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 16-bit
             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 32-bit 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 8802-3: 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 n-1. 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 32-bit 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 pseudo-code 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 Std1003.2-1992 (``POSIX.2'')
     specification.

HISTORY
     The cksum utility appeared in 4.4BSD.

OpenBSD 2.6                     April 28, 1995                               2