At first, I was quite confused by this challenge. The title was Japanese, and there was a YouTube link (but it didn’t seem to connect to the challenge). Once I opened the launcher, it was clear this was a blockchain challenge because it included an RPC URL, a private key, and a contract setup.
I’m used to seeing setups like this, meaning they require interacting with smart contracts. The problem was, I didn’t immediately understand the bug. So, to be honest, at this point, I asked ChatGPT for help analyzing the contract. From there, the problem began to become clear.
The contract handled the retransaction by storing the signature in the usedSignature variable. However, ECDSA signatures are malleable, meaning that for a valid signature (r, s, v), then a signature (r, N-s, v ⊕ 1), where N is the secp256k1 curve order, is also a valid signature and recovers to the same address, but with a different hash.
We use this exploit by reading the last signature from lastSignature and then computing a new signature that is the counterpart of the obtained signature. Submission with the counterpart signature will change the root, and the flag can be obtained.
FLAG
ARA7{isnt_signature_malleability_cool?_now_with_a_twist_of_validator_quorums}