CKSUM(1) OpenBSD Reference Manual CKSUM(1)NAME
cksum, sum - display file checksums and block counts
SYNOPSIScksum [-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
defaults 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.
Additionally, 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 supported 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
supported 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
additional 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
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
checking 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
significant 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 value, 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.
EXIT STATUS
The cksum and sum utilities exit 0 on success or >0 if an error occurred.
SEE ALSOmd5(1), sha1(1), sha256(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
appeared 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 4.9 November 7, 2010 OpenBSD 4.9
