I'm not asking you about the meaning of a common idiom. I'm only asking about what you mean. What are you trying to say.
> I think the standard lay person on a jury would not necessarily be open to a defense attorney attempting to explain that the word “decode” in goes against their base understanding of the word.
If they're not "open" to understand what Base64-decoding means, then they're not open to understand what the defendant did. Maybe that's true. If so it's a very deep indictment of the process. If what you are saying is true, the judge should not allow the prosecution to use the word "decode."
> To use a metaphor—if a state building is closed, but a person with certain knowledge knows that the windows on the building are easily opened
We don't need a metaphor. I used another actual example (not metaphor) of a encoding -- Morse code. A person who takes a transmission in Morse code and writes it down on paper has decoded it.
Is decoding Morse code allowed "without authorization"? Do I have a right to broadcast information on my HAM radio and then demand prosecution of any people who decode it?
It seems you would argue yes. Or else "no, but only because juries have heard of Morse but not Base64."
If the state, in this case, had used strings of "." and "-" to encode the data in Morse code, instead of Base64, do you think they would have the same case?
Or what if they had used Greek instead of English? (I think if you insisted to use a metaphor for encodings, that would actually be a good one.)