1. Check that to get the flag we must sign WITHDRAW in a multisignature together with the bank.

2. The multisignature is done by computing an aggregated key from the composition of the two keys using the binary group operation.

3. Use a so-called rogue-key attack to forge a signature that looks like it was signed by both users.

Original writeup (https://sectt.github.io/writeups/RealWorld19/crypto_bank/README).