May
01

## Google CTF – Jekyll (Crypto)

Can you access the admin page? You can look at the crypto here.

Summary: finding a preimage for a simple 64-bit ARX-based hash.

Here’s the code of the web server:

def jekyll32(data, seed): def mix(a, b, c): a &= 0xFFFFFFFF; b &= 0xFFFFFFFF; c &= 0xFFFFFFFF;   a -= b+c; a &= 0xFFFFFFFF; a ^= c >> 13 b -= c+a; b &= 0xFFFFFFFF; b ^=(a << 8)&0xFFFFFFFF c -= a+b; c &= 0xFFFFFFFF; c ^= b >> 13 a -= b+c; a &= 0xFFFFFFFF; a ^= c >> 12 b -= c+a; b &= 0xFFFFFFFF; b ^=(a << 16)&0xFFFFFFFF c -= a+b; c &= 0xFFFFFFFF; c ^= b >> 5 a -= b+c; a &= 0xFFFFFFFF; a ^= c >> 3 b -= c+a; b &= 0xFFFFFFFF; b ^=(a << 10)&0xFFFFFFFF c -= a+b; c &= 0xFFFFFFFF; c ^= b >> 15   return a, b, c   a = 0x9e3779b9 b = a c = seed length = len(data)   keylen = length while keylen >= 12: values = struct.unpack('<3I', data[:12]) a += values b += values c += values   a, b, c = mix(a, b, c) keylen -= 12 data = data[12:]   c += length   data += '\x00' * (12-len(data)) values = struct.unpack('<3I', data)   a += values b += values c += values   a, b, c = mix(a, b, c)   return c   def jekyll(data): return jekyll32(data, 0x60061e) | (jekyll32(data, 0x900913) << 32)   ... cookie = self.request.cookies.get('admin') if cookie is not None and jekyll(base64.b64decode(cookie)) == 0x203b1b70cb122e29: self.response.write('Hello admin!\n'+FLAG) else: self.response.write('Who are you?') ...

So we need to find preimage of 203b1b70cb122e29 with hash described by the jekyll function, which simply concatenates two calls to jekyll32 with different seeds.

The core of jekyll32 is the mix function. It takes three 32-bit workds and transforms them using ARX operations. Note that mix is easily invertible if we have all three values. However the jekyll32 function returns only the third value.

The message is processed in blocks of 12 bytes and is padded with at least one zero. Let’s see what we can do with one block. The hash then works like this:

$$\begin{split} jekyll32 & (m_1 || m_2 || m_3, seed) = \\ & mix( (\text{9e3779b9},\text{9e3779b9},seed + length) + (m_1, m_2, m_3) ). \end{split}$$

We can set some random values to the outputs $a, b$, and invert the $mix$ function. Then, we subtract the initial constants and deduce a message which results in the given triple $a, b, c$, where $c$ is equal to the 32-bit half of the hash. Now we can change the seed and compute the hash and check if it matches the other half. That is, we need $2^{33}$ evaluations of the $mix$ function.

However, there is a problem: at least one zero byte is added, so with one block we can control only 11 bytes. That is, when we invert the $mix$ function, we don’t control the least significant byte of the third word, which need to be equal to $seed + length$. Thus, we have to try $2^8$ times more. It is still doable, but takes quite a lot of time.

Let’s instead consider messages with two blocks. We won’t care about the second block, we will use only the fact that the first block is fully controlled by us. So we can actually let the second block be the zero pad. And the general scheme stays the same.

To sum up the attack:

• let $h_1, h_2$ be 32-bit halves of the target hash;
• choose random $a, b$;
• compute $t = mix^{-1}(a, b, h_1)$;
• subtract $length = 12$;
• compute $s = mix^{-1}(t – 12)$;
• deduce $m = s – (\text{9e3779b9},\text{9e3779b9},seed1)$;
• check if $jekyll32(m, seed_2) == h_2$.

We will have to repeat this around $2^{32}$ times, each time we do $4$ evaluations of $mix$ or $mix^{-1}$.

Here’s C++ code:

#include <bits/stdc++.h> // g++ brute.cpp -O3 -std=c++11 -o brute && time ./brute   struct State { uint32_t a, b, c; void mix() { a -= b+c; a ^= c >> 13; b -= c+a; b ^= a << 8; c -= a+b; c ^= b >> 13; a -= b+c; a ^= c >> 12; b -= c+a; b ^= a << 16; c -= a+b; c ^= b >> 5; a -= b+c; a ^= c >> 3; b -= c+a; b ^= a << 10; c -= a+b; c ^= b >> 15; } void unmix() { c ^= b >> 15; c += a+b; b ^= a << 10; b += c+a; a ^= c >> 3; a += b+c; c ^= b >> 5; c += a+b; b ^= a << 16; b += c+a; a ^= c >> 12; a += b+c; c ^= b >> 13; c += a+b; b ^= a << 8; b += c+a; a ^= c >> 13; a += b+c; } };   uint32_t STARTCONST = 0x9e3779b9; uint32_t LENGTH = 12; uint32_t SEED1 = 0x60061e; uint32_t SEED2 = 0x900913; uint32_t HASH1 = 0xcb122e29; uint32_t HASH2 = 0x203b1b70;   int main() { for(uint64_t a = 0; a < 1ll << 32; a++) { if ((a & 0xffffff) == 0) { printf("%08x\n", a); } State s = {a, 0x31337, HASH1}; s.unmix(); s.c -= LENGTH; // subtract message, but we set it to zeroes // so do nothing s.unmix();   uint32_t p; p = s.a - STARTCONST; p = s.b - STARTCONST; p = s.c - SEED1; s.a = p + STARTCONST; s.b = p + STARTCONST; s.c = p + SEED2; s.mix(); s.c += LENGTH; s.mix();   if (s.c == HASH2) { printf("GOOD: %08x %08x %08x\n", p, p, p); printf("PLAIN: "); for(int i = 0; i < 8; i++) printf("%02x", (char*)p + i); printf("\n"); } } return 0; }
GOOD: 5cc80e2e e7fee109 d6d486f1 PLAIN: 2e0ec85c09e1fee7f186d4d6 2m2.185s

The flag: CTF{diD_y0u_ruN_iT_0N_Y0uR_l4PtoP?}