You need to have the seahorse-plugins package installed. It is
already the case under Tails. Under Debian or Ubuntu, if you're not sure or
want to install it, you can issue the following commands:
sudo apt-get update sudo apt-get install seahorse-plugins
The development of seahorse-plugins has stopped. It hasn't been ported to Gnome 3. It is no more available on Debian starting from Wheezy and Ubuntu starting from version 11.10, Oneiric Ocelot. If you can't install it please try verifying the ISO using the command line.
If you are using Tails, you already have the signing key. Otherwise, first download Tails signing key:
Your browser should propose you to open it with "Import Key". Choose this action. It will add Tails signing key to your keyring, the collection of OpenPGP keys you already imported:
You will get notified will the following message:
Now, download the cryptographic signature corresponding to the ISO image you want to verify:
Your browser should propose you to open it with "Verify Signature". Choose this action to start the cryptographic verification:
Browse your files to select the Tails ISO image you want to verify. Then, the verification will start. It can take several minutes:
If the ISO image is correct you will get a notification telling you that the signature is good:
If the ISO image is not correct you will get a notification telling you that the signature is bad:
