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The blocking checksum is a checksum which controls the integrity of the collected data of such kind.
Usage of block checksums in POLYAS online voting systems
In online voting a block checksum is used for checking the cast ballots. The input ballots are sorted into blocks and comprise their own block checksum. Each new block is built on the basis of the previous block. The ballots inside one block are this way clearly sorted according to random classification. These classifications make sure that the allocation to time slots at which the ballots were cast, as well as the attribution to voter identites, is impossible. A block checksum is built on the basis of a collective checksum of the election. The additional goal of the checksum is this ballot block.
Checking the correct counting of the online ballot box with the help of block checksums
If you want to recount your ballot box, in order to verify the storage of ballots, you can also use the chekcsum of the election system as as well as the block checksum of the ballots. The block checksums have to match with the new ballot counting in order and value. If the block checksum matches the data from the ballot, the election result is verified.
POLYAS online voting system checks automatically for data integrity by means of block checksums and only provides the election result once it has been verified. You can find further information on separate verification of online election results with an independent tool here.