NAME
SHA256_Init,
SHA256_Update, SHA256_Final,
SHA256_End, SHA256_File,
SHA256_FileChunk,
SHA256_Data —
calculate the FIPS 180-2 ``SHA-256''
message digest
LIBRARY
library “libmd”
SYNOPSIS
#include
<sys/types.h>
#include <sha256.h>
void
SHA256_Init(SHA256_CTX
*context);
void
SHA256_Update(SHA256_CTX
*context, const void
*data, size_t
len);
void
SHA256_Final(unsigned
char digest[32],
SHA256_CTX *context);
char *
SHA256_End(SHA256_CTX
*context, char
*buf);
char *
SHA256_File(const
char *filename, char
*buf);
char *
SHA256_FileChunk(const
char *filename, char
*buf, off_t offset,
off_t length);
char *
SHA256_Data(const
void *data, unsigned int
len, char
*buf);
DESCRIPTION
TheSHA256_ 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
“fingerprint” of the input-data, which does not disclose the
actual input.
The
SHA256_Init(),
SHA256_Update(),
and SHA256_Final() functions are the core functions.
Allocate an SHA256_CTX, initialize it with
SHA256_Init(), run over the data with
SHA256_Update(), and finally extract the result
using SHA256_Final().
SHA256_End()
is a wrapper for
SHA256_Final()
which converts the return value to a 65-character (including the terminating
'\0') ASCII string which represents the 256 bits in hexadecimal.
SHA256_File()
calculates the digest of a file, and uses
SHA256_End() to return the result. If the file
cannot be opened, a null pointer is returned.
SHA256_FileChunk()
is similar to SHA256_File(), but it only calculates
the digest over a byte-range of the file specified, starting at
offset and spanning length
bytes. If the length parameter is specified as 0, or
more than the length of the remaining part of the file,
SHA256_FileChunk() calculates the digest from
offset to the end of file.
SHA256_Data() calculates the digest of a chunk of
data in memory, and uses SHA256_End() to return the
result.
When using
SHA256_End(),
SHA256_File(), or
SHA256_Data(),
the buf argument can be a null pointer, in which case
the returned string is allocated with
malloc(3) and subsequently must be explicitly deallocated using
free(3) after use. If the buf argument is
non-null it must point to at least 65 characters of buffer space.
SEE ALSO
HISTORY
These functions appeared in FreeBSD 4.0.
AUTHORS
The core hash routines were implemented by Colin Percival based on the published FIPS 180-2 standard.
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.