Hacktivitycon 2021

TRIFORCE

Solved By : choco

This is a CBC AES encryption (Chained Block Cipher)

Suppose the plain text P is 64 bits long It is then split into 4 parts (16 bit each)

There is an initializer iv that is 16 bits long and key k (16 bits long) The cipher text for first part of P is

C0 = E(P0 ^ iv,k) and following chain goes like this Ci = E(Pi ^ Ci-1,k) i > 0

The decryption does like this

Where P0 = D(C0,k) ^ iv and following decryption goes Pi = D(Ci,k) ^ Ci-1

If the key k and vi are same, a vulnerability arises During decryption, P0 = D(C0,k) ^ k P1 = D(C1,k) ^ C0 P2 = D(C2,k) ^ C1

if we replace C2 with C0 P2 = D(C0,k) ^ C1

xor the result with any of the two inputs will give the other input D(C0,k) = P2 ^ C1 P0 = D(C0,k) ^ k k = D(C0,k) ^ P0 therefore, k = P2 ^ C1 ^ P0 so since we know P0,P2 and C1, we can easily find k

Hence all we need to do is to replace the 3rd block of ciphertext with the first block of cipher text, Xor the result of 3rd plain text block with 2nd cipher text block and xor that result with 1st plaintext block to get the key

Do this three times with the different keys above to get the flag input: 616161616161616161616161616161616161616161616161616161616161616161616161616161616161616161616161

Code for replacing ciphertext:

def listToString(s): 
    str1 = ""   
    for ele in s: 
        str1 += ele   
    return str1 

def chunk(in_string,num_chunks):
    chunk_size = len(in_string)//num_chunks
    if len(in_string) % num_chunks: chunk_size += 1
    iterator = iter(in_string)
    for _ in range(num_chunks):
        accumulator = list()
        for _ in range(chunk_size):
            try: accumulator.append(next(iterator))
            except StopIteration: break
        yield ''.join(accumulator)
        
string = "e5cc7f05f279ca152d3fbd32d899b55fec8de5d9ae3d62bac51195ddedc0863462021d3c51a4a56555ca95ff4303b764a02a2f44327cb2007d46de117900720a"
l = list(chunk(string,4))
l[2] = l[0]
print(l)
out = listToString(l)
print(out)

code for getting hex for part of flag:

def chunk(in_string,num_chunks):
    chunk_size = len(in_string)//num_chunks
    if len(in_string) % num_chunks: chunk_size += 1
    iterator = iter(in_string)
    for _ in range(num_chunks):
        accumulator = list()
        for _ in range(chunk_size):
            try: accumulator.append(next(iterator))
            except StopIteration: break
        yield ''.join(accumulator)

string = "6227616161616161616161616161616161616161616161616161616161616161e8c6e5dfb46432e2c2499084e899d462e6af4534aed0627d75f825c096970f36"
l = list(chunk(string,4))
c = "ec8de5d9ae3d62bac51195ddedc08634"

print(hex(int(l[2],16)))

d = int(l[2],16) ^ int(c,16)
print(hex(d))
e = d ^ int(l[0],16)
print(hex(e)) ![](https://i.imgur.com/TgyoAnU.jpg)

do this and replace each cipher for each key each keys in hex = 666c61677b3831396639643864383337 32316163346334343262313635396633 36646632647d20202020202020202020

flag: **flag{819f9d8d83721ac4c442b1659f36df2d}**