The ciphertext of this crypto challenge is contained in the file decode.me.
The content of the file is:
```
554545532245{22434223_4223_42212322_55_234234313551_34553131423344}
```
knowing the format of the flag which is "AFFCTF{}, i started from it mapping the first letters:
* A=55
* F=45
* C=53
* T=22

Analyzing the mapping i understood that it was something similar to a decreasing mapping starting from 55, but with some gaps and i mapped the remaining letters taking the numbers in pairs and, with the following substitutions i came up with the right.

* E=51
* G=44
* H=43
* I=42
* L=35
* M=34
* N=33
* P=31
* S=23
* U=21

With these substitution the correct key is: **AFFCTF{THIS_IS_IUST_A_SIMPLE_MAPPING}**