back door n.
[common] A hole in the security of a system deliberately left
in place by designers or maintainers. The motivation for such
holes is not always sinister; some operating systems, for
example, come out of the box with privileged accounts intended
for use by field service technicians or the vendor's maintenance
programmers. Syn. trap door;
may also be called a `wormhole'. See also iron box, cracker, worm,
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 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. Your editor has heard two separate reports
that suggest that the crocked login did make it out of Bell Labs,
notably to BBN, and that it enabled at least one late-night login
across the network by someone using the login name `kt'.