| Independent Submission | M. Bäuerle |
| Internet-Draft | STZ Elektronik |
| Updates: 5537 (if approved) | May 31, 2017 |
| Intended status: Standards Track | |
| Expires: December 2, 2017 |
Cancel-Locks in Netnews articles
draft-baeuerle-netnews-cancel-lock-05
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.
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 December 2, 2017.
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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.
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].
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.
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>:
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>.
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.
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).
The Cancel-Lock header field contains hashes of secret strings. This secret strings can later be used to authenticate a cancel or supersede request.
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 or more provided that it positively authenticates the author. The injecting agent (or moderator) MUST NOT add this header field to a proto-article unless it is able to authenticate all cancelling or superseding attempts from the poster and automatically add a working Cancel-Key header field or extend an existing one for such proto-articles.
Other agents MUST NOT add this header field to articles or proto-articles that they process.
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.
Use cases for extending the Cancel-Lock header field body:
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.
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 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 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 correspond to that article.
Other agents MUST NOT alter this header field.
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 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 correspond 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.
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:
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. [Q1]Security review: Some existing implementations concatenates the <uid> part with <sec> instead of <mid>. This variant was not used to ensure that <sec> is directly used as HMAC key (to avoid confusion with the length considerations below).
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).
A posting agent, that uses a dedicated local secret <sec> for every user, should use an empty string for the <uid> part.
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). 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 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.
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=
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=
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=
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.
Manual checks 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=
$ 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=
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.
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 <c-key-string> element that was used to create the Cancel-Lock header field body of the original article. A successful second preimage attack would allow to create a new, different <c-key-string> element that, if used in the Cancel-Key header field body, matches a <c-lock-string> element in the Cancel-Lock header field body of the original article 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 <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:
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?
IANA has registered the following header fields in the Permanent Message Header Field Repository, in accordance with the procedures set out in [RFC3864]:
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-cancel-lock-parameters/>.
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 algorithm 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 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.
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.
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
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
| [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. |
| [RFC4648] | Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006. |
| [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. |
| [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. |
| [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. |
| [SHA] | National Institute of Standards and Technology, "Secure Hash Standard (SHS)", FIPS 180-4, DOI 10.6028/FIPS.180-4, August 2015. |
| [USEFOR-CANCEL-LOCK] | Lyall, S., "Cancel-Locks in Usenet articles.", Work in Progress, November 1998. |
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 Élie.