[W/K] :: back door

  Back door \Back" door"\
     A door in the back part of a building; hence, an indirect
     way. --Atterbury.
     [1913 Webster]

  back door
       n 1: a secret or underhand means of access (to a place or a
            position); "he got his job through the back door"
       2: an entrance at the rear of a building [syn: back entrance]

  105 Moby Thesaurus words for "back door":
     French door, afterpart, afterpiece, archway, back, back road,
     back seat, back side, back stairs, back street, back way,
     backstairs, barway, behind, bolt-hole, breech, bulkhead, by-lane,
     bypass, bypath, byroad, bystreet, byway, carriage entrance,
     cellar door, cellarway, clandestine, covert, covert way, detour,
     door, doorjamb, doorpost, doorway, escalier derobe, escape hatch,
     escape route, feline, front door, furtive, gate, gatepost, gateway,
     hatch, hatchway, heel, hidlings, hind end, hind part, hindhead,
     hole-and-corner, hugger-mugger, lintel, occiput, porch, portal,
     porte cochere, posterior, postern, privy, propylaeum, pylon, quiet,
     rear, rear end, rearward, reverse, roundabout way, scuttle,
     secret exit, secret passage, secret staircase, shifty, side door,
     side road, side street, skulking, slinking, slinky, sly, sneaking,
     sneaky, stealthy, stern, stile, storm door, surreptitious, tail,
     tail end, tailpiece, threshold, tollgate, trap, trap door,
     turnpike, turnstile, under-the-counter, under-the-table,
     undercover, underground, underground railroad, underground route,
     underhand, underhanded, unobtrusive
  
  

  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 Turing lecture that suggested this truly moby hack was later
     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'.
  
  

  back door
       
           (Or "{trap door", "{wormhole}").  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.
          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.  The infamous RTM worm of late 1988, for example,
          used a back door in the BSD Unix "sendmail(8)" utility.
       
          Ken Thompson's 1983 Turing Award lecture to the ACM
          revealed the existence of a back door in early Unix versions
          that may have qualified as the most fiendishly clever security
          hack of all time.  The C compiler contained code that would
          recognise 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 *recognise
          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 recognise 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 revealed this truly moby hack was published as
          ["Reflections on Trusting Trust", "Communications of the ACM
          27", 8 (August 1984), pp. 761--763].
       
          [{Jargon File]
       
          (1995-04-25)