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.\" 	From: Id: mdX.3,v 1.14 1999/02/11 20:31:49 wollman Exp
.\" $FreeBSD: release/9.1.0/lib/libmd/sha256.3 223582 2011-06-27 02:10:10Z cperciva $
.\"
.Dd September 14, 2005
.Dt SHA256 3
.Os
.Sh NAME
.Nm SHA256_Init ,
.Nm SHA256_Update ,
.Nm SHA256_Final ,
.Nm SHA256_End ,
.Nm SHA256_File ,
.Nm SHA256_FileChunk ,
.Nm SHA256_Data
.Nd calculate the FIPS 180-2 ``SHA-256'' message digest
.Sh LIBRARY
.Lb libmd
.Sh SYNOPSIS
.In sys/types.h
.In sha256.h
.Ft void
.Fn SHA256_Init "SHA256_CTX *context"
.Ft void
.Fn SHA256_Update "SHA256_CTX *context" "const unsigned char *data" "size_t len"
.Ft void
.Fn SHA256_Final "unsigned char digest[32]" "SHA256_CTX *context"
.Ft "char *"
.Fn SHA256_End "SHA256_CTX *context" "char *buf"
.Ft "char *"
.Fn SHA256_File "const char *filename" "char *buf"
.Ft "char *"
.Fn SHA256_FileChunk "const char *filename" "char *buf" "off_t offset" "off_t length"
.Ft "char *"
.Fn SHA256_Data "const unsigned char *data" "unsigned int len" "char *buf"
.Sh DESCRIPTION
The
.Li SHA256_
functions calculate a 256-bit cryptographic checksum (digest)
for any number of input bytes.
A cryptographic checksum is a one-way
hash function; that is, it is computationally impractical to find
the input corresponding to a particular output.
This net result is
a
.Dq fingerprint
of the input-data, which does not disclose the actual input.
.Pp
The
.Fn SHA256_Init ,
.Fn SHA256_Update ,
and
.Fn SHA256_Final
functions are the core functions.
Allocate an
.Vt SHA256_CTX ,
initialize it with
.Fn SHA256_Init ,
run over the data with
.Fn SHA256_Update ,
and finally extract the result using
.Fn SHA256_Final .
.Pp
.Fn SHA256_End
is a wrapper for
.Fn SHA256_Final
which converts the return value to a 65-character
(including the terminating '\e0')
.Tn ASCII
string which represents the 256 bits in hexadecimal.
.Pp
.Fn SHA256_File
calculates the digest of a file, and uses
.Fn SHA256_End
to return the result.
If the file cannot be opened, a null pointer is returned.
.Fn SHA256_FileChunk
is similar to
.Fn SHA256_File ,
but it only calculates the digest over a byte-range of the file specified,
starting at
.Fa offset
and spanning
.Fa length
bytes.
If the
.Fa length
parameter is specified as 0, or more than the length of the remaining part
of the file,
.Fn SHA256_FileChunk
calculates the digest from
.Fa offset
to the end of file.
.Fn SHA256_Data
calculates the digest of a chunk of data in memory, and uses
.Fn SHA256_End
to return the result.
.Pp
When using
.Fn SHA256_End ,
.Fn SHA256_File ,
or
.Fn SHA256_Data ,
the
.Fa buf
argument can be a null pointer, in which case the returned string
is allocated with
.Xr malloc 3
and subsequently must be explicitly deallocated using
.Xr free 3
after use.
If the
.Fa buf
argument is non-null it must point to at least 65 characters of buffer space.
.Sh SEE ALSO
.Xr md2 3 ,
.Xr md4 3 ,
.Xr md5 3 ,
.Xr ripemd 3 ,
.Xr sha 3
.Sh HISTORY
These functions appeared in
.Fx 6.0 .
.Sh AUTHORS
The core hash routines were implemented by Colin Percival based on
the published
.Tn FIPS 180-2
standard.
.Sh BUGS
No method is known to exist which finds two files having the same hash value,
nor to find a file with a specific hash value.
There is on the other hand no guarantee that such a method does not exist.