ROT-ROT encryption algorithm

[half geek]

In my infinite boredom, I was inspired to write a useless "encryption" algorithm.
It is named ROT-ROT after the infinitely useful ROT-26.

Your job is to write a function that can decrypt any message encrypted with the ROT-ROT encryption algorithm. There's the trick: you need to find out how it works before you can do anything with it.

You can write the solution in any language, as long as I can read it and run it on my Linux box.

The winners will get a free hug, cookie, AND gold star at the Championship Event in Atlanta, Georgia. All travel and related costs not inluded.

The character set used in these examples:
['a','b','c','d','e','f','g','h','i','j','k','l','m ','n','o','p','q','r','s','t','u','v','w','x','y', 'z','_']

Example 1 (They are ni longer what?):
wrvuukonfmqpzltzfypojqdcuurqck

Example 2 (What is the meaning of life?):
tovsrjqqihcqjuxwxa_eslburqidrqdihjxqgyb

Example 3 (What is the airspeed velocity of an unladen swallow?):
h_ggzyaetxjmdcyeiahlbaia_hzyyctaccpobsrvoesgkkx


P.S. The solution is indeed deceivingly simple.
P.P.S. I will probably post clues in the next week if nobody gets it.

[half geek]

Who is good at numerical derivatives?

[half geek]

Numerical derivatives are a hint.

Another hint:
A=0 and _=26

Maybe roboticists are not big on useless encryption...