Rating:
# Creds : Ariana, TuanLinh
# 1 Find ISITDTU position :
- This is easiest part, just need to find in the enc array that enc[i] == enc[i+2] and enc[i+3] == enc[i+5] (I = I and T = T), the start pos of ISITDTU is 36
- The S,T,U is near each other on the alphabet, so we can base on this to find m
# 2 Find m :
- We calculate the m1 and m2 :
m1 = enc[39]-enc[37] = 1110321085421447768275945874294757311370451552696385093861149689
So m1 = enc[39] - enc[37] = enc(T) - enc(S) > 0
And ord("T") - ord("S") = 1 or T > S
Call F(x) = (m * ord(x) + i)
Because m1 > 0, we can assume that F(T) and F(S) in same mod round.
=> m = m1
# 3 Find n :
m2 = enc[42] - enc[41] = F(U) - F(T) = -1204876429695607233906200723727474340332744346831302520996263862
So F(U) - F(T) < 0 mean F(U) is overflowed, that's why it result F(U) < F(T) when U > T
We know F(U) is overflowed, assume that it's 1 round above, then we got :
n = m2 - m1 = 2315197515117055002182146598022231651703195899527687614857413551
# 4 Find c :
c = n*k + enc[37] - ( m * ord("S"))
=> bruteforce k : k = 41
=> c = 1449370084268958114154753941529504723862204623391963277996853964
# 5 Generate alphabet and got flag :
