Starting from the name of the challenge we can suppose that it was encrypted with a shift cipher.
Knowing the format of the flag which is "AFFCTF{}" i analized the first part of the ciphertext which is "AGHFXK".
Seeing that the first mapping is A=A and the second G=F i tried a shift that started from 0 and it's increased by one at each iteration.
What i obtained wasn't what i expected, so i tried not to count the number values which are not encrypted and in that case the index is not increased.
Implementing this in a python program i came up with the right key which is: **"AFFCTF\{5ub5t1tut10n\}".**

```
ciphertext="AGHFXK5ai5b1cee10z"
alphabet = "abcdefghijklmnopqrstuvwxyz"
plaintext=""
i=0
ct=ciphertext.lower()
for l in ct:
if(l==str(5) or l==str(1) or l==str(0)):
plaintext+=l
continue
index=alphabet.find(l)
plaintext+=alphabet[index-i]
i=i+1

print(plaintext)
```