back door

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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].

back door

A secret way to take control of a computer. Also called "trap doors," back doors are built into software by the original programmer, who can gain access to the computer by entering a code locally or remotely. For example, a back door in an application would enable a person to activate either normal or hidden functions within the software. A back door in an operating system would provide access to all system functions in the computer. See Easter Egg, Back Orifice and one-way hash function.
References in periodicals archive ?
This system is penalising the provincial finalists which is unfair, and they're also doing their best to penalise the back-door guys as well," he claimed.
Surely not through that back-door system after he couldn't win a constituency because the nominations had closed.
The profit potential is huge - but deregulating this state-of-the-art technology for export could put a back-door key in the front pocket of spies and terrorists around the world.
The film's central conceit - a Bill Gates-coded software tycoon engineers a series of high-profile superhacks to foment hysteria over the threat of information terrorists, then exploits this fear to sell his Gatekeeper security software to banks, multinational corporations, and the government, allowing him unlimited back-door access to the system - resonates with the current "convergence of the commercial and the military sectors" in the war against data diddling.
And is the back-door mechanism in the "Loft" form - i.
So let's be clear that back-door prohibition is prohibition nonetheless," Johnston noted.