SSL_READ_EARLY_DATA(3) OpenSSL SSL_READ_EARLY_DATA(3)
NAME
SSL_set_max_early_data, SSL_CTX_set_max_early_data,
SSL_get_max_early_data, SSL_CTX_get_max_early_data,
SSL_set_recv_max_early_data, SSL_CTX_set_recv_max_early_data,
SSL_get_recv_max_early_data, SSL_CTX_get_recv_max_early_data,
SSL_SESSION_get_max_early_data, SSL_SESSION_set_max_early_data,
SSL_write_early_data, SSL_read_early_data, SSL_get_early_data_status,
SSL_allow_early_data_cb_fn, SSL_CTX_set_allow_early_data_cb,
SSL_set_allow_early_data_cb - functions for sending and receiving early
data
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 *s);
int SSL_CTX_set_recv_max_early_data(SSL_CTX *ctx, uint32_t recv_max_early_data);
uint32_t SSL_CTX_get_recv_max_early_data(const SSL_CTX *ctx);
int SSL_set_recv_max_early_data(SSL *s, uint32_t recv_max_early_data);
uint32_t SSL_get_recv_max_early_data(const SSL *s);
uint32_t SSL_SESSION_get_max_early_data(const SSL_SESSION *s);
int SSL_SESSION_set_max_early_data(SSL_SESSION *s, uint32_t max_early_data);
int SSL_write_early_data(SSL *s, const void *buf, size_t num, size_t *written);
int SSL_read_early_data(SSL *s, void *buf, size_t num, size_t *readbytes);
int SSL_get_early_data_status(const SSL *s);
typedef int (*SSL_allow_early_data_cb_fn)(SSL *s, void *arg);
void SSL_CTX_set_allow_early_data_cb(SSL_CTX *ctx,
SSL_allow_early_data_cb_fn cb,
void *arg);
void SSL_set_allow_early_data_cb(SSL *s,
SSL_allow_early_data_cb_fn cb,
void *arg);
DESCRIPTION
These functions are used to send and receive early data where TLSv1.3
has been negotiated. 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 be sent if a session has
previously been established with the server or when establishing a new
session using an out-of-band PSK, and only when the server is known to
support it. Additionally these functions can be used to send data from
the server to the client when the client has not yet completed the
authentication stage of the handshake.
Early data has weaker security properties than other data sent over an
SSL/TLS connection. In particular the data does not have forward
secrecy. There are also additional considerations around replay attacks
(see "REPLAY PROTECTION" below). For these reasons extreme care should
be exercised when using early data. For specific details, consult the
TLS 1.3 specification.
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 and care
should be taken when using this capability.
A server or client can determine whether the full handshake has been
completed or not by calling SSL_is_init_finished(3).
On the client side, the function SSL_SESSION_get_max_early_data() can
be used to determine if 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.
The function SSL_SESSION_set_max_early_data() sets the maximum number
of early data bytes that can be sent for a session. This would
typically be used when creating a PSK session file (see
SSL_CTX_set_psk_use_session_callback(3)). If using a ticket based PSK
then this is set automatically to the value provided by the server.
A client uses the function SSL_write_early_data() to send early data.
This function is similar to the SSL_write_ex(3) function, but with the
following differences. See SSL_write_ex(3) for information on how to
write bytes to the underlying connection, and how to handle any errors
that may arise. This page describes the differences between
SSL_write_early_data() and SSL_write_ex(3).
When called by a client, SSL_write_early_data() must be the first IO
function called on a new connection, i.e. it must occur before any
calls to SSL_write_ex(3), SSL_read_ex(3), SSL_connect(3),
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_data() call has completed successfully the client may
interleave calls to SSL_read_ex(3) and SSL_read(3) with calls to
SSL_write_early_data() as required.
If SSL_write_early_data() fails you should call SSL_get_error(3) to
determine the correct course of action, as for 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
SSL_connect(3) or SSL_do_handshake(3). Alternatively you can call a
standard write function such as SSL_write_ex(3), which will
transparently complete the connection and write the requested data.
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_data() function to receive early data
on a connection for which early data has been enabled using
SSL_CTX_set_max_early_data() or SSL_set_max_early_data(). As for
SSL_write_early_data(), this must be the first IO function called on a
connection, i.e. it must occur before any calls to SSL_write_ex(3),
SSL_read_ex(3), SSL_accept(3), SSL_do_handshake(3), or other similar
functions.
SSL_read_early_data() is similar to SSL_read_ex(3) with the following
differences. Refer to SSL_read_ex(3) for full details.
SSL_read_early_data() may return 3 possible values:
SSL_READ_EARLY_DATA_ERROR
This indicates an IO or some other error occurred. This should be
treated in the same way as a 0 return value from SSL_read_ex(3).
