POLYAS uses the block checksum to prevent cast votes in the ballot box from being manipulated. For this purpose, a block is formed after every 30 votes received. The votes are arranged at random, making it impossible to trace which vote was received at what time. The block is used to calculate an SHA-256 checksum.
The checksum is written to the electoral roll after each block. The second block includes the previous checksum. If a vote is manipulated, the checksum of the block and all subsequent checksums no longer match those of the ballot box.
Furthermore, every system has a checksum, so if a change had been made after the election was sealed, the checksum would change and manipulation would thus be apparent. After the selection, the verification tool can be used to inspect the checksums of the systems.