This is a standalone HMAC-SHA-256 implementation used to compute an
HMAC-SHA-256 message authentication code. The tool has originally
been developed as a second implementation for Libgcrypt to allow
comparing against the primary implementation and to be used for
internal consistency checks. It should not be used for sensitive data
because no mechanisms to clear the stack etc are used.
The code has been written in a highly portable manner and requires
only a few standard definitions to be provided in a config.h file.
hmac256 is commonly invoked as
hmac256 "This is my key" foo.txt
This compute the MAC on the file foo.txt using the key given on
the command line.