I thought about a device that can encrypt what you type character after character! You type a character it is encrypted directly between your keyboard and your computer. What you see on your screen is another character.
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So even if you have a keylogger malware you can send secure messages.
Are you saying that if I typed "x" that something other than "x" would be displayed? Or would the screen display "x"?
If it is the former, you may as well not display anything. And the arguments about the time to decrypt would be moot. If it is the latter, it would not actually prevent keyloggers. Someone with access to your computer would be able to know that you pressed "x". The decryption key would also need to be stored in RAM, so an attacker would be able to obtain the decryption key and decrypt messages. The only thing this setup would protect against would be a device that a keyboard is plugged into that plugs into your computer.