.Dd June 6, 2004
.Dt SHA1 256
.Os
.Sh NAME
.Nm sha256
.Nd calculate a message-digest fingerprint (checksum) for a file
.Sh SYNOPSIS
.Nm sha256
.Op Fl pqrtx
.Op Fl s Ar string
.Op Ar
.Sh DESCRIPTION
The
.Nm sha256
utility takes as input a message of arbitrary length and produces as
output a
.Dq fingerprint
or
.Dq message digest
of the input.
It is conjectured that it is computationally infeasible to
produce two messages having the same message digest, or to produce any
message having a given prespecified target message digest.
The
.Th SHA-256
algorithm is intended for digital signature applications, where a
large file must be
.Dq compressed
in a secure manner before being encrypted with a private
(secret)
key under a public-key cryptosystem such as
.Tn RSA .
.Pp
The following options may be used in any combination and must
precede any files named on the command line.
The hexadecimal checksum of each file listed on the command line is printed
after the options are processed.
.Bl -tag -width indent
.It Fl s Ar string
Print a checksum of the given
.Ar string .
.It Fl p
Echo stdin to stdout and append the checksum to stdout.
.It Fl q
Quiet mode - only the checksum is printed out.
Overrides the
.Fl r
option.
.It Fl r
Reverses the format of the output.
This helps with visual diffs.
Does nothing
when combined with the
.Fl ptx
options.
.It Fl t
Run a built-in time trial.
.It Fl x
Run a built-in test script.
.El
.Sh DIAGNOSTICS
The
.Nm sha256
utility exits 0 on success,
and 1 if at least one of the input files could not be read.
.Sh SEE ALSO
.Xr cksum 1 ,
.Xr md5 1
.Rs
.%A J. Burrows
.%T The Secure Hash Standard
.%O FIPS PUB 180-2
.Re
.Rs
.%A D. Eastlake and P. Jones
.%T US Secure Hash Algorithm 1
.%O RFC 3174
.Re
.Pp
Secure Hash Standard (SHS):
.Pa http://csrc.nist.gov/cryptval/shs.html .
.Sh ACKNOWLEDGMENTS
This program is placed in the public domain for free general use by
RSA Data Security.