Rewrite introduction to WoT

wiki/src/install/download/openpgp.mdwn

@@ -137,14 +137,21 @@ signing key through the OpenPGP Web of Trust</a>.
 Authenticate the signing key through the OpenPGP Web of Trust
 =============================================================
 
-Note that since all Tails releases are signed with the same key, you will not
-have to verify the key every time and the trust you might progressively build in
-it will be built once and for all. Still, you will have to check the ISO image
-every time you download a new one!
-
-If you want to be extra cautious and really authenticate Tails signing key in a
-stronger way than what standard HTTPS offers you, you will need to use the
-OpenPGP Web of Trust.
+The verification techniques presented until now ([[Firefox extension,
+BitTorrent|download/install]], or OpenPGP verification) all rely on some
+information being securely downloaded using HTTPS from our website:
+
+  - The *checksum* for the Firefox extension
+  - The *Torrent file* for BitTorrent
+  - The *Tails signing key* for the OpenPGP verification
+
+But, while doing so, you could download malicious information if our
+website is compromised or if you are victim of a [[man-in-the-middle
+attack|doc/about/warning#man-in-the-middle]].
+
+The OpenPGP verification allows you to verify the ISO image even better
+by also authenticating the Tails signing key through the OpenPGP Web of
+Trust, without having to trust your download.
 
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