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}**