SSL_READ_EARLY_DATA_SUCCESS
This indicates that early data was successfully read. This should
be treated in the same way as a 1 return value from SSL_read_ex(3).
You should continue to call SSL_read_early_data() to read more
data.
SSL_READ_EARLY_DATA_FINISH
This indicates that no more early data can be read. It may be
returned on the first call to SSL_read_early_data() if the client
has not sent any early data, or if the early data was rejected.
Once the initial SSL_read_early_data() call has completed successfully
(i.e. it has returned SSL_READ_EARLY_DATA_SUCCESS or
SSL_READ_EARLY_DATA_FINISH) then the server may choose to write data
immediately to the unauthenticated client using SSL_write_early_data().
If SSL_read_early_data() returned SSL_READ_EARLY_DATA_FINISH then in
some situations (e.g. if the client only supports TLSv1.2) the
handshake may have already been completed and calls to
SSL_write_early_data() are not allowed. Call SSL_is_init_finished(3) to
determine whether the handshake has completed or not. If the handshake
is still in progress then the server may interleave calls to
SSL_write_early_data() with calls to SSL_read_early_data() as required.
Servers must not call SSL_read_ex(3), SSL_read(3), SSL_write_ex(3) or
SSL_write(3) until SSL_read_early_data() has returned with
SSL_READ_EARLY_DATA_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 SSL_accept(3) or SSL_do_handshake(3). Alternatively
you can call a standard read function such as 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_data() has returned SSL_READ_EARLY_DATA_FINISH.
Only servers may call SSL_read_early_data().
Calls to SSL_read_early_data() may, in certain circumstances, complete
the connection immediately without further need to call a function such
as SSL_accept(3). This can happen if the client is using a protocol
version less than TLSv1.3. Applications can test for this by calling
SSL_is_init_finished(3). Alternatively, applications may choose to call
SSL_accept(3) anyway. 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 the server does not accept early
data; a server may indicate support for early data 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. The
max_early_data parameter specifies the maximum amount of early data in
bytes that is permitted to be sent on a single connection. 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. Generally a server application
will either use both of SSL_read_early_data() and
SSL_CTX_set_max_early_data() (or SSL_set_max_early_data()), or neither
of them, since there is no practical benefit from using only one of
them. If the maximum early data setting for a server is nonzero then
replay protection is automatically enabled (see "REPLAY PROTECTION"
below).
If the server rejects the early data sent by a client then it will skip
over the data that is sent. The maximum amount of received early data
that is skipped is controlled by the recv_max_early_data setting. If a
client sends more than this then the connection will abort. This value
can be set by calling SSL_CTX_set_recv_max_early_data() or
SSL_set_recv_max_early_data(). The current value for this setting can
be obtained by calling SSL_CTX_get_recv_max_early_data() or
SSL_get_recv_max_early_data(). The default value for this setting is
16,384 bytes.
The recv_max_early_data value also has an impact on early data that is
accepted. The amount of data that is accepted will always be the lower
of the max_early_data for the session and the recv_max_early_data
setting for the server. If a client sends more data than this then the
connection will abort.
The configured value for max_early_data on a server may change over
time as required. However, clients may have tickets containing the
previously configured max_early_data value. The recv_max_early_data
should always be equal to or higher than any recently configured
max_early_data value in order to avoid aborted connections. The
recv_max_early_data should never be set to less than the current
configured max_early_data value.
Some server applications may wish to have more control over whether
early data is accepted or not, for example to mitigate replay risks
(see "REPLAY PROTECTION" below) or to decline early_data when the
server is heavily loaded. The functions
SSL_CTX_set_allow_early_data_cb() and SSL_set_allow_early_data_cb() set
a callback which is called at a point in the handshake immediately
before a decision is made to accept or reject early data. The callback
is provided with a pointer to the user data argument that was provided
when the callback was first set. Returning 1 from the callback will
allow early data and returning 0 will reject it. Note that the OpenSSL
library may reject early data for other reasons in which case this
callback will not get called. Notably, the built-in replay protection
feature will still be used even if a callback is present unless it has
been explicitly disabled using the SSL_OP_NO_ANTI_REPLAY option. See
"REPLAY PROTECTION" below.
NOTES
The whole purpose of early data is to enable a client to start sending
data to the server before a full round trip of network traffic has
occurred. Application developers should ensure they consider
optimisation of the underlying TCP socket to obtain a performant
solution. For example Nagle's algorithm is commonly used by operating
systems in an attempt to avoid lots of small TCP packets. In many
scenarios this is beneficial for performance, but it does not work well
with the early data solution as implemented in OpenSSL. In Nagle's
algorithm the OS will buffer outgoing TCP data if a TCP packet has
already been sent which we have not yet received an ACK for from the
peer. The buffered data will only be transmitted if enough data to fill
an entire TCP packet is accumulated, or if the ACK is received from the
peer. The initial ClientHello will be sent in the first TCP packet
along with any data from the first call to SSL_write_early_data(). If
the amount of data written will exceed the size of a single TCP packet,
or if there are more calls to SSL_write_early_data() then that
additional data will be sent in subsequent TCP packets which will be
buffered by the OS and not sent until an ACK is received for the first
packet containing the ClientHello. This means the early data is not
actually sent until a complete round trip with the server has occurred
which defeats the objective of early data.
