Add documentation for the SSL_export_keying_material() function

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3735)

doc/man3/SSL_export_keying_material.pod

@@ -0,0 +1,61 @@
+=pod
+
+=head1 NAME
+
+SSL_export_keying_material - obtain keying material for application use
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ int SSL_export_keying_material(SSL *s, unsigned char *out, size_t olen,
+                                const char *label, size_t llen,
+                                const unsigned char *context,
+                                size_t contextlen, int use_context);
+
+=head1 DESCRIPTION
+
+During the creation of a TLS or DTLS connection shared keying material is
+established between the two endpoints. The function SSL_export_keying_material()
+enables an application to use some of this keying material for its own purposes
+in accordance with RFC5705.
+
+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.
+
+For a given SSL connection B<s>, B<olen> bytes of data will be written to
+B<out>. The application specific context should be supplied in the location
+pointed to by B<context> and should be B<contextlen> bytes long. Provision of
+a context is optional. If the context should be omitted entirely then
+B<use_context> should be set to 0. Otherwise it should be any other value. If
+B<use_context> is 0 then the values of B<context> and B<contextlen> are ignored.
+Note that a zero length context is treated differently to no context at all, and
+will result in different keying material being returned.
+
+An application specific label should be provided in the location pointed to by
+B<label> and should be B<llen> bytes long. Typically this will be a value from
+the IANA Exporter Label Registry
+(L<https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#exporter-labels>).
+Alternatively labels beginning with "EXPERIMENTAL" are permitted by the standard
+to be used without registration.
+
+Note that this function is only defined for TLSv1.0 and above, and DTLSv1.0 and
+above. Attempting to use it in SSLv3 will result in an error.
+
+=head1 RETURN VALUES
+
+SSL_export_keying_material() returns 0 or -1 on failure or 1 on success.
+
+=head1 COPYRIGHT
+
+Copyright 2017 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut

include/openssl/tls1.h

@@ -212,12 +212,12 @@ __owur int SSL_get_servername_type(const SSL *s);
  * as specified in RFC 5705. It writes |olen| bytes to |out| given a label and
  * optional context. (Since a zero length context is allowed, the |use_context|
  * flag controls whether a context is included.) It returns 1 on success and
- * zero otherwise.
+ * 0 or -1 otherwise.
  */
 __owur int SSL_export_keying_material(SSL *s, unsigned char *out, size_t olen,
-                               const char *label, size_t llen,
-                               const unsigned char *p, size_t plen,
-                               int use_context);
+                                      const char *label, size_t llen,
+                                      const unsigned char *context,
+                                      size_t contextlen, int use_context);
 
 int SSL_get_peer_signature_type_nid(const SSL *s, int *pnid);
 

ssl/ssl_lib.c

@@ -2556,15 +2556,15 @@ void SSL_get0_alpn_selected(const SSL *ssl, const unsigned char **data,
 
 int SSL_export_keying_material(SSL *s, unsigned char *out, size_t olen,
                                const char *label, size_t llen,
-                               const unsigned char *p, size_t plen,
+                               const unsigned char *context, size_t contextlen,
                                int use_context)
 {
     if (s->version < TLS1_VERSION && s->version != DTLS1_BAD_VER)
         return -1;
 
     return s->method->ssl3_enc->export_keying_material(s, out, olen, label,
-                                                       llen, p, plen,
-                                                       use_context);
+                                                       llen, context,
+                                                       contextlen, use_context);
 }
 
 static unsigned long ssl_session_hash(const SSL_SESSION *a)