md5(1) - NetBSD Manual Pages

CKSUM(1)                    NetBSD Reference Manual                   CKSUM(1)


NAME

     cksum, md5, sum - display file checksums and block counts


SYNOPSIS

     cksum [-m | [-o 1 | 2]] [file ...]
     sum [file ...]
     md5 [-p] [-t] [-x] [-s string] [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 md5 utility takes as input a message of arbitrary length and produces
     as output a 128-bit ``fingerprint'' or ``message digest'' of the input.
     It is conjectured that it is computationally infeasible to product two
     messages having the same message digest, or to produce any message having
     a given prespecified target message digest.  The MD5 algorithm is intend-
     ed for digital signature applications, where a large file must be
     ``compressed'' in a secure manner before being encrypted with a private
     (secret) key under a public-key encryption system such as RSA.

     The options are as follows:

     -m      Use the MD5 algorithm rather than the default one.

     -o      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 following options apply only when using the MD5 algorithm:

     -s string
             Print the MD5 checksum of the given string string.

     -p      Echo input from standard input to standard output, and append an
             MD5 checksum.

     -t      Run a built-in MD5 time trial.

     -x      Run a built-in MD5 test script.

     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 encoding
     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, and >0 if an error occurs.


SEE ALSO

     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 expected to conform to IEEE Std 1003.2-1992
     (``POSIX.2'').


HISTORY

     The cksum utility appeared in 4.4BSD.

NetBSD 1.5                      April 28, 1995                               2