64 Bit Safe

Steps

1. Open Ciphertext
2. Read hint, understand an 8-byte key is required. Also understand that it is a cyclic XOR problem
3. XOR first 7 bytes of the ciphertext with the first 7 known letters of the flag: fsuCTF{
4. Find missing last byte (letter) needed for the cyclic XOR
5. Write python script to automate XOR
6. Obtain flag

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Detailed Explanation (Including Screenshots)

The first thing I did was open the ciphertext: It gives this string:

161b1031353e125d401850533e493659475b3a460d14365a185b3a10551b59030d

From the hint I gathered that this is a cyclic XOR problem, so I can likely find the key by XORing the first 8 bytes with what I know from the flag.

We know the flag is in the format fsuCTF{…}, since I don’t know what’s inside, I can use the beginning: fsuCTF{ to find it:

The ASCII codes are:

f: 0x66
s: 0x73
u: 0x75
C: 0x43
T: 0x54
F: 0x46
{: 0x7b

I can XOR these with the first bytes of the ciphertext:

0x16 XOR 0x66 = p
0x1b XOR 0x73 = h
0x10 XOR 0x75 = e
0x31 XOR 0x43 = r
0x35 XOR 0x54 = a
0x3e XOR 0x46 = x
0x12 XOR 0x7b = i

I wrote a python script that does the cyclic XOR with the key “pheraxi”

Output:

I’m obviously missing the last byte of the key, so I modified my script to cycle the last letter from a-z until I find the flag:

Output:

It looks like m was the missing character giving the key “pheraxim” The flag is fsuCTF{00p5!_1_473_4ll_7h3_b4c0n}

I submitted and verified the flag