back door

back door n.  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 suggested the
   possibility of a back door in early UNIX versions that may have
   qualified as the most fiendishly clever security hack of all time
   (Thompson's presentation managed to be unclear on whether it was
   ever actually implemented or not).  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.