xref: /openssl/doc/man3/SSL_read_early_data.pod (revision da1c088f)
1=pod
2
3=head1 NAME
4
5SSL_set_max_early_data,
6SSL_CTX_set_max_early_data,
7SSL_get_max_early_data,
8SSL_CTX_get_max_early_data,
9SSL_set_recv_max_early_data,
10SSL_CTX_set_recv_max_early_data,
11SSL_get_recv_max_early_data,
12SSL_CTX_get_recv_max_early_data,
13SSL_SESSION_get_max_early_data,
14SSL_SESSION_set_max_early_data,
15SSL_write_early_data,
16SSL_read_early_data,
17SSL_get_early_data_status,
18SSL_allow_early_data_cb_fn,
19SSL_CTX_set_allow_early_data_cb,
20SSL_set_allow_early_data_cb
21- functions for sending and receiving early data
22
23=head1 SYNOPSIS
24
25 #include <openssl/ssl.h>
26
27 int SSL_CTX_set_max_early_data(SSL_CTX *ctx, uint32_t max_early_data);
28 uint32_t SSL_CTX_get_max_early_data(const SSL_CTX *ctx);
29 int SSL_set_max_early_data(SSL *s, uint32_t max_early_data);
30 uint32_t SSL_get_max_early_data(const SSL *s);
31
32 int SSL_CTX_set_recv_max_early_data(SSL_CTX *ctx, uint32_t recv_max_early_data);
33 uint32_t SSL_CTX_get_recv_max_early_data(const SSL_CTX *ctx);
34 int SSL_set_recv_max_early_data(SSL *s, uint32_t recv_max_early_data);
35 uint32_t SSL_get_recv_max_early_data(const SSL *s);
36
37 uint32_t SSL_SESSION_get_max_early_data(const SSL_SESSION *s);
38 int SSL_SESSION_set_max_early_data(SSL_SESSION *s, uint32_t max_early_data);
39
40 int SSL_write_early_data(SSL *s, const void *buf, size_t num, size_t *written);
41
42 int SSL_read_early_data(SSL *s, void *buf, size_t num, size_t *readbytes);
43
44 int SSL_get_early_data_status(const SSL *s);
45
46
47 typedef int (*SSL_allow_early_data_cb_fn)(SSL *s, void *arg);
48
49 void SSL_CTX_set_allow_early_data_cb(SSL_CTX *ctx,
50                                      SSL_allow_early_data_cb_fn cb,
51                                      void *arg);
52 void SSL_set_allow_early_data_cb(SSL *s,
53                                  SSL_allow_early_data_cb_fn cb,
54                                  void *arg);
55
56=head1 DESCRIPTION
57
58These functions are used to send and receive early data where TLSv1.3 has been
59negotiated. Early data can be sent by the client immediately after its initial
60ClientHello without having to wait for the server to complete the handshake.
61Early data can be sent if a session has previously been established with the
62server or when establishing a new session using an out-of-band PSK, and only
63when the server is known to support it. Additionally these functions can be used
64to send data from the server to the client when the client has not yet completed
65the authentication stage of the handshake.
66
67Early data has weaker security properties than other data sent over an SSL/TLS
68connection. In particular the data does not have forward secrecy. There are also
69additional considerations around replay attacks (see L</REPLAY PROTECTION>
70below). For these reasons extreme care should be exercised when using early
71data. For specific details, consult the TLS 1.3 specification.
72
73When a server receives early data it may opt to immediately respond by sending
74application data back to the client. Data sent by the server at this stage is
75done before the full handshake has been completed. Specifically the client's
76authentication messages have not yet been received, i.e. the client is
77unauthenticated at this point and care should be taken when using this
78capability.
79
80A server or client can determine whether the full handshake has been completed
81or not by calling L<SSL_is_init_finished(3)>.
82
83On the client side, the function SSL_SESSION_get_max_early_data() can be used to
84determine if a session established with a server can be used to send early data.
