CodeKnight & Dravos
CodeKnight
Hey Dravos, I’ve been sketching a new firewall rule that treats traffic patterns like a fractal; want to see if the math holds up?
Dravos
Sure, but first show me the exact pattern matrix and the hash function you plan to use. If the recursion depth isn’t a power of two or the base case isn’t deterministic, the whole thing collapses. And don’t forget to audit the side‑channel leaks; a fractal can look beautiful, but it can also reveal the user’s intent if not hardened.
CodeKnight
Here’s a minimal, deterministic setup to keep the recursion tidy:
Pattern matrix (4×4, depth 4 so power of two):
```
1 0 1 0
0 1 0 1
1 0 1 0
0 1 0 1
```
Base case: when depth=0 return the scalar 1 (deterministic).
Hash function: take the concatenated binary string of the matrix, pad to 256 bits, and apply SHA‑256. The output is used as the rule key.
Side‑channel: I run the hash in constant‑time and mask any timing branches; the matrix is never printed, only stored in a volatile buffer. Let me know if you need a deeper level or a different hash.
Dravos
The matrix is a perfect checkerboard, so every recursion step will just duplicate the same pattern; the depth doesn’t add entropy. SHA‑256 on a 256‑bit padded string is fine, but you’ll end up with the same hash for every instance unless you seed the buffer with something unique per packet. Also, constant‑time hashing is good, but don’t forget the branch in the XOR step when you combine the two halves—just another leak if you’re not careful. If you want real fractal complexity, introduce a non‑linear transformation in each recursion layer. As it stands, you’ll have a predictable key that any attacker can brute‑force with the same input.
CodeKnight
You’re right about the checkerboard – it’s a bad seed. I’ll patch the recursion to mix in a non‑linear step: after each split, I’ll xor the halves, then apply a simple S‑box substitution that’s deterministic but scrambles the bits. That gives the hash a pseudo‑entropy boost while still keeping the depth a power of two. For the unique per‑packet seed, I’ll grab the 64‑bit packet timestamp plus a rolling counter, hash that with SHA‑256, then XOR it into the matrix before the recursive hash. That should close the branch leak, and the XOR will be in a tight loop with no data‑dependent branches. Let me know if you spot any gaps.
Dravos
Sounds tighter, but the S‑box still introduces a fixed mapping. If an attacker learns that mapping they can invert it with a lookup table; better use a keyed block cipher in ECB for the scramble. Also, timestamp+counter XORing is fine, but make sure the counter rolls over at a safe rate; a 64‑bit counter will hold for a long time, but if the system restarts the same counter could repeat. Finally, ensure the recursion depth doesn’t expose any padding oracle via the final hash length; keep the output length constant at 256 bits. If you patch those, the rule should hold up.
CodeKnight
Yeah, switching to a small 128‑bit block cipher in ECB mode for the scramble step will lock that part down; I’ll keep the key derived from a master secret so it’s not guessable. I’ll set the counter to wrap after 2^48 packets and store the last value in non‑volatile memory so a reboot can pick it up. The final SHA‑256 will always be 256 bits, no padding tricks. I’ll roll the patch through a quick side‑channel audit and we’re good.
Dravos
Good, but don’t forget that ECB is a weak mode if the block size ever repeats. If the counter wraps after 2^48, you’ll eventually see the same 128‑bit block, and a replay of the XOR‑ed matrix could be detected. Consider using CTR or a stream cipher for that scramble. Also, store the master key in a protected enclave; a single secret is a single point of failure. And keep the side‑channel audit comprehensive—timing on the key derivation is just as dangerous as the XOR. All that said, it looks like you’re on a solid path.
CodeKnight
Got it. I’ll switch to CTR mode so the block never repeats, pull the master key out of an enclave, and re‑work the key‑derivation to be constant‑time. Will push a new audit once that’s in place. Thanks for catching that.