.\"
.\" Copyright (C) 2006 Red Hat, Inc. All Rights Reserved.
.\" Copyright (C) 2016 Intel Corporation. All rights reserved.
.\"
.\" This program is free software; you can redistribute it and/or
.\" modify it under the terms of the GNU General Public License
.\" as published by the Free Software Foundation; either version
.\" 2 of the License, or (at your option) any later version.
.\"
.TH KEYCTL_DH_COMPUTE 3 "07 Apr 2016" Linux "Linux Key Management Calls"
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.SH NAME
keyctl_dh_compute \- Compute a Diffie-Hellman shared secret or public key
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.SH SYNOPSIS
.nf
.B #include <keyutils.h>
.sp
.BI "long keyctl_dh_compute(key_serial_t " private ", key_serial_t " prime ,
.BI "key_serial_t " base ", char *" buffer ", size_t " buflen ");"
.sp
.BI "long keyctl_dh_compute_alloc(key_serial_t " private,
.BI "key_serial_t " prime ", key_serial_t " base ", void **" _buffer ");"
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.SH DESCRIPTION
.BR keyctl_dh_compute ()
computes a Diffie-Hellman public key or shared secret.  That computation is:
.IP
.I base
^
.I private
( mod
.I prime
)
.P
When
.I base
is a key containing the shared generator value, the remote public key is
computed.  When
.I base
is a key containing the remote public key, the shared secret is computed.
.P
.IR base ", " private ", and " prime
must all refer to
.BR user -type
keys containing the parameters for the computation.  Each of these keys must
grant the caller
.B read
permission in order for them to be used.
.P
.I buffer
and
.I buflen
specify the buffer into which the computed result will be placed.
.I buflen
may be zero, in which case the buffer is not used and the minimum buffer length
is fetched.
.P
.BR keyctl_dh_compute_alloc ()
is similar to
.BR keyctl_dh_compute ()
except that it allocates a buffer big enough to hold the payload data and
places the data in it.  If successful, a pointer to the buffer is placed in
.IR *_buffer .
The caller must free the buffer.
.P
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.SH RETURN VALUE
On success
.BR keyctl_dh_compute ()
returns the amount of data placed into the buffer when
.I buflen
is non-zero.  When
.I buflen
is zero, the minimum buffer length to hold the data is returned.
.P
On success
.BR keyctl_dh_compute_alloc ()
returns the amount of data in the buffer.
.P
On error, both functions set errno to an appropriate code and return the value
.BR -1 .
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.SH ERRORS
.TP
.B ENOKEY
One of the keys specified is invalid or not readable.
.TP
.B EINVAL
The buffer pointer is invalid or buflen is too small.
.TP
.B EOPNOTSUPP
One of the keys was not a valid user key.
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.SH LINKING
This is a library function that can be found in
.IR libkeyutils .
When linking,
.B -lkeyutils
should be specified to the linker.
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.SH SEE ALSO
.BR keyctl (1),
.br
.BR keyctl (2),
.br
.BR keyctl (3),
.br
.BR keyutils (7)