Quickly updating a Tails ISO image

... by patching the ISO image file you already have, avoiding downloading the whole new version.

Thanks to such a 'cold' (not live) upgrade you are protected from some types of (security breaches).

Pre-requisites

Before booting on the new ISO image please check that it is identical to the official one published by the Tails project! In order to do so you MUST already have:

the official OpenPGP public signing key in your GPG keyring. Easy way: download this key then invoke gpg --import https://tails.net/tails-signing.key
If you don't know anything about all this and don't want to explore then the present document is not for you, therefore upgrade Tails using the standard procedure. If you don't know and want to explore you may first install Tails from Debian or Ubuntu using the command line and GnuPG (then be prepared to benefit from this document).
an ISO image file of some previous version of Tails
an executable xdelta3.

Process

You will download a VCDIFF patchfile, created by me (using the 'xdelta3' software), which describes the differences between the old version of the Tails image you already have, and the new one.

For example a file named 'tails-amd64-6.15.iso_to_6.15.1.xdelta3' enables you to patch the Tails AMD 6.15 ISO image into a 6.15.1 version. It is useless if you don't already have the file named 'tails-amd64-6.15.iso'

Available xdelta3 patchfiles.

Usage

stop Tails (because during this process you will modify the ISO image it uses!)
on a command line (the directory containing your Tails ISO image file must be the current directory), invoke the 6 following commands:
1. export Tails_PREVIOUS_VERSION=6.15 # replace '6.15' by the version number of the image file you already have
2. export Tails_VERSION=6.15.1 # replace '6.15.1' by the version number of the newest version
3. wget https://tails.net/torrents/files/tails-amd64-${Tails_VERSION}.iso.sig # download the official GPG seal
4. wget http://makarevitch.com/Tails/upd_xdelta/tails-amd64-${Tails_PREVIOUS_VERSION}.iso_to_${Tails_VERSION}.xdelta3 # download the pertinent patchfile
5. xdelta3 -d -stails-amd64-${Tails_PREVIOUS_VERSION}.iso tails-amd64-${Tails_PREVIOUS_VERSION}.iso_to_${Tails_VERSION}.xdelta3 tails-amd64-${Tails_VERSION}.iso # apply the patchfile
6. gpg --verify tails-amd64-${Tails_VERSION}.iso.sig # check the new image file
If it does NOT produce a gpg: Good signature from "Tails developers (offline long-term identity key)" message then something failed (it never happened to me nor to numerous friends) and DO NOT USE the new image.