SYSV checksum

The SYSV checksum algorithm was a commonly used, legacy checksum algorithm. It has been implemented in UNIX System V and is also available through the sum command line utility.

This algorithm is useless on a security perspective, and is weaker than the CRC-32 cksum for error detection.

Description of the algorithm
The main part of this algorithm is simply adding up all bytes in a 32-bit sum. As a result, this algorithm has the characteristics of a simple sum: As a result, many common changes to text data are not detected by this method.
 * re-arranging the same bytes in another order (e.g. moving text from one place to another place) does not change the checksum.
 * increasing one byte and decreasing another byte by the same amount does not change the checksum.
 * adding or removing zero bytes does not change the checksum.

The FreeBSD pseudocode for this algorithm is:

The last part folds the value into 16 bits.