Control message#cancel

The verb cancel ( engl. cancel " cancel " ) referred to Usenet conscious, premature deletion of an article. The term is ambiguous.

News Reader, so the programs that you participate in Usenet, in general, a function ( menu, button, etc.) have called " Cancel Message ", "Cancel Usenet Message" etc., to produce a cancel message and submit.

News server understood by the Cancel function, however, the pure deletion, ie the removal of an item from the item memory of the news server. The automatic processing of incoming messages is a fact Cancel fitting end functionality. Alternatives to Cancel messages are NoCeM and supersede.

Cancel message

A Cancel message is automatically analyzable by a software request to delete a specific item locally with you. It belongs to the group of control messages and different from ordinary postings by a line in the header ( where the sender, subject, newsgroup, date available, etc.) with the following syntax:

Control: cancel < [email protected] > This message can not be read in the appropriate newsgroup, but asks to delete the item with the message ID < [email protected] >. Many news server sort Cancel messages in the pseudo - newsgroup control or control.cancel.

It is usual, but not necessary, to make the subject of a cancel message as follows:

Subject: cmsg cancel < [email protected] > rogue cancel

Rogue cancel is the translation of third-party cancel.

According to RFC 1036 should an item only from the author or the administrator of the server on which the article was fed to Usenet, be canceled. Since the publication of RFC 1036 in December 1987, the practice has changed slightly. Rogue cancel be tolerated today to remove spam. The extensive guidelines for this, however, were not elevated to the status of an RFC. Procedural requirements and conditions (eg Breidbart index) are individually determined by the hierarchies.

But there are also voices that consider every rogue cancel as inadmissible interference in the expression of other participants. So * Cancel all messages are about in the hierarchy free. Prohibited.

In German-speaking countries this debate on freedom of expression nor by the peculiarity of the word rogue cancel is difficult because it suggests a duality with Eigencancel. The proposed in RFC -1036 administrator's privilege to Admincancel is here following the instinct of language considered as a subset of rogue cancel. In fact, the original concept of third-party cancel speaks but of the third kind of Cancelns.

Major news servers allow very flexible manner according to a number of different criteria, which of these recommendations should not be followed and what. However, there is also news server, which allow to prevent abuse no cancels.

Cancel -Watch

Cancel -Watch is a simple process to send to the author of the post concerned a notification (email ) upon arrival of a cancel message.

The prerequisite is a Message-ID with a unique, ie used by any other user, Fully Qualified Domain Name. This eliminates the need to maintain a database used message IDs.

Cancel-Lock & Key Cancel

Cancel-Lock is a mechanism to prevent unauthorized Cancel and Supersedes. It is used in draft-ietf - usefor -cancel -lock- 01 (dated November 1998) described and is not widespread.

The method relies on the irreversibility of a hash function. In Draft only Secure Hash Algorithm is mentioned. The format of Cancel lock itself is not limited to a specific hash function.

Cancel-Lock is easier to implement than the other control messages ( newgroup, rmgroup, checkgroups ) used encryption with PGP. Especially not a database of public keys is required.

However, it is not guaranteed that a cancel message arrives after the message to be deleted. If a Cancel -Bot runs on a server, and this generated immediately upon receipt of a ( spam ) article, a Cancel Message will only further spread this. The original message can be as a roundabout way ( slow servers, bad connection, servers that do not run Cancel ) to cancel the message arrive.

Expiration

When submitting an article, a matching pair of Cancel-Lock and Cancel -Key is generated. The Cancel-Lock will be published with the article. The Cancel key remains secret for the time being.

In a later case you need to Cancel or Supersedes the right to Cancel-Lock the target postings Cancel Key is enclosed. Servers that implement the method, by deleting Cancel-Lock protected article only if the Cancel or Supersedes present a correct Cancel -Key.

Few newsreader implement Cancel-Lock:

• Wildebeest
• slrn
• Tin

However, Cancel-Lock can be set also from the news server software. Since the method provides for the possibility existing locks to add further, can article so more than one Cancel-Lock (or more than one - Cancel Key ) have. As in the review only one of the key must match one of the locks to admin stopped by the automatic insertion of the possibility of admin -cancel open.

Algorithm

The relationship between key and lock is set by the Draft.

Lock = encode_base64 ( sha1 (key) ) Since the key messages in Cancel or Supersedes published eventually, every posting must be protected by an individual lock. Theoretically, one could leave the key generated by a random number generator, but then would have to keep records about posting to what fits which key.

The draft recommends instead the method applied to HMAC Message-ID and a secret. Since the message ID is different by definition, one obtains for every posting an individual lock and only a mystery for all postings must still remember.

The following Perl program generates a key / lock pair analogous to the canlock based on the library implementation of slrn. The variable $ secret corresponds to the contents of the specified file by cansecret_file.

#! / usr / bin / perl -w use MIME :: Base64 (); use Digest :: SHA1 (); use Digest :: hmac_sha1 (); my $ secret = ' secret password \ n "; my $ message_id = ' < [email protected] >'; my $ digest = Digest :: hmac_sha1 :: hmac_sha1 ($ message_id, $ secret); my $ cancel_key = MIME :: Base64 :: encode ($ digest ,''); my $ cancel_lock = MIME :: Base64 :: encode ( Digest :: SHA1 :: sha1 ($ cancel_key ) ,''); printf " Cancel Key: sha1 :% s \ n ", $ cancel_key; printf " Cancel-Lock: sha1 :% s \ n ", $ cancel_lock; The following program checks a given key / lock pair.

#! / usr / bin / perl -w use MIME :: Base64 qw ( encode_base64 decode_base64 ); use Digest :: SHA1 qw ( sha1 ); my $ cancel_key = ' sha1: 5xiMFrYJK5pJjJEPpTjofnjdSEI ='; my $ cancel_lock = ' sha1: dQK3bshKibllVXZJPNXfj/N2zlU ='; my $ verify = encode_base64 ( sha1 ( substr ($ cancel_key, 5) ) ,''); if ( substr ($ cancel_lock, 5) eq $ verify)    {Print "Cancel - Cancel-Lock Key matches. \ N "; } else    {Print "Cancel -Key does not match Cancel-Lock \ n. "; } NoCeM

NoCeM is hardly a widespread alternative to rogue cancel. The artificial word is the English word combination No See 'Em and is modeled as [, nou'si: əm ] pronounced.

NoCeM messages are signed with an asymmetric encryption format PGP / INLINE and contain a type specification such as SPAM or MMF. This allows server operators to selectively allow certain messages automatically evaluate from trusted senders.

In contrast to a Cancel Message can NoCeM a message concerning any number of target messages.

NoCeM messages are usually evaluated by external programs. In contrast to Cancel messages subsequent or repeated evaluation is therefore easily.

The set for rogue cancel conventions as Breidbart index have no relevance for NoCeM. However, the conditions ( installation of the software, import the PGP key and configuration of the type specification ) to ensure that only a small minority of the servers NoCeM considered.

Footnotes

• Usenet