A Key Is A Metal Password sticker

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While it's not strictly true (a key is more of a token than a password), they are both authenticators, and there's actually not that much difference between them. After all, if you write down your password on a slip of paper and then forget it, it becomes something you have rather than something you know. Likewise, if you memorize the bitting on a key, then with the right tools you could push the pins in a lock up the right amount from memory.