VERYBABYREV

fun fact: verybabyrev backwards is verybabyrev

• File : verybabyrev

Here the flag typed is xored xith itself and compared with a string in the code :

compare[0] = flag[0] ^ flag[1]
for n > 1 :
compare[n] = compare[n-1] ^ flag[n]

We extract the compared string and knowing the first letter ('r') we can xor again each character with its neighbour and decipher the flag.

• File : solve.py

import struct

cmp = [
0x45481d1217111313,
0x095f422c260b4145,
0x541b56563d6c5f0b,
0x585c0b3c2945415f,
0x402a6c54095d5f00,
0x4b5f4248276a0606,
0x6c5e5d432c2d4256,
0x6b315e434707412d,
0x5e54491c6e3b0a5a,
0x2828475e05342b1a,
0x060450073b26111f,
0x0a774803050b0d04
]

cmp = b''.join([ struct.pack('<Q',elt) for elt in cmp ])

result = 'r'

for i,elt in enumerate(cmp):
    result += chr(elt^ord(result[i]))

print(result)

$ python solve.py
rarctf{3v3ry_s1ngl3_b4by-r3v_ch4ll3ng3_u535_x0r-f0r_s0m3_r34s0n_4nd_1-d0nt_kn0w_why_dc37158365}

flag : rarctf{3v3ry_s1ngl3_b4by-r3v_ch4ll3ng3_u535_x0r-f0r_s0m3_r34s0n_4nd_1-d0nt_kn0w_why_dc37158365}