Quote:
Originally Posted by Gudy
Not necessarily. One way to achieve this would be a two-step process where you encrypt each book with a key unique to that book, then encrypt the key needed to decrypt the book with the PID. If the mobipocket format has a fixed place to store the decryption key four times for each of the four PIDs allowed, the size doesn't need to change.
If you assume a 2048 bit key for encrypting/decrypting the book, then storing four encrypted versions of that key takes 1 kB, which isn't that much.
I don't know if this is what happens, but IIRC, this is how gnupg works if you encrypt an email for multiple recipients (except that gnupg doesn't have a fixed number of "slots", so each additional recipient you encrypt the message for enlarges the whole thing by a couple hundred bytes).
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There's a 16-byte primary key (the same across all Secure Mobipocket ebooks) and a 10-byte PID key. That gives 128 bits in the hidden key and 80 bits in the PID key. I suppose you should study how the various decryption python scripts handle it all.
Derek