SHELL CTF - rev/Tea [268]

SHELL CTF 2022

A beginner-friendly CTF, Hosted By S.H.E.L.L

We got the 11 position on the scoreboard under team name 0xCha0s

And got first blood on this challange ;)

tea

analyzing the main function it ask the user for the flag then call another 4 functions

addSugar();
addTea();
addMilk();
strainAndServe();

walking threw each one of the them

addSugar

spilt odd index chars from the even index chars then concatinate them together

addTea

will perform some kind of subtraction encryption

addMilk

will split the string and concatinate it with diffrent order

strainAndServe

check if the result is correct

writing python script to reverse all of this

enc_flag = [104, 108, 96, 99, 78, 89, 101, 96, 93, 109, 37, 53, 48, 103, 89, 104, 117, 103, 111, 119, 126, 51, 52, 105, 82, 59, 99, 114, 99, 55, 53, 105]

for i in range(32 >> 1):
    enc_flag[i] -= 3 * int(i / -2)

for i in range(32 >> 1, len(enc_flag)):
    enc_flag[i] -= int(i / 6)


flag_l = enc_flag[0: len(enc_flag) // 2]
flag_r = enc_flag[len(enc_flag) // 2:]

flag = ''
for i in range(16):
    flag += chr(flag_r[i]) + chr(flag_l[i])

print(flag)

flag shellctf{T0_1nfiNi7y_4nD_B3y0nd}