Add documentation for SRTP functions

=pod

=head1 NAME

SSL_CTX_set_tlsext_use_srtp,

SSL_set_tlsext_use_srtp,

SSL_get_srtp_profiles,

SSL_get_selected_srtp_profile

- Configure and query SRTP support

=head1 SYNOPSIS

 #include <openssl/srtp.h>

 int SSL_CTX_set_tlsext_use_srtp(SSL_CTX *ctx, const char *profiles);

 int SSL_set_tlsext_use_srtp(SSL *ssl, const char *profiles);

 STACK_OF(SRTP_PROTECTION_PROFILE) *SSL_get_srtp_profiles(SSL *ssl);

 SRTP_PROTECTION_PROFILE *SSL_get_selected_srtp_profile(SSL *s);

=head1 DESCRIPTION

SRTP is the Secure Real-Time Transport Protocol. OpenSSL implements support for

the "use_srtp" DTLS extension defined in RFC5764. This provides a mechanism for

establishing SRTP keying material, algorithms and parameters using DTLS. This

capability may be used as part of an implementation that conforms to RFC5763.

OpenSSL does not implement SRTP itself or RFC5763. Note that OpenSSL does not

support the use of SRTP Master Key Identifiers (MKIs). Also note that this

extension is only supported in DTLS. Any SRTP configuration will be ignored if a

TLS connection is attempted.

An OpenSSL client wishing to send the "use_srtp" extension should call

SSL_CTX_set_tlsext_use_srtp() to set its use for all SSL objects subsequently

created from an SSL_CTX. Alternatively a client may call

SSL_set_tlsext_use_srtp() to set its use for an individual SSL object. The

B<profiles> parameters should point to a NUL-terminated, colon delimited list of

SRTP protection profile names.

The currently supported protection profile names are:

=over 4

=item SRTP_AES128_CM_SHA1_80

This corresponds to SRTP_AES128_CM_HMAC_SHA1_80 defined in RFC5764.

=item SRTP_AES128_CM_SHA1_32

This corresponds to SRTP_AES128_CM_HMAC_SHA1_32 defined in RFC5764.

=item SRTP_AEAD_AES_128_GCM

This corresponds to the profile of the same name defined in RFC7714.

=item SRTP_AEAD_AES_256_GCM

This corresponds to the profile of the same name defined in RFC7714.

=back

Supplying an unrecognised protection profile name will result in an error.

An OpenSSL server wishing to support the "use_srtp" extension should also call

SSL_CTX_set_tlsext_use_srtp() or SSL_set_tlsext_use_srtp() to indicate the

protection profiles that it is willing to negotiate.

The currently configured list of protection profiles for either a client or a

server can be obtained by calling SSL_get_srtp_profiles(). This returns a stack

of SRTP_PROTECTION_PROFILE objects. The memory pointed to in the return value of

this function should not be freed by the caller.

After a handshake has been completed the negotiated SRTP protection profile (if

any) can be obtained (on the client or the server) by calling

SSL_get_selected_srtp_profile(). This function will return NULL if no SRTP

protection profile was negotiated. The memory returned from this function should

not be freed by the caller.

If an SRTP protection profile has been sucessfully negotiated then the SRTP

keying material (on both the client and server) should be obtained via a call to

L<SSL_export_keying_material(3)>. This call should provide a label value of

"EXTRACTOR-dtls_srtp" and a NULL context value (use_context is 0). The total

length of keying material obtained should be equal to two times the sum of the

master key length and the salt length as defined for the protection profile in

use. This provides the client write master key, the server write master key, the

client write master salt and the server write master salt in that order.

=head1 RETURN VALUES

SSL_CTX_set_tlsext_use_srtp() and SSL_set_tlsext_use_srtp() return 0 on success

or 1 on error.

SSL_get_srtp_profiles() returns a stack of SRTP_PROTECTION_PROFILE objects on

success or NULL on error or if no protection profiles have been configured.

SSL_get_selected_srtp_profile() returns a pointer to an SRTP_PROTECTION_PROFILE

object if one has been negotiated or NULL otherwise.

=head1 SEE ALSO

L<SSL_export_keying_material(3)>

=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

# ifndef OPENSSL_NO_SRTP

__owur int SSL_CTX_set_tlsext_use_srtp(SSL_CTX *ctx, const char *profiles);

__owur int SSL_set_tlsext_use_srtp(SSL *ctx, const char *profiles);

__owur int SSL_set_tlsext_use_srtp(SSL *ssl, const char *profiles);

__owur STACK_OF(SRTP_PROTECTION_PROFILE) *SSL_get_srtp_profiles(SSL *ssl);

__owur SRTP_PROTECTION_PROFILE *SSL_get_selected_srtp_profile(SSL *s);