Cryptography is hard. There exist people with full-time jobs, who are dedicated to breaking your cryptography. Enter Ledger Donjon.
Recent vulnerabilities discovered and responsibly disclosed by Ledger Donjon:
Charles Guillemet, CTO @ Ledger
Jean-Baptiste Bédrune, Head of Security Research @ Ledger
Victor Servant, Security Engineer @ Ledger
Pure Python Elliptic Curve library by Cedric Mesnil (co-founder @ Ledger): http://cslashm.github.io/ECPy