Checksum

A checksum is a small block of data derived from a larger block of digital data so that errors introduced during transmission or storage can be detected. A sender or storage system computes the checksum with a checksum function and keeps it beside the data; a receiver or reader computes it again and compares, and a mismatch shows that the data changed. Checksums verify integrity but are not relied on to verify authenticity. The simplest form is the longitudinal parity check, which combines fixed-size words with a bitwise exclusive or. Position-dependent designs such as Fletcher's checksum, Adler-32 and cyclic redundancy checks also weigh where each word sits, so they catch reordered or inserted words. Check digits and parity bits are special cases for short values, and some error-correcting codes build on checksums so the original data can be recovered.