The challenge give us a file "decode.me" with the following content:
> 554545532245{22434223_4223_42212322_55_234234313551_34553131423344}
I assumed that substitution cipher is used.
Trying to fit the content of the file in flag format *AFFCTF{...}*, I assumed that each group of 2 numbers stands for 1 character, and I replaced the numbers in this way:
55 -> A
53 -> C
45 -> F
22 -> T
A F F C T F { T 43 42 23_42 23_42 21 23  T _ A _23 42 34 31 35 51_34  A  31 31 42 33 44}
Then, I assumed that the first two words inside the parentheses are "THIS" and "IS".
So, I replaced the numbers in this way:
43 -> H
42 -> I 
23 -> S
A  F  F  C  T  F {T  H  I  S _I  S _I  21 S  T _A _S  I  34 31 35 51_34 A  31 31 I  33 44}
Going ahead by assuming the most probable characters, since this seems to be a kind of decreasing mapping, I replaced the remaining numbers like this:
51 -> E
44 -> G
35 -> L
34 -> M
33 -> N
31 -> P
21 -> U
The flag is: **AFFCTF{THIS_IS_IUST_A_SIMPLE_MAPPING}**