The BSD checksum algorithm was a commonly used, legacy

checksum A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify data ...

algorithm. It has been implemented in old

BSD The Berkeley Software Distribution or Berkeley Standard Distribution (BSD) is a discontinued operating system based on Research Unix, developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berke ...

and is also available through the sum command line utility. This algorithm is useless from a security perspective, and is weaker than the

CRC-32 A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to digital data. Blocks of data entering these systems get a short ''check value'' attached, based on ...

cksum cksum is a command in Unix and Unix-like operating systems that generates a checksum value for a file or stream of data. The cksum command reads each file given in its arguments, or standard input if no arguments are provided, and outputs the f ...

for error detection. — manual pages from

GNU GNU () is an extensive collection of free software (383 packages as of January 2022), which can be used as an operating system or can be used in parts with other operating systems. The use of the completed GNU tools led to the family of operat ...

coreutils The GNU Core Utilities or coreutils is a package of GNU software containing implementations for many of the basic tools, such as cat, ls, and rm, which are used on Unix-like operating systems. In September 2002, the ''GNU coreutils'' were ...

Computation of the BSD checksum

Below is the relevant part of the

sum source code ( GPL licensed). It computes a 16-bit checksum by adding up all bytes (8-bit words) of the input data stream. In order to avoid many of the weaknesses of simply adding the data, the checksum accumulator is circular rotated to the right by one bit at each step before the new char is added. int bsdChecksumFromFile(FILE *fp) /* The file handle for input data */

Description of the algorithm

As mentioned above, this algorithm computes a checksum by segmenting the data and adding it to an accumulator that is circular right shifted between each summation. To keep the accumulator within return value bounds, bit-masking with 1's is done. Example: Calculating a 4-bit checksum using 4-bit sized segments (

big-endian In computing, endianness, also known as byte sex, is the order or sequence of bytes of a word of digital data in computer memory. Endianness is primarily expressed as big-endian (BE) or little-endian (LE). A big-endian system stores the most si ...

)
Input: 101110001110 -> three segments: 1011, 1000, 1110. ''Iteration 1:'' segment: 1011 checksum: 0000 bitmask: 1111 a) Apply circular shift to the checksum: 0000 -> 0000 b) Add checksum and segment together, apply bitmask onto the obtained result: 0000 + 1011 = 1011 -> 1011 & 1111 = 1011 ''Iteration 2:'' segment: 1000 checksum: 1011 bitmask: 1111 a) Apply circular shift to the checksum: 1011 -> 1101 b) Add checksum and segment together, apply bitmask onto the obtained result: 1101 + 1000 = 10101 -> 10101 & 1111 = 0101 ''Iteration 3:'' segment: 1110 checksum: 0101 bitmask: 1111 a) Apply circular shift to the checksum: 0101 -> 1010 b) Add checksum and segment together, apply bitmask onto the obtained result: 1010 + 1110 = 11000 -> 11000 & 1111 = 1000 ''Final checksum:'' 1000

References

{{Reflist

Sources

official FreeBSD sum source code

coreutils download page
--- find and unpack the newest version of the coreutils package, read src/sum.c Checksum algorithms