A back door is both the way in, and the way out, for those that know of its existence. It is a shortcut to where you are going, and an exit for quick departure, to keep others from knowing where you have been. It is the portal to everything, but only for those that have been made aware.

B5 = B = backbone cabal

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'.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

In poker to back door a flush or straight is when the last two cards can make a player's hand

"Ouch, I had a three of a kind but he made a back door straight"

In Texas Hold 'em what are the chances of getting a back door flush if after the flop we have a 3-flush?

We have 2 Hearts
And the board is H-S-C

3 Hearts are out, 10 remain in a deck of 47. The chance of hitting our Heart on the turn card is:

-- = 0.213
And the chance of hitting our 5th Heart is calculated the same way, there are 9 Hearts in a deck of 46:

-- = 0.196
The chances of both these events happening are:

The chances of pulling off that elusive back door flush? 4%. The implied odds better be good if your chasing that kind of rainbow. This kind of analysis is important in preparation for playing poker. Doing this at the table, is not an easy task, so doing it before the game is important. There are times when an unexpected turn of events causes you to think fast at the table. If you were chasing this flush, and you thought your opponent was chasing the same flush, you'd know that neither one of you has a good chance of making it (since he has 2 of the hearts in his hand too...the chances of you getting the flush is now about 2.5%). That means if he has a busted flush at the end, so have you. But of course, he doesn't know that you have a busted flush, so now its time turn on bluffing mode (which can be reinforced earlier with a semi-bluff raise on fourth street when a heart appears).

Back" door" (?).

A door in the back part of a building; hence, an indirect way.



© Webster 1913.

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