CKSUM(1)                   OpenBSD Reference Manual                   CKSUM(1)

NAME
     cksum, sum - display file checksums and block counts

SYNOPSIS
     cksum [-bpqrtx] [-a algorithms] [-c [checklist ...]] [-o 1 | 2]
           [-s string] [file ...]
     sum [-bpqrtx] [-a algorithms] [-c [checklist ...]] [-o 1 | 2] [-s string]
           [file ...]

DESCRIPTION
     The cksum utility writes to the standard output a single line for each
     input file.  The format of this line varies with the algorithm being used
     as follows:

     cksum   The output line consists of three whitespace separated fields: a
             CRC checksum, the number of octets in the input, and name of the
             file or string.  If no file name is specified, the standard input
             is used and no file name is written.

     sum     The output line consists of three whitespace separated fields: a
             CRC checksum, the number of kilobytes in the input, and name of
             the file or string.  If no file name is specified, the standard
             input is used and no file name is written.

     sysvsum
             The output line consists of three whitespace separated fields: a
             CRC checksum, the number of 512-byte blocks in the input, and
             name of the file or string.  If no file name is specified, the
             standard input is used and no file name is written.

     all others
             The output line consists of four whitespace separated fields: the
             name of the algorithm used, the name of the file or string in
             parentheses, an equals sign, and the cryptographic hash of the
             input.  If no file name is specified, the standard input is used
             and only the cryptographic hash is output.

     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:

     -a algorithms
             Use the specified algorithm(s) instead of the default (cksum).
             Supported algorithms include cksum, md4, md5, rmd160, sha1,
             sha256, sha384, sha512, sum, and sysvsum.  Multiple algorithms
             may be specified, separated by a comma or whitespace.  Addition-
             ally, multiple -a options may be specified on the command line.
             Case is ignored when matching algorithms.  The output format may
             be specified on a per-algorithm basis by using a single-character
             suffix, e.g. ``sha256b''.  If the algorithm has a `b' suffix, the
             checksum will be output in base64 format.  If the algorithm has
             an `x' suffix, the checksum will be output in hex format.  If an
             algorithm with the same output format is repeated, only the first
             instance is used.  Note that output format suffixes are not sup-
             ported for the cksum, sum and sysvsum algorithms.

     -b      Output checksums in base64 notation, not hexadecimal by default.
             A `b' or `x' suffix on the algorithm will override this default.
             This option is ignored for the cksum, sum and sysvsum algorithms,
             which do not use hexadecimal output.

     -c [checklist ...]
             Compares all checksums contained in the file checklist with newly
             computed checksums for the corresponding files.  Output consists
             of the digest used, the file name, and an OK or FAILED for the
             result of the comparison.  This will validate any of the support-
             ed checksums.  If no file is given, stdin is used.  The -c option
             may not be used in conjunction with more than a single -a option.

     -o 1 | 2
             Use historic algorithms instead of the (superior) default one
             (see below).

     -p      Echoes stdin to stdout and appends the checksum to stdout.

     -q      Only print the checksum (quiet mode) or if used in conjunction
             with the -c flag, only print the failed cases.

     -r      Reverse the format of the hash algorithm output, making it match
             the checksum output format.

     -s string
             Prints a checksum of the given string.

     -t      Runs a built-in time trial.  Specifying -t multiple times results
             in the number of rounds being multiplied by 10 for each addition-
             al flag.

     -x      Runs a built-in test script.

     Algorithm 1 (aka sum) is the algorithm used by historic BSD systems as
     the sum 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 (aka sysvsum) 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 re-
     placed 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 en-
     coding 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 other available algorithms are described in their respective man
     pages in section 3 of the manual.

     The cksum and sum utilities exit 0 on success or >0 if an error occurred.

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 Std 1003.1-2008 (``POSIX'')
     specification.

     All the flags are extensions to that specification.

HISTORY
     A sum command appeared in Version 2 AT&T UNIX.  The cksum utility ap-
     peared in 4.4BSD.

CAVEATS
     Do not use the cksum, md4, md5, sum, or sysvsum algorithms to verify file
     integrity.  An attacker can trivially produce modified payload that has
     the same checksum as the original version.  Use a cryptographic checksum
     instead.

OpenBSD 4.7                     August 16, 2009                              3
