unaesgcm is An AES-256-GCM de-/encryption utility specializing on
`aesgcm`-scheme URLs as described in the [OMEMO media sharing XMPP
extension (XEP-0454)](https://xmpp.org/extensions/xep-0454.html) and as
implemented in modern XMPP clients. The latter meaning, in particular,
that the initialization vector size is not restricted to 96 bits.

The utility consists of:

* `unaesgcm` command for decrypting local files;
* `aesgcm-open` command for fetching URLs, decrypting and opening files
  with associated applications;
* a desktop entry handling `aesgcm` URLs using a GUI-friendly flavor of
  the above command;
* `aesgcm` command for encrypting local files that exists mostly for
  symmetry.

The utility is intended mainly for development/debugging, as the URLs
with this scheme are supposed to be invisible to XMPP users, in ideal.