85If the session cannot be used then this function will return 0. Otherwise it
86will return the maximum number of early data bytes that can be sent.
87
88The function SSL_SESSION_set_max_early_data() sets the maximum number of early
89data bytes that can be sent for a session. This would typically be used when
90creating a PSK session file (see L<SSL_CTX_set_psk_use_session_callback(3)>). If
91using a ticket based PSK then this is set automatically to the value provided by
92the server.
93
94A client uses the function SSL_write_early_data() to send early data. This
95function is similar to the L<SSL_write_ex(3)> function, but with the following
96differences. See L<SSL_write_ex(3)> for information on how to write bytes to
97the underlying connection, and how to handle any errors that may arise. This
98page describes the differences between SSL_write_early_data() and
99L<SSL_write_ex(3)>.
100
101When called by a client, SSL_write_early_data() must be the first IO function
102called on a new connection, i.e. it must occur before any calls to
103L<SSL_write_ex(3)>, L<SSL_read_ex(3)>, L<SSL_connect(3)>, L<SSL_do_handshake(3)>
104or other similar functions. It may be called multiple times to stream data to
105the server, but the total number of bytes written must not exceed the value
106returned from SSL_SESSION_get_max_early_data(). Once the initial
107SSL_write_early_data() call has completed successfully the client may interleave
108calls to L<SSL_read_ex(3)> and L<SSL_read(3)> with calls to
109SSL_write_early_data() as required.
110
111If SSL_write_early_data() fails you should call L<SSL_get_error(3)> to determine
112the correct course of action, as for L<SSL_write_ex(3)>.
113
114When the client no longer wishes to send any more early data then it should
115complete the handshake by calling a function such as L<SSL_connect(3)> or
116L<SSL_do_handshake(3)>. Alternatively you can call a standard write function
117such as L<SSL_write_ex(3)>, which will transparently complete the connection and
118write the requested data.
119
120A server may choose to ignore early data that has been sent to it. Once the
121connection has been completed you can determine whether the server accepted or
122rejected the early data by calling SSL_get_early_data_status(). This will return
123SSL_EARLY_DATA_ACCEPTED if the data was accepted, SSL_EARLY_DATA_REJECTED if it
124was rejected or SSL_EARLY_DATA_NOT_SENT if no early data was sent. This function
125may be called by either the client or the server.
126
127A server uses the SSL_read_early_data() function to receive early data on a
128connection for which early data has been enabled using
129SSL_CTX_set_max_early_data() or SSL_set_max_early_data(). As for
130SSL_write_early_data(), this must be the first IO function
131called on a connection, i.e. it must occur before any calls to
132L<SSL_write_ex(3)>, L<SSL_read_ex(3)>, L<SSL_accept(3)>, L<SSL_do_handshake(3)>,
133or other similar functions.
134
135SSL_read_early_data() is similar to L<SSL_read_ex(3)> with the following
136differences. Refer to L<SSL_read_ex(3)> for full details.
137
138SSL_read_early_data() may return 3 possible values:
139
140=over 4
141
142=item SSL_READ_EARLY_DATA_ERROR
143
144This indicates an IO or some other error occurred. This should be treated in the
145same way as a 0 return value from L<SSL_read_ex(3)>.
146
147=item SSL_READ_EARLY_DATA_SUCCESS
148
149This indicates that early data was successfully read. This should be treated in
150the same way as a 1 return value from L<SSL_read_ex(3)>. You should continue to
151call SSL_read_early_data() to read more data.
152
153=item SSL_READ_EARLY_DATA_FINISH
154
155This indicates that no more early data can be read. It may be returned on the
156first call to SSL_read_early_data() if the client has not sent any early data,
157or if the early data was rejected.
