The BSD checksum algorithm was a commonly used, legacy checksum algorithm. It has been implemented in old BSD and is also available through the sum command line utility.
This algorithm is useless from a security perspective, and is weaker than the CRC-32cksum for error detection.[1][2]
Below is the relevant part of the GNU 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.
intbsdChecksumFromFile(FILE*fp)/* The file handle for input data */{intchecksum=0;/* The checksum mod 2^16. */for(intch=getc(fp);ch!=EOF;ch=getc(fp)){checksum=(checksum>>1)+((checksum&1)<<15);checksum+=ch;checksum&=0xffff;/* Keep it within bounds. */}returnchecksum;}
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)
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:
checksum, algorithm, commonly, used, legacy, checksum, algorithm, been, implemented, also, available, through, command, line, utility, this, algorithm, useless, from, security, perspective, weaker, than, cksum, error, detection, contents, computation, descript. The BSD checksum algorithm was a commonly used legacy checksum algorithm It has been implemented in old BSD 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 cksum for error detection 1 2 Contents 1 Computation of the BSD checksum 2 Description of the algorithm 3 References 4 SourcesComputation of the BSD checksum EditBelow is the relevant part of the GNU 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 int checksum 0 The checksum mod 2 16 for int ch getc fp ch EOF ch getc fp checksum checksum gt gt 1 checksum amp 1 lt lt 15 checksum ch checksum amp 0xffff Keep it within bounds return checksum Description of the algorithm EditAs 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 Input 101110001110 gt three segments 1011 1000 1110 Iteration 1 segment 1011 checksum 0000 bitmask 1111 a Apply circular shift to the checksum 0000 gt 0000 b Add checksum and segment together apply bitmask onto the obtained result 0000 1011 1011 gt 1011 amp 1111 1011 Iteration 2 segment 1000 checksum 1011 bitmask 1111 a Apply circular shift to the checksum 1011 gt 1101 b Add checksum and segment together apply bitmask onto the obtained result 1101 1000 10101 gt 10101 amp 1111 0101 Iteration 3 segment 1110 checksum 0101 bitmask 1111 a Apply circular shift to the checksum 0101 gt 1010 b Add checksum and segment together apply bitmask onto the obtained result 1010 1110 11000 gt 11000 amp 1111 1000 Final checksum 1000References Edit sum 1 manual pages from GNU coreutils sum 1 FreeBSD General Commands ManualSources Editofficial FreeBSD sum source code official GNU sum manual page GNU sum source code Retrieved from https en wikipedia org w index php title BSD checksum amp oldid 1132117581, wikipedia, wiki, book, books, library,
