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A checksum is a data protection mechanism, usually intermediate in power between parity and a cyclic redundancy check. (In some cases the term 'checksum' is applied to what is techically a CRC.)

A checksum usually is simply the sum of all the data items in a block of data (hence the name). Some checksums use slightly more elaborate math; e.g. the checksums typically used in the TCP/IP protocol suite involve adding carries back in, and those used in PUP rotate the sum one position as each word is added in.