158
159=back
160
161Once the initial SSL_read_early_data() call has completed successfully (i.e. it
162has returned SSL_READ_EARLY_DATA_SUCCESS or SSL_READ_EARLY_DATA_FINISH) then the
163server may choose to write data immediately to the unauthenticated client using
164SSL_write_early_data(). If SSL_read_early_data() returned
165SSL_READ_EARLY_DATA_FINISH then in some situations (e.g. if the client only
166supports TLSv1.2) the handshake may have already been completed and calls
167to SSL_write_early_data() are not allowed. Call L<SSL_is_init_finished(3)> to
168determine whether the handshake has completed or not. If the handshake is still
169in progress then the server may interleave calls to SSL_write_early_data() with
170calls to SSL_read_early_data() as required.
171
172Servers must not call L<SSL_read_ex(3)>, L<SSL_read(3)>, L<SSL_write_ex(3)> or
173L<SSL_write(3)>  until SSL_read_early_data() has returned with
174SSL_READ_EARLY_DATA_FINISH. Once it has done so the connection to the client
175still needs to be completed. Complete the connection by calling a function such
176as L<SSL_accept(3)> or L<SSL_do_handshake(3)>. Alternatively you can call a
177standard read function such as L<SSL_read_ex(3)>, which will transparently
178complete the connection and read the requested data. Note that it is an error to
179attempt to complete the connection before SSL_read_early_data() has returned
180SSL_READ_EARLY_DATA_FINISH.
181
182Only servers may call SSL_read_early_data().
183
184Calls to SSL_read_early_data() may, in certain circumstances, complete the
185connection immediately without further need to call a function such as
186L<SSL_accept(3)>. This can happen if the client is using a protocol version less
187than TLSv1.3. Applications can test for this by calling
188L<SSL_is_init_finished(3)>. Alternatively, applications may choose to call
189L<SSL_accept(3)> anyway. Such a call will successfully return immediately with no
190further action taken.
191
192When a session is created between a server and a client the server will specify
193the maximum amount of any early data that it will accept on any future
194connection attempt. By default the server does not accept early data; a
195server may indicate support for early data by calling
196SSL_CTX_set_max_early_data() or
197SSL_set_max_early_data() to set it for the whole SSL_CTX or an individual SSL
198object respectively. The B<max_early_data> parameter specifies the maximum
199amount of early data in bytes that is permitted to be sent on a single
200connection. Similarly the SSL_CTX_get_max_early_data() and
201SSL_get_max_early_data() functions can be used to obtain the current maximum
202early data settings for the SSL_CTX and SSL objects respectively. Generally a
203server application will either use both of SSL_read_early_data() and
204SSL_CTX_set_max_early_data() (or SSL_set_max_early_data()), or neither of them,
205since there is no practical benefit from using only one of them. If the maximum
206early data setting for a server is nonzero then replay protection is
207automatically enabled (see L</REPLAY PROTECTION> below).
208
209If the server rejects the early data sent by a client then it will skip over
210the data that is sent. The maximum amount of received early data that is skipped
211is controlled by the recv_max_early_data setting. If a client sends more than
212this then the connection will abort. This value can be set by calling
213SSL_CTX_set_recv_max_early_data() or SSL_set_recv_max_early_data(). The current
214value for this setting can be obtained by calling
215SSL_CTX_get_recv_max_early_data() or SSL_get_recv_max_early_data(). The default
216value for this setting is 16,384 bytes.
217
218The recv_max_early_data value also has an impact on early data that is accepted.
219The amount of data that is accepted will always be the lower of the
220max_early_data for the session and the recv_max_early_data setting for the
221server. If a client sends more data than this then the connection will abort.
222
223The configured value for max_early_data on a server may change over time as
224required. However, clients may have tickets containing the previously configured
225max_early_data value. The recv_max_early_data should always be equal to or
226higher than any recently configured max_early_data value in order to avoid
227aborted connections. The recv_max_early_data should never be set to less than
228the current configured max_early_data value.
