Historically, back doors have often lurked in systems longer than
anyone expected or planned, and a few have become widely known.
Ken Thompson's 1983 Turing Award lecture to the ACM admitted the
existence of a back door in early Unix versions that may have
qualified as the most fiendishly clever security hack of all time.
In this scheme, the C compiler contained code that would recognize
when the `login' command was being recompiled and insert some
code recognizing a password chosen by Thompson, giving him entry to
the system whether or not an account had been created for him.

Normally such a back door could be removed by removing it from the
source code for the compiler and recompiling the compiler. But to
recompile the compiler, you have to *use* the compiler -- so
Thompson also arranged that the compiler would *recognize when
it was compiling a version of itself*, and insert into the
recompiled compiler the code to insert into the recompiled
`login' the code to allow Thompson entry -- and, of course, the
code to recognize itself and do the whole thing again the next time
around! And having done this once, he was then able to recompile
the compiler from the original sources; the hack perpetuated itself
invisibly, leaving the back door in place and active but with no
trace in the sources.

The talk that suggested this truly moby hack was published as
"Reflections on Trusting Trust", "Communications of the ACM
27", 8 (August 1984), pp. 761--763 (text available at
http://www.acm.org/classics). Ken Thompson has since
confirmed that this hack was implemented and that the Trojan Horse
code did appear in the login binary of a Unix Support group
machine. Ken says the crocked compiler was never distributed.