An integrity protection method to verify data, by using transmission errors. A checksum is an extra byte attached to data that is to be stored or transmitted. A checksum does not ensure transmitted data is secure, only that it is intact.
A sender simply divides all the data up into two-character numbers, then adds all the numbers together. The receiver makes the same calculation, and checks the calculated checksum with the transmitted checksum. If the figure does not match the data has been corrupted.