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[IEEE Std1003.21992 (``POSIX.2'')]
CKSUM(1) OpenBSD Reference Manual CKSUM(1)
NAME
cksum, sum  display file checksums and block counts
SYNOPSIS
cksum [o 1  2] [file ...]
sum [file ...]
DESCRIPTION
The cksum utility writes to the standard output three whitespace separat
ed fields for each input file. These fields are a checksum CRC, the to
tal number of octets in the file and the file name. If no file name is
specified, the standard input is used and no file name is written.
The sum utility is identical to the cksum utility, except that it de
faults to using historic algorithm 1, as described below. It is provided
for compatibility only.
The options are as follows:
o 1  2
Use historic algorithms instead of the (superior) default one.
Algorithm 1 is the algorithm used by historic BSD systems as the
sum(1) algorithm and by historic AT&T System V UNIX systems as
the sum algorithm when using the r option. This is a 16bit
checksum, with a right rotation before each addition; overflow is
discarded.
Algorithm 2 is the algorithm used by historic AT&T System V UNIX
systems as the default sum algorithm. This is a 32bit checksum,
and is defined as follows:
s = sum of all bytes;
r = s % 2^16 + (s % 2^32) / 2^16;
cksum = (r % 2^16) + r / 2^16;
Both algorithm 1 and 2 write to the standard output the same
fields as the default algorithm except that the size of the file
in bytes is replaced with the size of the file in blocks. For
historic reasons, the block size is 1024 for algorithm 1 and 512
for algorithm 2. Partial blocks are rounded up.
The default CRC used is based on the polynomial used for CRC error check
ing in the networking standard ISO 88023: 1989 The CRC checksum encod
ing is defined by the generating polynomial:
G(x) = x^32 + x^26 + x^23 + x^22 + x^16 + x^12 +
x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
Mathematically, the CRC value corresponding to a given file is defined by
the following procedure:
The n bits to be evaluated are considered to be the coefficients of
a mod 2 polynomial M(x) of degree n1. These n bits are the bits
from the file, with the most significant bit being the most signif
icant bit of the first octet of the file and the last bit being the
least significant bit of the last octet, padded with zero bits (if
necessary) to achieve an integral number of octets, followed by one
or more octets representing the length of the file as a binary val
ue, least significant octet first. The smallest number of octets
capable of representing this integer are used.
M(x) is multiplied by x^32 (i.e., shifted left 32 bits) and divided
by G(x) using mod 2 division, producing a remainder R(x) of degree
<= 31.
The coefficients of R(x) are considered to be a 32bit sequence.
The bit sequence is complemented and the result is the CRC.
The cksum and sum utilities exit 0 on success or >0 if an error occurred.
WARNING
Do not use sum or cksum to detect hostile binary modifications. An at
tacker can trivially produce backdoored daemons which have the same CRC
as the standard versions. Use a cryptographic checksum (such as MD5) in
stead.
SEE ALSO
md5(1), rmd160(1), sha1(1)
The default calculation is identical to that given in pseudocode in the
following ACM article.
Dilip V. Sarwate, "Computation of Cyclic Redundancy Checks Via Table
Lookup", Communications of the ACM, August 1988.
STANDARDS
The cksum utility is compliant with the IEEE Std1003.21992 (``POSIX.2'')
specification.
HISTORY
The cksum utility appeared in 4.4BSD.
OpenBSD 2.6 April 28, 1995 2
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts Institute of Technology, Free Software Foundation, FreeBSD Inc., and others. 
(Corrections, notes, and links courtesy of RocketAware.com)
FreeBSD Sources for cksum(1) OpenBSD sources for cksum(1)
Up to: Data integrity and Security, Checksums and Digests  cryptography, message digests, etc.
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[IEEE Std1003.21992 (``POSIX.2'')]
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