
[RFC5537].
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 7, 2017.
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
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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 . . . . . . . . . . . . . . . . . . . . . . . 4
3. Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Adding an initial Cancel-Lock header field to a proto-
article . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Extending the Cancel-Lock header field of a proto-article 5
3.3. Adding a Cancel-Key header field to a proto-article . . . 5
3.4. Check a Cancel-Key header field . . . . . . . . . . . . . 5
4. Calculating the key data . . . . . . . . . . . . . . . . . . 6
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Obsolete Syntax . . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Appendix B. Document History (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 11
B.1. Changes since draft-ietf-usefor-cancel-lock-01 . . . . . 11
B.2. Changes since draft-ietf-usefor-cancel-lock-00 . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
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
removes or replaces 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)
an OpenPGP signature is suited.
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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) and the letters "ue" in Baden-Wuerttemberg as an
u-umlaut (U+00FC, "ü" in XML).
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:
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 a <code-string> based on a <scheme>:
scheme = %s"sha-256" / 1*20scheme-char / obs-scheme
scheme-char = LOWER / DIGIT / "-"
code-string = 1*base64-octet
base64-octet = ALPHA / DIGIT / "+" / "/" / "="
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 value
is mandatory to implement
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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.
Values for <scheme> defined in future updates to this document are
length limited to 20 characters. This and the case-sensitivity are
defined to simplify the checks.
2.1. Cancel-Lock
cancel-lock = "Cancel-Lock:" SP c-lock *(CFWS c-lock) [CFWS]
c-lock = scheme ":" code-string
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.
The <code-string> in <c-lock> is the Base64 encoded output of a hash
operation (defined by <scheme>) of the Base64 encoded key that is
intended to authenticate the person or agent that created or
processed respectively the article before injection. Because of the
one-way nature of the hash operation the key is not revealed.
2.2. Cancel-Key
cancel-key = "Cancel-Key:" SP c-key *(CFWS c-key) [CFWS]
c-key = scheme ":" code-string
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.
The <code-string> in <c-key> is the Base64 encoded key that was used
to create the Cancel-Lock header field as defined in Section 2.1 of
the original article.
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
provided that it positively authenticates the author. The injecting-
agent (or moderator) MUST NOT add this header to an article unless it
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is able to authenticate all remove or replace attempts from the
poster and automatically add a working Cancel-Key header field 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 (prior to the article being injected) further
processing the proto-article (moderators and injecting-agents) MAY
append a single <c-lock> element to those already in the header.
No more than one <c-lock> element SHOULD be added by each agent that
processes the proto-article.
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
The 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 "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 a <c-key> element
that correspond to those 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 remove or
replace 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.
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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 HMAC(mid+sec) to create the key
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 simply check whether it matches one of the
Cancel-Locks.
Common libraries like OpenSSL can be used for the cryptographic
operations.
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5. Examples
Matching pair of Cancel-Lock and Cancel-Key header fields:
Cancel-Lock: sha-256:RrKLp7YCQc9T8HmgSbxwIDlnCDWsgy1awqtiDuhedRo=
Cancel-Key: sha-256:sSkDke97Dh78/d+Diu1i3dQ2Fp/EMK3xE2GfEqZlvK8=
Legacy variant:
Cancel-Lock: sha1:bNXHc6ohSmeHaRHHW56BIWZJt+4=
Cancel-Key: ShA1:aaaBBBcccDDDeeeFFF
Manual checks using the OpenSSL command line tools in a POSIX shell:
$ 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 allowed other
<scheme> values and more liberal (case insensitive) syntax than is
allowed in this version. The following values for <scheme> are now
deprecated and SHOULD not be generated anymore. Serving agents
SHOULD still accept them for a transition period as long as the
corresponding hash function is not considered unsafe. See Section 7
for details.
obs-scheme = "md5" / "sha1"
<obs-scheme> MUST be parsed case-insensitive.
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 matches an arbitary Cancel-
Lock is not helpful to break the authentication system defined in
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this document (if a specific article is defined as target). Only
collateral damage like arbitrary deletion or spam is possible.
Currently there are no known practicable preimage and second preimage
attacks against the hash functions MD5 and SHA1. Therefore there is
no hurry to replace them. The reasons why this document specify
SHA-256 (aka SHA2-256) 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 extremly slowly too
(newsreaders from the last millenium are still in heavy use).
What is defined today should be strong enough for at least the
next decades.
o The collision resistance of MD5 and SHA1 is already broken,
therefore they are 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(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
The Hash Algorithm registry is maintained by IANA. The registry is
available at <http://www.iana.org/assignments/hash-function-text-
names/>.
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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,
<http://www.rfc-editor.org/info/rfc2045>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://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, <http://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,
<http://www.rfc-editor.org/info/rfc5537>.
[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>.
9.2. Informative References
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc3174>.
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[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,
<http://www.rfc-editor.org/info/rfc6151>.
[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,
<http://www.rfc-editor.org/info/rfc6234>.
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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
<https://tools.ietf.org/html/draft-ietf-usefor-cancel-lock-01>. 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.
Appendix B. Document History (to be removed by RFC Editor before
publication)
B.1. 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 3.4.
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.2. 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.
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o Changed paragraph 2 of 3.1 to discourage injection-agents from
adding the header.
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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