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SHA256(3)		 BSD Library Functions Manual		     SHA256(3)

     SHA256_Init, SHA256_Update, SHA256_Final, SHA256_End, SHA256_File,
     SHA256_FileChunk, SHA256_Data -- calculate	the FIPS 180-2 ``SHA-256''
     message digest

     Message Digest (MD4, MD5, etc.) Support Library (libmd, -lmd)

     #include <sys/types.h>
     #include <sha256.h>

     SHA256_Init(SHA256_CTX *context);

     SHA256_Update(SHA256_CTX *context,	const unsigned char *data,
	 size_t	len);

     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);

     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 in-
     put corresponding to a particular output.	This net result	is a
     "fingerprint" of the input-data, which does not disclose the actual in-

     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	ex-
     tract 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 re-
     turned.  SHA256_FileChunk() is similar to SHA256_File(), but it only cal-
     culates 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 argu-
     ment can be a null	pointer, in which case the returned string is allo-
     cated with	malloc(3) and subsequently must	be explicitly deallocated us-
     ing free(3) after use.  If	the buf	argument is non-null it	must point to
     at	least 65 characters of buffer space.

     md4(3), md5(3), ripemd(3),	sha(3),	sha256(3), sha512(3), skein(3)

     These functions appeared in FreeBSD 6.0.

     The core hash routines were implemented by	Colin Percival based on	the
     published FIPS 180-2 standard.

     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				April 26, 2016				   BSD


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