How Many Common, Unique Letters Do *You* Think There Are?

[Jorge_Stolfi]

That anyone who is literate can read without learning a different alphabet or cryptography technique.

I think that the definition is wider than that, no?  I would say that the "plaintext" is more like "the text that was meant to be hidden by the encryption".  For example, it could be a list of the coordinates of the enemy's submarines.  Only some naval intelligence guy will be able to "read" that, maybe.

And that supposes that the text was encrypted.  No one can read the Linear A tablets, but they are still "plaintext" because the authors were not trying to hide the text.

You know about the "Navajo encryption", right?  Would you consider a  transcript of one of their messages a "plaintext"?  If not, what would be the "plaintext"?

All the best, --stolfi