Via the Package Menu, users may download packages from package
archives. Such archives are specified by the variable
package-archives, whose default value lists the archives
hosted on GNU ELPA and
non-GNU ELPA. This section describes
how to set up and maintain a package archive.
The value of this variable is an alist of package archives recognized by the Emacs package manager.
Each alist element corresponds to one archive, and should have the
form (id . location), where id is the name of
the archive (a string) and location is its base location
(a string).
If the base location starts with ‘http:’ or ‘https:’, it is treated as an HTTP(S) URL, and packages are downloaded from this archive via HTTP(S) (as is the case for the default GNU archive).
Otherwise, the base location should be a directory name. In this case, Emacs retrieves packages from this archive via ordinary file access. Such local archives are mainly useful for testing.
A package archive is simply a directory in which the package files, and associated files, are stored. If you want the archive to be reachable via HTTP, this directory must be accessible to a web server; See Interfacing to an archive web server.
A convenient way to set up and update a package archive is via the
package-x library. This is included with Emacs, but not loaded
by default; type M-x load-library RET package-x RET to
load it, or add (require 'package-x) to your init file.
See Lisp Libraries in The GNU Emacs Manual.
After you create an archive, remember that it is not accessible in the
Package Menu interface unless it is in package-archives.
Maintaining a public package archive entails a degree of responsibility. When Emacs users install packages from your archive, those packages can cause Emacs to run arbitrary code with the permissions of the installing user. (This is true for Emacs code in general, not just for packages.) So you should ensure that your archive is well-maintained and keep the hosting system secure.
One way to increase the security of your packages is to sign them using a cryptographic key. If you have generated a private/public gpg key pair, you can use gpg to sign the package like this:
gpg -ba -o file.sig file
For a single-file package, file is the package Lisp file; for a multi-file package, it is the package tar file. You can also sign the archive’s contents file in the same way. Make the .sig files available in the same location as the packages. You should also make your public key available for people to download; e.g., by uploading it to a key server such as https://pgp.mit.edu/. When people install packages from your archive, they can use your public key to verify the signatures.
A full explanation of these matters is outside the scope of this manual. For more information on cryptographic keys and signing, see GnuPG in The GNU Privacy Guard Manual. Emacs comes with an interface to GNU Privacy Guard, see EasyPG in Emacs EasyPG Assistant Manual.