229
230Some server applications may wish to have more control over whether early data
231is accepted or not, for example to mitigate replay risks (see L</REPLAY PROTECTION>
232below) or to decline early_data when the server is heavily loaded. The functions
233SSL_CTX_set_allow_early_data_cb() and SSL_set_allow_early_data_cb() set a
234callback which is called at a point in the handshake immediately before a
235decision is made to accept or reject early data. The callback is provided with a
236pointer to the user data argument that was provided when the callback was first
237set. Returning 1 from the callback will allow early data and returning 0 will
238reject it. Note that the OpenSSL library may reject early data for other reasons
239in which case this callback will not get called. Notably, the built-in replay
240protection feature will still be used even if a callback is present unless it
241has been explicitly disabled using the SSL_OP_NO_ANTI_REPLAY option. See
242L</REPLAY PROTECTION> below.
243
244These functions cannot currently be used with QUIC SSL objects.
245SSL_set_max_early_data(), SSL_set_recv_max_early_data(), SSL_write_early_data(),
246SSL_read_early_data(), SSL_get_early_data_status() and
247SSL_set_allow_early_data_cb() fail if called on a QUIC SSL object.
248
249=head1 NOTES
250
251The whole purpose of early data is to enable a client to start sending data to
252the server before a full round trip of network traffic has occurred. Application
253developers should ensure they consider optimisation of the underlying TCP socket
254to obtain a performant solution. For example Nagle's algorithm is commonly used
255by operating systems in an attempt to avoid lots of small TCP packets. In many
256scenarios this is beneficial for performance, but it does not work well with the
257early data solution as implemented in OpenSSL. In Nagle's algorithm the OS will
258buffer outgoing TCP data if a TCP packet has already been sent which we have not
259yet received an ACK for from the peer. The buffered data will only be
260transmitted if enough data to fill an entire TCP packet is accumulated, or if
261the ACK is received from the peer. The initial ClientHello will be sent in the
262first TCP packet along with any data from the first call to
263SSL_write_early_data(). If the amount of data written will exceed the size of a
264single TCP packet, or if there are more calls to SSL_write_early_data() then
265that additional data will be sent in subsequent TCP packets which will be
266buffered by the OS and not sent until an ACK is received for the first packet
267containing the ClientHello. This means the early data is not actually
268sent until a complete round trip with the server has occurred which defeats the
269objective of early data.
270
271In many operating systems the TCP_NODELAY socket option is available to disable
272Nagle's algorithm. If an application opts to disable Nagle's algorithm
273consideration should be given to turning it back on again after the handshake is
274complete if appropriate.
275
276In rare circumstances, it may be possible for a client to have a session that
277reports a max early data value greater than 0, but where the server does not
278support this. For example, this can occur if a server has had its configuration
279changed to accept a lower max early data value such as by calling
280SSL_CTX_set_recv_max_early_data(). Another example is if a server used to
281support TLSv1.3 but was later downgraded to TLSv1.2. Sending early data to such
282a server will cause the connection to abort. Clients that encounter an aborted
283connection while sending early data may want to retry the connection without
284sending early data as this does not happen automatically. A client will have to
285establish a new transport layer connection to the server and attempt the SSL/TLS
286connection again but without sending early data. Note that it is inadvisable to
287retry with a lower maximum protocol version.
288
289=head1 REPLAY PROTECTION
290
291When early data is in use the TLS protocol provides no security guarantees that
292the same early data was not replayed across multiple connections. As a
293mitigation for this issue OpenSSL automatically enables replay protection if the
294server is configured with a nonzero max early data value. With replay
295protection enabled sessions are forced to be single use only. If a client
296attempts to reuse a session ticket more than once, then the second and
297subsequent attempts will fall back to a full handshake (and any early data that
298was submitted will be ignored). Note that single use tickets are enforced even
299if a client does not send any early data.
