cc.h: Reorder the declarations.

[u/mdw/catacomb] / hashsum.1

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.TH hashsum 1 "29 July 2000" "Straylight/Edgeware" "Catacomb cryptographic library"
.SH NAME
hashsum \- compute and verify cryptographic checksums of files
.SH SYNOPSIS
.B hashsum
.RB [ \-f0ecbpv ]
.RB [ \-a
.IR algorithm ]
.RB [ \-E
.IR encoding ]
.IR files ...
.SH DESCRIPTION
The
.B hashsum
program generates and verifies cryptographic checksums (hashes) of
files.  A number of hashing algorithms are available.
.PP
The
.B hashsum
program's options and output were originally designed to be upwardly
compatible with the GNU
.BR md5sum (1)
program, but the two have diverged somewhat.  See the
.B "COMPATIBILITY NOTES"
section of this manual for details.
.PP
Usually,
.B hashsum
generates checksums of a collection of files named either on the command
line or read from standard input, and write their hashes to standard
output using a simple file format.  However, given the
.B \-c
option, it will read in files in its usual output format and verify that
the named files have the reported hashes.
.SS "Options"
The
.B hashsum
program understands the following options:
.TP
.B "\-h, \-\-help"
Prints a help message to standard output and exits successfully.
.TP
.B "\-V, \-\-version"
Prints the program's version number to standard output and exits
successfully.
.TP
.B "\-u, \-\-usage"
Prints a brief usage summary to standard output and exits successfully.
.TP
.BR "\-l, \-\-list " [ \fIitem ...]
Show lists of hash functions and encodings supported.
.TP
.BI "\-a, \-\-algorithm=" alg
Use the hash algorithm
.IR alg .
If this option is not given, a default hashing algorithm is selected:
see
.B "Hashing algorithms"
below.
.TP
.BI "\-E, \-\-encoding=" encoding
Use the given
.I encoding
to represent hashes in the output.  This is not interoperable with other
programs, but it's handy, e.g., for building sha1 URNs.  The encodings
recognized are
.B hex
(the default),
.B base64
and
.BR base32 .
Type
.B hashsum \-\-list enc
for a list of supported encodings.
.TP
.B "\-f, \-\-files"
Each input file is considered to be a list of filenames which should be
read and hashed.  By default, the filenames are considered to be
whitespace-separated, although control characters can be escaped (see
.B "Escaping control characters"
below).
.TP
.B "\-0, \-\-null"
In conjunction with the
.B \-f
option above, reads null-terminated filenames, as emitted by GNU
.BR find (1)'s
.B \-print0
option, rather than whitespace-delimited filenames.  If the
.B \-c
option is also given, each named in the list is a list of filename/hash
pairs to be checked.
.TP
.B "\-e, \-\-escape"
Escape control characters (see
.B "Escaping control characters"
below) in filenames when generating output.  Escaped
output is not compatible with
.BR md5sum (1),
but copes better with files containing newlines and other strange
control characters.
.TP
.B "\-c, \-\-check"
Check hashes.  Each input file is assumed to be in
.BR hashsum 's
output format.  It is read, and
.B hashsum
will verify that each named file has the correct hash.  Assuming that
the hash list is authentic (e.g., it has been digitally signed, or
obtained via some secure medium), this provides strong assurance that
the files listed have not been tampered with.
.TP
.B "\-b, \-\-binary"
Assume that the files to be hashed are binary files.  This doesn't make
any difference in Unix systems, although it might on other platforms
which draw a distinction.
.TP
.B "\-p, \-\-progress"
Display a progress indicator while hashing large files.  The progress
indicator is written to standard error.
.TP
.B "\-v, \-\-verbose"
In conjunction with the
.B \-c
option above, be verbose when checking files.
.PP
If no filenames are given on the command line, standard input is read.
Standard input does not have a filename.
.SS "Output format"
There are three types of line in
.BR hashsum 's
output format:
.IR directives ,
.IR "file lines" ,
and
.IR rubbish .
.PP
A
.I directive
begins with a hash
.RB (` # ')
character.  These directives are currently understood:
.TP
.BI "#hash " alg
Subsequent hashes in this file were generated using the algorithm
.IR alg .
.TP
.BI "#encoding " encoding
Subsequent hashes in this file are represented using the named
.IR encoding .
.TP
.BI "#escape"
Filenames in subsequence lines are written using the `escaped' format,
described below.
.PP
A
.I "file line"
consists of a hash, in the requested encoding, followed by a space, a
.IR flag ,
and the filename.  The
.I flag
is either a star
.RB (` * ')
to indicate that the file should be read in binary mode, or a space.
The rest of the line contains the filename.
.PP
A
.I rubbish
line is one which doesn't look like a directive or a file line.  Rubbish
lines are ignored.  Hence, you can apply PGP clear-signing to a
.B hashsum
file without preventing it from being read.
.SS "Escaping control characters"
When reading filenames to hash from a list of files or an escaped hash
list, the following rules are obeyed:
.hP \*o
An escaped string cannot contain unescaped, unquoted whitespace
characters.  If such a character is found, the string is considered to
have ended.
