I think you're misusing the word "obscure" here. Private keys are not obscure, they're private. You can steal them but you can hardly reverse-engineer them. Finding how a specific model of ATM works is an obscure piece of information but it's not private: people who sell,buy,repair, maintain,design this ATM have this piece of information. Giving enough will and effort one can reverse engineer this information from the ATM itself.
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I think you're misusing the word "obscure" here. Private keys are not obscure, they're private. You can steal them but you can hardly reverse-engineer them. Finding how a specific model of ATM works is an obscure piece of information but it's not private: people who sell,buy,repair, maintain,design this ATM have this piece of information. Giving enough will and effort one can reverse engineer this information from the ATM itself.