Redundancy check

A redundancy check is extra symbols added to a message for the purposes of error detection or error correction.

A check digit is a digit used as a redundancy check. The UPC barcode has a check digit computed by multiplying digits by alternately 3 and 1 and adding them up. For instance:

070617006092
0+0+1+0+6+9 =6
 7+6+7+0+0+2=2
3*6+2=0,

so the check digit, which is the 2 at the end, is correct.

A longitudinal redundancy check is computed by exclusive-oring all the bytes in a message.

A cyclic redundancy check is computed by shifting an accumulator left, inserting the next data byte, and dividing the accumulator by a binary polynomial.