The whitepaper is currently under construction.
Please see the related item in the issue tracker for further details & to join the discussion/work: hyperledger-cacti#2691
To access the old version of the whitepaper, you can use an older commit of the
mainbranch, such as this one: https://github.com/hyperledger-cacti/cacti/blob/7bb39576080592919bea0ac89646b32105e1748e/whitepaper/whitepaper.md