feat(presenting): add a warning about authenticate-and-encrypt

content/tips/presenting.md

@@ -76,6 +76,11 @@ Resulting in:
 Moreover, you have access to the whole latex ecosystem, especially those for
 neat illustrations such as [tikzpingus].
 
+**Note:** I have to say that the above technique is unsafe under fairly
+reasonnable assumptions. Long story short you should sign first *then* encrypt
+and not do both in parallel. Please see [this paper](https://ia.cr/2001/045)
+from Crypto’01 if you want a more detailed explanation.
+
 ### Customisation
 
 It is also quite easy to customise slides with beamer. For instance, with