The outset of the edge of cipher

This is an entry into the 16th Fortnightly Challenge :

Person 1: Look at the paper.

Person 2: Yes, I see, I see, but I don't know what cipher this is.

Person 1: Okay, let me give you a hint: The outset of airplane becomes the edge of airspace.

Person 2: What does that even mean?

Person 1: And the opposite.

Person 2: What opposite?

Person 1: The outset of backyard is outset.

Person 2: What?

Person 1: And the opposite.

Person 2: Seriously, what does that mean?

Person 1: The outset of car is the edge of plane.

Person 2: I have no idea what this means.

Person 1: Remember, and the opposite is true, too!

Person 2: Okay, only now did I get it. I know what cipher this is.

Sadly, you do not have the previously mentioned paper. Can you find what cipher this is?

• Well, ROT-13 kinda works for the last example, but not really for the others :(
– user14478
Commented Sep 24, 2016 at 15:29
• Not yet, I must be missing something obvious... But I might give it another try soon because of the bounty (if no one else solved it by then) :P
– user14478
Commented Sep 27, 2016 at 21:37

As mentioned by Lukas Rotter in comments, this could be something to do with

the simple ROT13 cipher.

Specifically, it could work as follows.

"Outset" and "edge" both refer to the initial letter of a word, while "[X] is [Y]" means that the transformed/enciphered version of X is Y and "[X] becomes [Y]" means that the transformed/enciphered version of X becomes Y when the cipher is applied again.

The outset of airplane becomes the edge of airspace.

The enciphered version of A(irplane) is N, which becomes A(irspace) again upon re-enciphering.

The outset of backyard is outset.

The enciphered version of B(ackyard) is O(utset).

The outset of car is the edge of plane.

The enciphered version of C(ar) is P(lane).

And the opposite!

Unlike most things in life, the ROT13 cipher is self-reversing: if you do it twice, you get back to where you started. So C enciphering to P, for instance, means that P enciphers to C.

• Damn it, I thought that edge was supposed to be the first and last x letters from the word, especially considering that car becomes pne when ROT-13-ing. Many others and I really missed the obvious, since OP also referred to the first letter of a sentence as "margin" in another puzzle. :P
– user14478
Commented Sep 29, 2016 at 11:22
• Congrats, rand al'thor! @LukasRotter, hopefully you'll have better luck next time! Commented Sep 29, 2016 at 12:17