300
301The replay protection mechanism relies on the internal OpenSSL server session
302cache (see L<SSL_CTX_set_session_cache_mode(3)>). When replay protection is
303being used the server will operate as if the SSL_OP_NO_TICKET option had been
304selected (see L<SSL_CTX_set_options(3)>). Sessions will be added to the cache
305whenever a session ticket is issued. When a client attempts to resume the
306session, OpenSSL will check for its presence in the internal cache. If it exists
307then the resumption is allowed and the session is removed from the cache. If it
308does not exist then the resumption is not allowed and a full handshake will
309occur.
310
311Note that some applications may maintain an external cache of sessions (see
312L<SSL_CTX_sess_set_new_cb(3)> and similar functions). It is the application's
313responsibility to ensure that any sessions in the external cache are also
314populated in the internal cache and that once removed from the internal cache
315they are similarly removed from the external cache. Failing to do this could
316result in an application becoming vulnerable to replay attacks. Note that
317OpenSSL will lock the internal cache while a session is removed but that lock is
318not held when the remove session callback (see L<SSL_CTX_sess_set_remove_cb(3)>)
319is called. This could result in a small amount of time where the session has
320been removed from the internal cache but is still available in the external
321cache. Applications should be designed with this in mind in order to minimise
322the possibility of replay attacks.
323
324The OpenSSL replay protection does not apply to external Pre Shared Keys (PSKs)
325(e.g. see SSL_CTX_set_psk_find_session_callback(3)). Therefore, extreme caution
326should be applied when combining external PSKs with early data.
327
328Some applications may mitigate the replay risks in other ways. For those
329applications it is possible to turn off the built-in replay protection feature
330using the B<SSL_OP_NO_ANTI_REPLAY> option. See L<SSL_CTX_set_options(3)> for
331details. Applications can also set a callback to make decisions about accepting
332early data or not. See SSL_CTX_set_allow_early_data_cb() above for details.
333
334=head1 RETURN VALUES
335
336SSL_write_early_data() returns 1 for success or 0 for failure. In the event of a
337failure call L<SSL_get_error(3)> to determine the correct course of action.
338
339SSL_read_early_data() returns SSL_READ_EARLY_DATA_ERROR for failure,
340SSL_READ_EARLY_DATA_SUCCESS for success with more data to read and
341SSL_READ_EARLY_DATA_FINISH for success with no more to data be read. In the
342event of a failure call L<SSL_get_error(3)> to determine the correct course of
343action.
344
345SSL_get_max_early_data(), SSL_CTX_get_max_early_data() and
346SSL_SESSION_get_max_early_data() return the maximum number of early data bytes
347that may be sent.
348
349SSL_set_max_early_data(), SSL_CTX_set_max_early_data() and
350SSL_SESSION_set_max_early_data() return 1 for success or 0 for failure.
351
352SSL_get_early_data_status() returns SSL_EARLY_DATA_ACCEPTED if early data was
353accepted by the server, SSL_EARLY_DATA_REJECTED if early data was rejected by
354the server, or SSL_EARLY_DATA_NOT_SENT if no early data was sent.
355
356=head1 SEE ALSO
357
358L<SSL_get_error(3)>,
359L<SSL_write_ex(3)>,
360L<SSL_read_ex(3)>,
361L<SSL_connect(3)>,
362L<SSL_accept(3)>,
363L<SSL_do_handshake(3)>,
364L<SSL_CTX_set_psk_use_session_callback(3)>,
365L<ssl(7)>
366
367=head1 HISTORY
368
369All of the functions described above were added in OpenSSL 1.1.1.
370
371=head1 COPYRIGHT
372
373Copyright 2017-2023 The OpenSSL Project Authors. All Rights Reserved.
374
375Licensed under the Apache License 2.0 (the "License").  You may not use
376this file except in compliance with the License.  You can obtain a copy
377in the file LICENSE in the source distribution or at
378L<https://www.openssl.org/source/license.html>.
379
380=cut
381