openssl/doc/man3/SSL_read_early.pod

=pod

=head1 NAME

SSL_set_max_early_data,
SSL_CTX_set_max_early_data,
SSL_get_max_early_data,
SSL_CTX_get_max_early_data,
SSL_SESSION_get_max_early_data,
SSL_write_early,
SSL_read_early,
SSL_get_early_data_status
- functions for sending and receiving early data

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_CTX_set_max_early_data(SSL_CTX *ctx, uint32_t max_early_data);
 uint32_t SSL_CTX_get_max_early_data(const SSL_CTX *ctx);
 int SSL_set_max_early_data(SSL *s, uint32_t max_early_data);
 uint32_t SSL_get_max_early_data(const SSL_CTX *s);
 uint32_t SSL_SESSION_get_max_early_data(const SSL_SESSION *s);

 int SSL_write_early(SSL *s, const void *buf, size_t num, size_t *written);

 int SSL_read_early(SSL *s, void *buf, size_t num, size_t *readbytes);

 int SSL_get_early_data_status(const SSL *s);

=head1 DESCRIPTION

These functions are used to send and recieve early data. Early data can be sent
by the client immediately after its initial ClientHello without having to wait
for the server to complete the handshake. Early data can only be sent if a
session has previously been established with the server, and the server is known
to support it.

Early data has weaker security properties than other data sent over an SSL/TLS
connection. In particular the data is not forward secret and the server has no
guarantees that the same early data was not replayed across multiple
connections. For this reason extreme care should be exercised when using early
data.

When a server receives early data it may opt to immediately respond by sending
application data back to the client. Data sent by the server at this stage is
done before the full handshake has been completed. Specifically the client's
authentication messages have not yet been received, i.e. the client is
unauthenticated at this point.

A server or client can determine whether the full handshake has been completed
or not by calling L<SSL_is_init_finished(3)>.

[[TODO(TLS1.3): The server uses SSL_write_ex()/SSL_write() to send data to an
unauthenticated client. Should we create a separate function for this to avoid
accidents??]]

On the client side the function SSL_SESSION_get_max_early_data() can be used to
determine whether a session established with a server can be used to send early
data. If the session cannot be used then this function will return 0. Otherwise
it will return the maximum number of early data bytes that can be sent.

A client uses the function SSL_write_early() to send early data. This function
works in the same way as the L<SSL_write_ex(3)> function, but with a few
differences. Refer to the L<SSL_write_ex(3)> documentation for
information on how to write bytes to the underlying connection, and how to
handle any errors that may arise. This page will detail the differences between
SSL_write_early() and L<SSL_write_ex(3)>.

SSL_write_early() must be the first IO function called on a new connection, i.e.
it must occur before any calls to L<SSL_write_ex(3)>, L<SSL_read_ex(3)>,
L<SSL_connect(3)>, L<SSL_do_handshake(3)> or other similar functions. It may be
called multiple times to stream data to the server, but the total number of
bytes written must not exceed the value returned from
SSL_SESSION_get_max_early_data(). Once the initial SSL_write_early() call has
completed successfully the client may interleave calls to L<SSL_read_ex(3)> and
L<SSL_read(3)> with calls to SSL_write_early() as required.

If SSL_write_early() fails you should call L<SSL_get_error(3)> to determine the
correct course of action, as for L<SSL_write_ex(3)>.

When the client no longer wishes to send any more early data then it should
complete the handshake by calling a function such as L<SSL_connect(3)> or
L<SSL_do_handshake(3)>. Alternatively you can call a standard write function
such as L<SSL_write_ex(3)>, which will transparently complete the connection and
write the requested data.

Only clients may call SSL_write_early().

A server may choose to ignore early data that has been sent to it. Once the
connection has been completed you can determine whether the server accepted or
rejected the early data by calling SSL_get_early_data_status(). This will return
SSL_EARLY_DATA_ACCEPTED if the data was accepted, SSL_EARLY_DATA_REJECTED if it
was rejected or SSL_EARLY_DATA_NOT_SENT if no early data was sent. This function
may be called by either the client or the server.

