.TH LIBKECCAK_ZEROCOPY_DIGEST 3 LIBKECCAK
.SH NAME
libkeccak_zerocopy_digest - Complete the hashing of a message without copying
.RB ( ADVANCED )
.SH SYNOPSIS
.LP
.nf
#include <libkeccak.h>

void libkeccak_zerocopy_digest(struct libkeccak_state *\fIstate\fP, void *\fImsg\fP, size_t \fImsglen\fP,
                               size_t \fIbits\fP, const char *\fIsuffix\fP, void *\fIhashsum\fP);
.fi
.P
Link with
.IR -lkeccak .
.SH DESCRIPTION
The
.BR libkeccak_zerocopy_digest ()
function absorbs the last part of (or all of) a message,
and returns the hash of the entire message. The last part
of the message is specified by the
.I msg
parameter, and its byte-size is specified by the
.I msglen
parameter. If all of the message has already be processed
by calls to the
.BR libkeccak_zerocopy_update (3)
function (with the same pointer on
.IR state ,)
.I msg
and
.I msglen
should be set to
.I NULL
and 0, respectively.
.PP
If the message is not comprised a whole number of bytes,
the number of bits, modulus 8, in the message should be
specified in the
.I bits
parameter.
.I msglen
must only count the number of whole bytes, that is, the
floor of the number of bits in the message divided by 8.
.PP
.I suffix
should be a NUL-terminated string of ASCII '1':s and '0':s,
representing the bits that should be appended to the
message. If this string is empty,
.I NULL
may be used instead. This is used to select hash algorithm.
For pure Keccak,
.I NULL
or \(dq\(dq is used. For the other algorithms the constants
.B LIBKECCAK_SHA3_SUFFIX
(for SHA-3),
.B LIBKECCAK_RAWSHAKE_SUFFIX
(for RawSHAKE), and
.B LIBKECCAK_SHAKE_SUFFIX
(for SHAKE), or the return of the
.BR libkeccak_cshake_suffix (3)
function (for cSHAKE), are used.
.PP
The hash of the message will be stored to
.IR hashsum ,
unless
.IR hashsum
is
.IR NULL
(which increases the performance of the call.) A total of
.RI (( state->n
+ 7) / 8) bytes will be written to the beginning of
.IR hashsum .
Therefore,
.I hashsum
needs at least an allocation size of that number of bytes.
.PP
.BR libkeccak_zerocopy_digest ()
will write at and beyond
.IR &msg[msglen] .
The caller must make sure that enough memory is allocated
for the
.I suffix
as well as padding of at least 2 bits, for
.IR msg .
The sum of
.IR msglen ,
the bits specified in
.IR suffix ,
and the padded, shall, in bytes, be an integer multiple of
the bitrate divided by eight, which is returned by the
.BR libkeccak_zerocopy_chunksize (3)
function.
.SH RETURN VALUES
The
.BR libkeccak_zerocopy_digest ()
function does not return a value.
.SH ERRORS
The
.BR libkeccak_zerocopy_digest ()
function cannot fail.
.SH NOTES
Calling the
.BR libkeccak_zerocopy_digest (3)
function after the
.BR libkeccak_update (3)
or
.BR libkeccak_fast_update (3)
functions, with the same
.I state
argument, may cause the message to be misread.
.SH NOTES
For cSHAKE, the
.BR libkeccak_cshake_initialise (3),
must be called, once, immediately after
state initialisation; before the first
call to any of the
.BR libkeccak_fast_update (),
.BR libkeccak_zerocopy_update (),
.BR libkeccak_update (),
and
.BR libkeccak_zerocopy_digest ()
functions.
.SH SEE ALSO
.BR libkeccak_state_initialise (3),
.BR libkeccak_cshake_initialise (3),
.BR libkeccak_zerocopy_chunksize (3),
.BR libkeccak_fast_update (3),
.BR libkeccak_zerocopy_update (3),
.BR libkeccak_update (3),
.BR libkeccak_cshake_suffix (3),
.BR libkeccak_digest (3),
.BR libkeccak_zerocopy_digest (3),
.BR libkeccak_simple_squeeze (3),
.BR libkeccak_fast_squeeze (3),
.BR libkeccak_squeeze (3)