ikev1: Add an option to accept unencrypted ID/HASH payloads

Even in Main Mode, some Sonicwall boxes seem to send ID/HASH payloads in
unencrypted form, probably to allow PSK lookup based on the ID payloads. We
by default reject that, but accept it if the
charon.accept_unencrypted_mainmode_messages option is set in strongswan.conf.

Initial patch courtesy of Paul Stewart.

conf/options/charon.opt

@@ -8,6 +8,21 @@ charon {}
 	**charon-cmd** instead of **charon**). For many options defaults can be
 	defined in the **libstrongswan** section.
 
+charon.accept_unencrypted_mainmode_messages = no
+	Accept unencrypted ID and HASH payloads in IKEv1 Main Mode.
+
+	Accept unencrypted ID and HASH payloads in IKEv1 Main Mode.
+
+	Some implementations send the third Main Mode message unencrypted, probably
+	to find the PSKs for the specified ID for authentication. This is very
+	similar to Aggressive Mode, and has the same security implications: A
+	passive attacker can sniff the negotiated Identity, and start brute forcing
+	the PSK using the HASH payload.
+
+	It is recommended to keep this option to no, unless you know exactly
+	what the implications are and require compatibility to such devices (for
+	example, some SonicWall boxes).
+
 charon.block_threshold = 5
 	Maximum number of half-open IKE_SAs for a single peer IP.