SHA256(3)              FreeBSD Library Functions Manual              SHA256(3)

NAME
     SHA224_Init, SHA224_Update, SHA224_Final, SHA224_End, SHA224_File,
     SHA224_FileChunk, SHA224_Data, SHA256_Init, SHA256_Update, SHA256_Final,
     SHA256_End, SHA256_File, SHA256_FileChunk, SHA256_Data - calculate the
     FIPS 180-2 ``SHA-256'' (or SHA-224) message digest

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

SYNOPSIS
     #include <sys/types.h>
     #include <sha224.h>

     void
     SHA224_Init(SHA224_CTX *context);

     void
     SHA224_Update(SHA224_CTX *context, const unsigned char *data,
         size_t len);

     void
     SHA224_Final(unsigned char digest[32], SHA224_CTX *context);

     char *
     SHA224_End(SHA224_CTX *context, char *buf);

     char *
     SHA224_File(const char *filename, char *buf);

     char *
     SHA224_FileChunk(const char *filename, char *buf, off_t offset,
         off_t length);

     char *
     SHA224_Data(const unsigned char *data, unsigned int len, char *buf);

     #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() 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(), which will also erase the
     SHA256_CTX.

     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.

     SHA224 is identical SHA256, except it has slightly different
     initialization vectors, and is truncated to a shorter digest.

ERRORS
     The SHA256_End() function called with a null buf argument may fail and
     return NULL if:

     [ENOMEM]           Insufficient storage space is available.

     The SHA256_File() and SHA256_FileChunk() may return NULL when underlying
     open(2), fstat(2), lseek(2), or SHA256_End(2) fail.

SEE ALSO
     md4(3), md5(3), ripemd(3), sha(3), sha512(3), skein(3)

HISTORY
     These functions appeared in FreeBSD 6.0.

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.

FreeBSD 13.1-RELEASE-p6          July 20, 2018         FreeBSD 13.1-RELEASE-p6