In many operating systems the TCP_NODELAY socket option is available to
disable Nagle's algorithm. If an application opts to disable Nagle's
algorithm consideration should be given to turning it back on again
after the handshake is complete if appropriate.
In rare circumstances, it may be possible for a client to have a
session that reports a max early data value greater than 0, but where
the server does not support this. For example, this can occur if a
server has had its configuration changed to accept a lower max early
data value such as by calling SSL_CTX_set_recv_max_early_data().
Another example is if a server used to support TLSv1.3 but was later
downgraded to TLSv1.2. Sending early data to such a server will cause
the connection to abort. Clients that encounter an aborted connection
while sending early data may want to retry the connection without
sending early data as this does not happen automatically. A client will
have to establish a new transport layer connection to the server and
attempt the SSL/TLS connection again but without sending early data.
Note that it is inadvisable to retry with a lower maximum protocol
version.
REPLAY PROTECTION
When early data is in use the TLS protocol provides no security
guarantees that the same early data was not replayed across multiple
connections. As a mitigation for this issue OpenSSL automatically
enables replay protection if the server is configured with a nonzero
max early data value. With replay protection enabled sessions are
forced to be single use only. If a client attempts to reuse a session
ticket more than once, then the second and subsequent attempts will
fall back to a full handshake (and any early data that was submitted
will be ignored). Note that single use tickets are enforced even if a
client does not send any early data.
The replay protection mechanism relies on the internal OpenSSL server
session cache (see SSL_CTX_set_session_cache_mode(3)). When replay
protection is being used the server will operate as if the
SSL_OP_NO_TICKET option had been selected (see SSL_CTX_set_options(3)).
Sessions will be added to the cache whenever a session ticket is
issued. When a client attempts to resume the session, OpenSSL will
check for its presence in the internal cache. If it exists then the
resumption is allowed and the session is removed from the cache. If it
does not exist then the resumption is not allowed and a full handshake
will occur.
Note that some applications may maintain an external cache of sessions
(see SSL_CTX_sess_set_new_cb(3) and similar functions). It is the
application's responsibility to ensure that any sessions in the
external cache are also populated in the internal cache and that once
removed from the internal cache they are similarly removed from the
external cache. Failing to do this could result in an application
becoming vulnerable to replay attacks. Note that OpenSSL will lock the
internal cache while a session is removed but that lock is not held
when the remove session callback (see SSL_CTX_sess_set_remove_cb(3)) is
called. This could result in a small amount of time where the session
has been removed from the internal cache but is still available in the
external cache. Applications should be designed with this in mind in
order to minimise the possibility of replay attacks.
The OpenSSL replay protection does not apply to external Pre Shared
Keys (PSKs) (e.g. see SSL_CTX_set_psk_find_session_callback(3)).
Therefore, extreme caution should be applied when combining external
PSKs with early data.
Some applications may mitigate the replay risks in other ways. For
those applications it is possible to turn off the built-in replay
protection feature using the SSL_OP_NO_ANTI_REPLAY option. See
SSL_CTX_set_options(3) for details. Applications can also set a
callback to make decisions about accepting early data or not. See
SSL_CTX_set_allow_early_data_cb() above for details.
RETURN VALUES
SSL_write_early_data() returns 1 for success or 0 for failure. In the
event of a failure call SSL_get_error(3) to determine the correct
course of action.
SSL_read_early_data() returns SSL_READ_EARLY_DATA_ERROR for failure,
SSL_READ_EARLY_DATA_SUCCESS for success with more data to read and
SSL_READ_EARLY_DATA_FINISH for success with no more to data be read. In
the event of a failure call 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(), SSL_CTX_set_max_early_data() and
SSL_SESSION_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.
SEE ALSO
SSL_get_error(3), SSL_write_ex(3), SSL_read_ex(3), SSL_connect(3),
SSL_accept(3), SSL_do_handshake(3),
SSL_CTX_set_psk_use_session_callback(3), ssl(7)
HISTORY
All of the functions described above were added in OpenSSL 1.1.1.
COPYRIGHT
Copyright 2017-2020 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
<https://www.openssl.org/source/license.html>;.
1.1.1k 2021-03-25 SSL_READ_EARLY_DATA(3)
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