A full video walkthrough can be found here: https://youtu.be/OPYt55LVwSM?t=1162

Solution:

The server is doing a XOR with some ciphertext and a 16byte key.

To get the key we can load up gdb and run:

x/s keygen(0xdeedbeef)

Then we can copy the ciphertext and key into a python file and do the xor:

key = [10, 52, 237, 211, 5, 251, 97, 153, 224, 185, 168, 241, 13, 233, 3, 198]
data = [
    73,
    91,
    131,
    180,
    119,
    154,
    21,
    236,
    140,
    216,
    220,
    152,
    98,
    135,
    112,
    230,
    101,
    90,
    205,
    183,
    96,
    152,
    19,
    224,
    144,
    205,
    193,
    159,
    106,
    201,
    119,
    174,
    111,
    20,
    139,
    191,
    100,
    156,
    79,
    185,
    183,
    209,
    193,
    157,
    104,
    201,
    112,
    174,
    101,
    70,
    153,
    243,
    109,
    158,
    19,
    252,
    192,
    208,
    219,
    209,
    116,
    134,
    118,
    180,
    42,
    71,
    154,
    182,
    96,
    143,
    65,
    235,
    133,
    206,
    201,
    131,
    105,
    199,
    9,
    204,
    96,
    87,
    153,
    181,
    126,
    169,
    82,
    239,
    133,
    203,
    219,
    152,
    99,
    142,
    92,
    175,
    121,
    107,
    139,
    166,
    107,
    164,
    5,
    246,
    191,
    212,
    199,
    131,
    104,
    200,
    126,
    0,
]
import pwn

print(pwn.xor(data, key))

b'Congratulations on decrypting the flag. While short here is your sweet reward.\n\njctf{R3versing_is_fun_do_more!}\xc6'