To modify a past block, an attacker would have to redo the proof-of-work of the block and all blocks after it and then catch up with and surpass the work of the honest nodes.
Ukuguqula ibhulokhi edlule, umhlaseli kuzofanela enze kabusha ubufakazi bokusebenza kwebhulokhi nawo wonke amabhulokhi ngemuva kwawo bese ehlangabezana futhi edlula umsebenzi wezindawo ezithembekile.