door; may also be called a `wormhole'. See also [927]iron box,
[928]cracker, [929]worm, [930]logic bomb.

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
[931]http://www.acm.org/classics). Ken Thompson has since confirmed
that this hack was implemented and that the Trojan Horse code did