ProSign 3

Big kudos to ConnorM on the Cryptohack Discord for the help. This challenge is super sneaky, as the implementation looks very sound, and it bears great resemblance to the example of the Python-ecdsa module. I learnt two lessons from this. First, please do code fuzzing carefully - I was very close to the actual solution but simply missed the crucial idea. Second, do not make assumptions about one’s code - vulnerabilities can start from something very silly....