.hP \*o
A backslash
.RB (` \e ')
escapes the following character.  If the character is one of
.RB ` a ',
.RB ` b ',
.RB ` f ',
.RB ` n ',
.RB ` r ',
.RB ` t ',
or
.RB ` v ',
it is replaced by the control character for an audible alert, backspace,
form-feed, newline, carriage return, horizontal tab or vertical tab
respectively; other escaped characters are unchanged, although they lose
any special meaning they might have had.
.hP \*o
A section of text may be quoted by surrounding it by
.BR ' ... ' ,
.BR """" ... """" ,
or
.BR ` ... '
pairs.  Within a quoted section, whitespace characters may appear
unescaped.  The backslash may be used to quote control characters or the
quoting characters as usual.
.hP \*o
A word beginning with a hash
.RB (` # ')
character is considered to begin a
.I comment
which extends to the end of the current line.  The hash character may be
escaped as usual.
.SS "Hashing algorithms"
The
.B hashsum
program understands several hashing algorithms:
.TP
.BR md2
Designed by Ron Rivest, although I don't know when, and described in
RFC1319, MD2 is a really old and slow hash function.  Its security is
suspect too: only its checksum stands between it and collision-finding
attacks.  Use of MD2 is not recommended, though it's still used in
various standards.
.TP
.BR md4 " and " md5
Designed by Ron Rivest in 1990 and 1992 respectively and described in
RFCs 1186, 1320 and 1321, these two early hash functions are efficient
but cryptographically suspect: the MD4 algorithm has been shown not to
be collision-resistant and there are `pseudo-collisions' in MD5.
Despite this,
.B md5
has been used heavily since its introduction and is still popular.  MD4
is still useful when a fast non-cryptographic hash is wanted.
.TP
.B sha
Designed by the US National Security Agency as part of the Digital
Signature Standard, SHA-1 provides a longer output than
.B md4
and
.BR md5 ,
and is seen as being more secure.
.TP
.BR rmd128 ", " rmd160 ", " rmd256 " and " rmd320
Designed by Antoon Bosselaers, Hans Dobbertin and Bart Preneel in 1996
as a replacement for the earlier RIPEMD algorithm, RIPEMD160 provides
the same length output as SHA-1, but has been designed in the open by
experts.  RIPEMD28 is a shortened version of RIPEMD160 designed as a
drop-in replacement for MD4, MD5 and the old RIPEMD.  The 256 and
320-bit versions are efficient double-width extensions of the 128 and
160-bit hashes, although they may not offer any additional security.
.TP
.B tiger
Designed by Ross Anderson and Eli Biham to take advantage of 64-bit
processors, Tiger seems to be an efficient and strong hash function.
It's a relatively new algorithm, however, and should probably be
approached with an open-minded caution.
.TP
.BR sha256 ", " sha384 " and " sha512
Designed by the US National Security Agency to provide security
commensurate with the Advanced Encryption Standard, these hash functions
provide long outputs.  SHA-256 is fairly quick, though the longer
variants are slower on 32-bit hardware since they require 64-bit
arithmetic.  They're all very new at the moment, and should be
approached with an open-minded caution.
.PP
The default hashing algorithm is determined by looking at the name by
which it was invoked passed to it in
.BR argv[0] :
if it has the form
.RI ` alg \c
.BR sum '
where
.I alg
is the name of a hash function, that hash becomes the default.  (Hence,
.B hashsum
can be used as a drop-in replacement for
.BR md5sum (1).)
If the program name doesn't match an algorithm, then
.B md5
is selected for compatibility with files generated by
.BR md5sum (1).
.PP
Note that the same default algorithm is used for both generating new
output files and checking existing ones.  If the algorithm is forced by
the
.B \-a
option,
.B hashsum
will emit a
.RB ` #hash '
directive in its output.
.SH "COMPATIBILITY NOTES"
Once upon a time, there was only the
.BR md5sum (1)
utility.  As its name suggested, it calculated MD5 hashes of files.  MD5
was shown to be weak, so the author wrote
.B hashsum
to do the same job with other, hopefully stronger, hash functions.  The
original
.B hashsum
program tried hard to be compatible with GNU
.BR md5sum (1),
but the latter has itself changed in incompatible ways since then;
.B hashsum
has intentionally not changed to match.
.PP
The following
.B hashsum
features are not found in the GNU Coreutils hashing utilities.
.hP
Filename escaping (the
.B \-e
option).
.hP
Magic comment lines in hash data to indicate algorithm selection, hash
encoding, and filename escaping.
.hP
Base-64 and Base-32 output.
.PP
Other differences are as follows.
.hP
Originally, if GNU
.B md5sum
was invoked without any filename arguments, it would print only the hash
of its stdin to stdout, which was very convenient for scripts which
manipulate hashes in nontrivial ways.  This behaviour was later changed,
and now the GNU Coreutils hashing utilities always print a filename or
.RB ` \- '
after the hash.  The
.B hashsum
program follows the original
.B md5sum
behaviour, and doesn't print a filename if no files were listed on the
command line.
.SH "SEE ALSO"
.BR md5sum (1),
.BR dsig (1),
.BR catsign (1),
.BR catcrypt (1).
.SH "AUTHOR"
Mark Wooding, <mdw@distorted.org.uk>