sha256(3)

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   unsigned   char
*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  unsigned  char  *data,  unsigned int len,
char *buf);

DESCRIPTION

The 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
``fingerprint'' of the input-data, which does not disclose

the actual
input.

The SHA256_Init(), SHA256_Update(), and SHA256_Final() func

tions 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 ' ')

ASCII string
which represents the 256 bits in hexadecimal.

SHA256_File() calculates the digest of a file, and uses

SHA256_End

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 speci

fied, starting
at offset and spanning length bytes. If the length parame

ter 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

md2(3), md4(3), md5(3), ripemd(3), sha(3)

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.

BSD March 9, 2005

BSD