SYSV checksum

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

Newer checksum algorithms

The manual page of the GNU sum utility program (that implements the BSD checksum algorithm) states: "sum is provided for compatibility; the cksum program is preferable in new applications."

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 (disadvantages and advantages) of a simple sum:

• 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.

As a result, many common changes to text data are not detected by this method.

The last two lines of the algorithm reduce the total sum to a 16-bit number.

Sources

• official GNU sum manual page
• coreutils download page --- find and unpack the newest version of the coreutils package, read src/sum.c