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First, we went about determining what the circuit evaluated. It was easy to figure out that the circuit was evaluating equality by running the program. However it was much harder to understand exactly what the binary was doing, that is, whether the ELF actually implemented the protocol as expected.

The binary was Rust and a mess to reverse engineer thanks tokio , so we instead looked at the network traffic between a client and server both run locally. Guessing that the protocol was implemented as we understood it, we examined the packets sent and received by the client.

After running the client multiple times with the same and then different secrets but with the same instance of the server running we could see that there was little change in the data we assumed to be the garbled circuit. This allowed us to infer that the server uses the same garbled circuit multiple times. One key aspect of the garbled circuit protocol is that, in its general form, it is single use only.

Here, the multiple uses of the same garbled circuit and the same secret allow us to input multiple secrets that can leak information about servers encrypted input. However, the problem with this attack is that it is necessary to see the intermediate gate outputs of the circuit, not just the final output. So to figure out the gate outputs we decided to step through the binary as it was evaluating the circuit, which necessitated finding where in the binary it did this. After several failed approaches and too much time staring at decompiled Rust we figured out we could target it at where the evaluation decrypts the table entries for each gate.

Using gdb we found where the program decrypted each table entry, and broke after each decryption. Notice how the third line has bits without obvious pattern. As one can see, the third line has padding 0xпїЅ This allows us to identify it as a successful decryption.

This is where we noticed something interesting. In the gate output in the case with same valued initial inputs i. This behavior was unexpected, and seems pretty intentional to save us the trouble of having to reverse the circuit. From here, the challenge was easy. Itching to know what other players tried and how they reached the flag? But the trick, of course, is figuring out who these hackers are.

In CTF, this is done through cut-throat competition. We select pre-qualifying events according to several considerations. We always look for quality events that present a variety of interesting challenges to their participants. We look for both prominent events with an established history and promising up-and-comers.

And we have an additional metric: connecting the various global hacker communities.