Editing Checksum (section)

===Parity byte or parity word===
The simplest checksum algorithm is the so-called [[longitudinal redundancy check|longitudinal parity check]], which breaks the data into "words" with a fixed number {{math|''n''}} of bits, and then computes the bitwise [[exclusive or]] (XOR) of all those words.  The result is appended to the message as an extra word. In simpler terms, for {{math|''n''}}=1 this means adding a bit to the end of the data bits to guarantee that there is an even number of '1's. To check the integrity of a message, the receiver computes the bitwise exclusive or of all its words, including the checksum; if the result is not a word consisting of {{math|''n''}} zeros, the receiver knows a transmission error occurred.<ref>{{Cite web |last=Fairhurst |first=Gorry |date=2014 |title=Checksums & Integrity Checks |url=https://erg.abdn.ac.uk/users/gorry/eg3576/checksums.html |access-date=March 11, 2022 |archive-date=April 8, 2022 |archive-url=https://web.archive.org/web/20220408011213/https://erg.abdn.ac.uk/users/gorry/eg3576/checksums.html |url-status=live }}</ref>

With this checksum, any transmission error which flips a single bit of the message, or an odd number of bits, will be detected as an incorrect checksum.  However, an error that affects two bits will not be detected if those bits lie at the same position in two distinct words.  Also swapping of two or more words will not be detected. If the affected bits are independently chosen at random, the probability of a two-bit error being undetected is {{math|1/''n''}}.