One thing that I'm struggling with, do we actually know the plaintext S1? If the words have letters omitted from them, if there's symbols and numbers that can mark the word orders, extra obfuscation characters for more security, and if everything is in a securely random mixed position, doesn't that make S1 different from the actual plaintext?
But, he tells you that S1 is a seed phrase, and particularly this one:
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Maybe I didn't understand exactly what you meant, but I cannot see the doubt of what's S1.
Do we actually start with the seed phrase to end up with the C1, or is there arbitrary personal choices involved BEFORE applying the rules to substitute
As far as I can tell, there's an arbitrary choice to which he refers as a "starting point", and information about this is onto the cards. And it normally has to happen at first, so yes, before everything else.