Anybody looking at this still that I can share my thought process with?
Surely 
Pretty easy challenge I should say, my phone calculator was enough to get a valid reverse equation
I know this is not so hard a challenge, but i’m going to start to be more active in the community starting from now
Anyone who need help, R is here
You have to use math and reverse the equotation.
The -18 can be done right away on the char as multiplication binds more then + or - so the first step is to substract 18.
Next step is to invert the modulus operation for which you have to find the so calles modulus inverse.
That means a mod b can be solved by finding the equivalent of a mod m = b where a*b = 1 mod m also known as a^-1
See: Online calculator: Modular Multiplicative Inverse Calculator
You will find out the inverse modulus for char * 123 % 256 is char * 179 % 256.
So here is a proper code to solve it:
def dec(msg):
pt = []
for char in msg:
# inverse modulus of 123 is 179
char = (179 * (char - 18)) % 256
pt.append(char)
return bytes(pt)
with open('msg.enc') as f:
ct = bytes.fromhex(f.read())
pt = dec(ct)
print(pt)
I really wonder what calculator you have that allows you to run through the advanced euclidian algorithm to find the multiplative inverse? xD
Android’s calculator is overpowered, I know 