.\"	$OpenBSD: SSL_export_keying_material.3,v 1.2 2018/03/23 05:50:30 schwarze Exp $
.\"	OpenSSL a599574b Jun 28 17:18:27 2017 +0100
.\"	OpenSSL 23cec1f4 Jun 21 13:55:02 2017 +0100
.\"
.\" This file was written by Matt Caswell <matt@openssl.org>.
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.Dd $Mdocdate: March 23 2018 $
.Dt SSL_EXPORT_KEYING_MATERIAL 3
.Os
.Sh NAME
.Nm SSL_export_keying_material
.Nd obtain keying material for application use
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft int
.Fo SSL_export_keying_material
.Fa "SSL *s"
.Fa "unsigned char *out"
.Fa "size_t olen"
.Fa "const char *label"
.Fa "size_t llen"
.Fa "const unsigned char *context"
.Fa "size_t contextlen"
.Fa "int use_context"
.Fc
.Sh DESCRIPTION
During the creation of a TLS or DTLS connection,
shared keying material is established between the two endpoints.
The function
.Fn SSL_export_keying_material
enables an application to use some of this keying material
for its own purposes in accordance with RFC 5705.
.Pp
An application may need to securely establish the context
within which this keying material will be used.
For example, this may include identifiers for the application session,
application algorithms or parameters, or the lifetime of the context.
The context value is left to the application but must be the same on
both sides of the communication.
.Pp
For a given SSL connection
.Fa s ,
.Fa olen
bytes of data will be written to
.Fa out .
The application specific context should be supplied
in the location pointed to by
.Fa context
and should be
.Fa contextlen
bytes long.
Provision of a context is optional.
If the context should be omitted entirely, then
.Fa use_context
should be set to 0.
Otherwise it should be any other value.
If
.Fa use_context
is 0, then the values of
.Fa context
and
.Fa contextlen
are ignored.
.Pp
In TLSv1.2 and below, a zero length context is treated differently
from no context at all, and will result in different keying material
being returned.
.Pp
An application specific label should be provided in the location pointed
to by
.Fa label
and should be
.Fa llen
bytes long.
Typically this will be a value from the
.Lk https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#exporter-labels "IANA Exporter Label Registry" .
.Pp
Alternatively, labels beginning with "EXPERIMENTAL" are permitted by the
standard to be used without registration.
.Sh RETURN VALUES
.Fn SSL_export_keying_material
returns 1 on success or 0 or -1 on failure.
.Sh HISTORY
.Fn SSL_export_keying_material
first appeared in OpenSSL 1.0.1 and has been available since
.Ox 5.3 .