is handled symmetrically, i.e. an unsigned rmgroup message is used as formalized proposal. Objections to the proposal are then expressed with a newgroup. Example Control: rmgroup comp.object.moderated Subject: cmsg rmgroup comp.object.moderated In 1995 the Church of Scientology attempted to silence criticism by sending mass "rmgroup" messages to Usenet servers targeting alt.religion.scientology, an example of the church's continuing efforts to suppress material critical of Scientology on the Internet. Most servers discarded the message, and those that did not were quickly sent "newgroup" messages reestablishing the newsgroup. checkgroups A checkgroups message lists all groups of a hierarchy. Example Control: checkgroups Subject: cmsg checkgroups Example conforming to RFC 5537 Control: checkgroups de !de.alt #2009021301 Obsolete message types The ihave/sendme protocol was obsoleted by NNTP. Answering control messages with large emails can be exploited for a Denial of service attack. Thus news servers stopped implementing long before it was declared obsolete by RFC 5537. Security considerations Header field "Approved:" Messages of type and are ignored unless there is an "Approved" line in the same message header. Newsservers traditionally allow only selected users to send articles with these lines. As long as there were only a handful of Usenet sites this provided sufficient protection against abuse. Digital signature The format of "Arpa Internet Text Messages" is the common base for Usenet and E-mail. The format provides no means of authentication. Various extensions adding a digital signature were developed to prevent forgeries. For control message a special format is required since the essential information is in the header fields. Pgpcontrol was originally designed for PGP but also works with OpenPGP. Hierarchy keys Newsgroup maintenance of the main Usenet hierarchies (Big 8 and regional hierarchies) is done through signed control messages. Each hierarchy has unique key that is guarded by the hierarchy founders (or their successors). Most newsservers are configured to both automatically execute controls signed with the right key and ignore anything else. Theoretically this system is also applicable to cancel messages. However, it would not only require a key pair for every Usenet user but also that the respective public key is known to every news server. Cancel-lock is much simpler, but neither commonly accepted, nor implemented in popular news servers and newsreaders. Archiving Control messages are typically not shown in the target newsgroup. Instead many servers put them into pseudo newsgroups like control. Google Groups provides no means to read or write control messages.