Flag Checker

Description

Flag Checker [Reverse Engineering]

We found this cryptic Python script that validates the user's flag, but we're having trouble understanding the code. Can you find the correct flag that passes through the program?

pyrev.py

Solution

Clean up the script and decrypt:

def enc(flag):
    if isinstance(flag, str): 
        flag = [(ord(c) - 27) ^ 15 for c in flag]
    else:
        flag = [(c - 27) ^ 15 for c in flag]

    return flag

def dec(flag):
    if isinstance(flag, str): 
        flag = [(ord(c) ^ 15) + 27 for c in flag]
    else:
        flag = [(c ^ 15) + 27 for c in flag]
        
    return flag

def swap(flag):
    a, b, c, d, e, f = 6, 17, 8, 10, 12, 9
    flag[a], flag[f] = flag[f], flag[a]
    flag[d], flag[c] = flag[c], flag[d]
    flag[b], flag[e] = flag[e], flag[b]
    return flag

def rev(flag):
    return flag[0: len(flag) // 2] + flag[len(flag) // 2: len(flag)][::-1]

def tostr(flag):
    return ''.join(map(chr, flag))


flag_enc = [55, 33, 52, 40, 35, 56, 86, 90, 66, 111, 81, 26, 23, 75, 109, 26, 88, 90, 75, 67, 92, 25, 87, 88, 92, 84, 23, 88]
flag = dec(flag_enc) # Step 1 
flag = rev(flag)     # Step 2
flag = swap(flag)    # Step 3
flag = tostr(flag)
print(flag)

Note: To reverse the encyption of the script actions must be done in backwards.

Flag: SIVBGR{pyth0n_r3v3rs1ng_pr0}