Writeup: FlareOn 2021: 002 - known

1. TLDR

2. Input data

The challenge file is here. Password: flare.

The available files are:

  UnlockYourFiles.exe
  Files/capa.png.encrypted           
  Files/cicero.txt.encrypted         
  Files/commandovm.gif.encrypted     
  Files/critical_data.txt.encrypted  
  Files/flarevm.jpg.encrypted        
  Files/latin_alphabet.txt.encrypted 

3. File analysis

In the file we find a base64 encoded string that is a hint:

$ echo KD4wXzApPiBJdCdzIGRhbmdlcm91cyB0byBh
ZGQrcm9yIGFsb25lISBUYWtlIHRoaXMgPCgwXzA8KQo= | base64 -d
(>0_0)> It's dangerous to add+ror alone! Take this <(0_0<)

4. Code analysis

We find the function responsible for decrypting a packet of 8 bytes:

The decryption algorithm was implemented as follows:

p[i] = rol(key[i] ^ c[i], i) - i

The transformation yields an encryption algorithm:

c[i] =  ror(p[i] + i, i) ^ key[i]

It is therefore necessary to recover the 8-byte key

5. Reading the flag

We know from the file names that one of the encrypted files contains the Latin alphabet. We can therefore find the key by looking for one that corresponded to a known ciphertext.

I therefore implemented an encryption function:

def enrypt(plaintext, key):
    max_bits = 8
    ror = lambda val, r_bits, max_bits: \
        ((val & (2**max_bits-1)) >> r_bits%max_bits) | \
        (val << (max_bits-(r_bits%max_bits)) & (2**max_bits-1))
    i = 0
    ciphertext = []
    for (p,k) in zip(plaintext, key):
        c = ror(ord(p) + (i%8), (i%8), max_bits) ^ k
        ciphertext.append(c)
        i = i+1
    return ciphertext

I then wrote code that finds the next bytes of the key:

def read_first_qword_buffer(filename):
    buffer = None
    with open(filename, 'rb') as f:
        buffer = f.read(8)
    return buffer

encrypted_file_header = read_first_qword_buffer('Files/latin_alphabet.txt.encrypted')

key = []
ciphertext = None

for i in range(0,8):
    for key_char_candidate in range(0,256):
        key_candidate = key + [key_char_candidate]
        ciphertext = enrypt('ABCDEFGH', key_candidate)
        if all(a==b for (a,b) in zip(ciphertext, encrypted_file_header)):
            key = key_candidate
            break

print(''.join([chr(k) for k in key ]))

As a result of the script, the keys were recovered:

No1Trust

After running the UnlockYourFiles.exe program and entering the key, the encrypted files were recovered:

  Files/capa.png           
  Files/cicero.txt         
  Files/commandovm.gif     
  Files/critical_data.txt  
  Files/flarevm.jpg        
  Files/latin_alphabet.txt 

In the file critical_data.txt there was a flag:

(>0_0)> You_Have_Awakened_Me_Too_Soon_EXE@flare-on.com <(0_0<)