[All WriteUps for this CTF here](https://github.com/ipv6-feet-under/WriteUps-S.H.E.L.L.CTF21): https://github.com/ipv6-feet-under/WriteUps-S.H.E.L.L.CTF21![S.H.E.L.L.CTF](https://raw.githubusercontent.com/ipv6-feet-under/WriteUps-S.H.E.L.L.CTF21/main/banner.png)
# Subsi
In this challenge we are given an encrypted flag:```HITSS{5X65Z1ZXZ10F_E1LI3J}```
and a python script that contains the encryption algorithm:```pythonalpha = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ{}_1234567890'key   = 'QWERTPOIUYASDFGLKJHZXCVMNB{}_1234567890'
text = <flag>
def encrypter(text,key):    encrypted_msg = ''    for i in text:        index = alpha.index(i)        encrypted_msg += key[index]    # print(encrypted_msg)    return encrypted_msg```
The algortihm looks like a simple substitution of characters. Even the key that was used is given.With that information we can put the information in a decoder and decrypt the flag:
![subsi](https://github.com/ipv6-feet-under/WriteUps-S.H.E.L.L.CTF21/blob/main/Cryptography/Subsi/subsi.png?raw=true)
The flag can now be turned in:```SHELL{5U65T1TUT10N_C1PH3R}```