Re: Not A Backdoor
I think you are only partly justified in saying that. Ofttimes a cryptographer*¹ has or guesses at a crib—some plain text he knows or shrewdly expects to have been included in the original message—and uses that as a lever to begin teasing out the key, thereafter decrypting the whole message. Indeed, feeding cribs into an adversary's information system can be helpful. Let Station X, known to be using Cipher69, learn of your grave concern about the ship "Wazottliqueeg" on its mission to deliver vital "Sponzagurgs" and hope that they soon after transmit a message to HQ (preferably triggering a chain of concerned conversations throughout their network) and you have seeded an unusual crib into his commo which just might help you crack his encryption.
Pursuing that example a little further, if you have introduced yourself, or are aware of, mathematical weaknesses in Cipher69 (NOT the same as knowing the key or having an alternate key, something I think not all commenters here have understood: sorry) then you are in a vastly better position to use those weaknesses and the crib to prise open the whole caboodle.
*¹ "Cryptographer" in this case being the mathematicians and coders who wrote a code-breaking program
Biting the hand that feeds IT