A server uses the SSL_read_early() function to receive early data on a
connection. As for SSL_write_early() this must be the first IO function called
on a connection, i.e. it must occur before any calls to L<SSL_write_ex(3)>,
L<SSL_read_ex(3)>, L<SSL_accept(3)>, L<SSL_do_handshake(3)>, or other similar
functions.

SSL_read_early() works in the same way as L<SSL_read_ex(3)> except for the
differences noted here. Refer to the L<SSL_read_ex(3)> documentation for full
details.

SSL_read_early() may return 3 possible values:

=over 4

=item SSL_READ_EARLY_ERROR

This indicates an IO or some other error occured. This should be treated in the
same way as a 0 return value from L<SSL_read_ex(3)>.

=item SSL_READ_EARLY_SUCCESS

This indicates that early data was successfully read. This should be treated in
the same way as a 1 return value from L<SSL_read_ex(3)>. You should continue to
call SSL_read_early() to read more data.

=item SSL_READ_EARLY_FINISH

This indicates that no more early data can be read. It may be returned on the
first call to SSL_read_early() if the client has not sent any early data, or
if the early data was rejected.

=back

Once the initial SSL_write_early() call has completed successfully the client
may interleave calls to L<SSL_write_ex(3)> and L<SSL_write(3)> with calls to
SSL_read_early() as required. As noted above data sent via L<SSL_write_ex(3)> or
L<SSL_write(3)> in this way is sent to an unauthenticated client.

Servers must not call L<SSL_read_ex(3)> or L<SSL_read(3)> until SSL_read_early()
has returned with SSL_READ_EARLY_FINISH. Once it has done so the connection to
the client still needs to be completed. Complete the connection by calling a
function such as L<SSL_accept(3)> or L<SSL_do_handshake(3)>. Alternatively you
can call a standard read function such as L<SSL_read_ex(3)>, which will
transparently complete the connection and read the requested data. Note that it
is an error to attempt to complete the connection before SSL_read_early() has
returned SSL_READ_EARLY_FINISH.

Only servers may call SSL_read_early().

Calls to SSL_read_early() may, in certain circumstances, complete the connection
immediately without further need to call a function such as L<SSL_accept(3)>.
Applications can test for this by calling L<SSL_is_init_finished(3)>.
Alternatively, applications may choose to call L<SSL_accept(3)> anway. Such a
call will successfully return immediately with no further action taken.

When a session is created between a server and a client the server will specify
the maximum amount of any early data that it will accept on any future
connection attempt. By default this is approximately 16k. A server may override
this default value by calling SSL_CTX_set_max_early_data() or
SSL_set_max_early_data() to set it for the whole SSL_CTX or an individual SSL
object respectively. Similarly the SSL_CTX_get_max_early_data() and
SSL_get_max_early_data() functions can be used to obtain the current maximum
early data settings for the SSL_CTX and SSL objects respectively.

In the event that the current maximum early data setting for the server is
different to that originally specified in a session that a client is resuming
with then the lower of the two values will apply.

=head1 RETURN VALUES

SSL_write_early() returns 1 for success or 0 for failure. In the event of a
failure call L<SSL_get_error(3)> to determine the correct course of action.

SSL_read_early() returns SSL_READ_EARLY_ERROR for failure,
SSL_READ_EARLY_SUCCESS for success with more data to read and
SSL_READ_EARLY_FINISH for no more to data be read. In the event of a failure
call L<SSL_get_error(3)> to determine the correct course of action.

SSL_get_max_early_data(), SSL_CTX_get_max_early_data() and
SSL_SESSION_get_max_early_data() return the maximum number of early data bytes
that may be sent.

SSL_set_max_early_data() and SSL_CTX_set_max_early_data() return 1 for success
or 0 for failure.

SSL_get_early_data_status() returns SSL_EARLY_DATA_ACCEPTED if early data was
accepted by the server, SSL_EARLY_DATA_REJECTED if early data was rejected by
the server, or SSL_EARLY_DATA_NOT_SENT if no early data was sent.

=head1 SEE ALSO

L<SSL_get_error(3)>,
L<SSL_write_ex(3)>,
L<SSL_read_ex(3)>,
L<SSL_connect(3)>,
L<SSL_accept(3)>,
L<SSL_do_handshake(3)>,
L<ssl(7)>

=head1 HISTORY

All of the functions described above were added in OpenSSL 1.1.1.

=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