Conlangs as secret communication styles

I recently got to thinking about this subject, after (re-)discovering one of the
earliest known Conlang projects: the 12th Century Lingua Ignota, which was
apparently never intended for general release. The subject fascinates me --
sort of the opposite end of the spectrum from auxlang projects (especially
Esperanto, the rights to which were released for general usage, rather than
kept for the author's benefit.) Or sci-fi/fantasy languages -- which may not
necessarily be released for public consumption like Klingon has been, but at
least have sample texts available in whatever fictional works they belong to.
 
I've heard of examples of conlangs created specifically for the author's own diary
(for the express purpose of keeping the contents secret.) I've also heard
rumors that security agencies (read: NSA or CIA in the United States) create
conlangs as a form of cipher.
 
Has anybody on the list created a "secret language" such as this? I suppose this
might be counter-productive to a list like this (where the entire purpose is
sharing your work,) but I wouldn't be surprised if the practice is relatively
widespread ...
 
:Chris
 
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