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:
arena brisk seminar tool risk cat despair repeat seat property cattle later
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.
I mean, IF some part of the plaintext actually looks like this, I'm just making this up by giving an example:
to4ol%.
1prop0e#
se@mi3n
....
Upon deciphering a part of the given C1, you could see there's tool 4, prope 10 (for property), semin 3 (for seminar), and that would be enough for you to make out the rest of your 12 word seed phrase.
If the plaintext looks like above, then the challenge immediately becomes even harder, even to figure out how to go backwards. This would mean we are working with a more obfuscated, altered, reordered, and with missing parts of the actual plaintext (if smaller number of letters are used for each word).