1SSL_READ_EARLY_DATA(3) OpenSSL SSL_READ_EARLY_DATA(3)
2
6 SSL_set_max_early_data, SSL_CTX_set_max_early_data,
7 SSL_get_max_early_data, SSL_CTX_get_max_early_data,
8 SSL_set_recv_max_early_data, SSL_CTX_set_recv_max_early_data,
9 SSL_get_recv_max_early_data, SSL_CTX_get_recv_max_early_data,
10 SSL_SESSION_get_max_early_data, SSL_SESSION_set_max_early_data,
11 SSL_write_early_data, SSL_read_early_data, SSL_get_early_data_status,
12 SSL_allow_early_data_cb_fn, SSL_CTX_set_allow_early_data_cb,
13 SSL_set_allow_early_data_cb - functions for sending and receiving early
14 data
15
17 #include <openssl/ssl.h>
18 int SSL_CTX_set_max_early_data(SSL_CTX *ctx, uint32_t max_early_data);
20 uint32_t SSL_CTX_get_max_early_data(const SSL_CTX *ctx);
21 int SSL_set_max_early_data(SSL *s, uint32_t max_early_data);
22 uint32_t SSL_get_max_early_data(const SSL *s);
23 int SSL_CTX_set_recv_max_early_data(SSL_CTX *ctx, uint32_t recv_max_early_data);
25 uint32_t SSL_CTX_get_recv_max_early_data(const SSL_CTX *ctx);
26 int SSL_set_recv_max_early_data(SSL *s, uint32_t recv_max_early_data);
27 uint32_t SSL_get_recv_max_early_data(const SSL *s);
28 uint32_t SSL_SESSION_get_max_early_data(const SSL_SESSION *s);
30 int SSL_SESSION_set_max_early_data(SSL_SESSION *s, uint32_t max_early_data);
31 int SSL_write_early_data(SSL *s, const void *buf, size_t num, size_t *written);
33 int SSL_read_early_data(SSL *s, void *buf, size_t num, size_t *readbytes);
35 int SSL_get_early_data_status(const SSL *s);
37 typedef int (*SSL_allow_early_data_cb_fn)(SSL *s, void *arg);
40 void SSL_CTX_set_allow_early_data_cb(SSL_CTX *ctx,
42 SSL_allow_early_data_cb_fn cb,
43 void *arg);
44 void SSL_set_allow_early_data_cb(SSL *s,
45 SSL_allow_early_data_cb_fn cb,
46 void *arg);
47
49 These functions are used to send and receive early data where TLSv1.3
50 has been negotiated. Early data can be sent by the client immediately
51 after its initial ClientHello without having to wait for the server to
52 complete the handshake. Early data can be sent if a session has
53 previously been established with the server or when establishing a new
54 session using an out-of-band PSK, and only when the server is known to
55 support it. Additionally these functions can be used to send data from
56 the server to the client when the client has not yet completed the
57 authentication stage of the handshake.
58 Early data has weaker security properties than other data sent over an
60 SSL/TLS connection. In particular the data does not have forward
61 secrecy. There are also additional considerations around replay attacks
62 (see "REPLAY PROTECTION" below). For these reasons extreme care should
63 be exercised when using early data. For specific details, consult the
64 TLS 1.3 specification.
65 When a server receives early data it may opt to immediately respond by
67 sending application data back to the client. Data sent by the server at
68 this stage is done before the full handshake has been completed.
69 Specifically the client's authentication messages have not yet been
70 received, i.e. the client is unauthenticated at this point and care
71 should be taken when using this capability.
72 A server or client can determine whether the full handshake has been
74 completed or not by calling SSL_is_init_finished(3).
75 On the client side, the function SSL_SESSION_get_max_early_data() can
77 be used to determine if a session established with a server can be used
78 to send early data. If the session cannot be used then this function
79 will return 0. Otherwise it will return the maximum number of early
80 data bytes that can be sent.
81 The function SSL_SESSION_set_max_early_data() sets the maximum number
83 of early data bytes that can be sent for a session. This would
84 typically be used when creating a PSK session file (see
85 SSL_CTX_set_psk_use_session_callback(3)). If using a ticket based PSK
86 then this is set automatically to the value provided by the server.
87 A client uses the function SSL_write_early_data() to send early data.
89 This function is similar to the SSL_write_ex(3) function, but with the
90 following differences. See SSL_write_ex(3) for information on how to
91 write bytes to the underlying connection, and how to handle any errors
92 that may arise. This page describes the differences between
93 SSL_write_early_data() and SSL_write_ex(3).
94 When called by a client, SSL_write_early_data() must be the first IO
96 function called on a new connection, i.e. it must occur before any
97 calls to SSL_write_ex(3), SSL_read_ex(3), SSL_connect(3),
98 SSL_do_handshake(3) or other similar functions. It may be called
99 multiple times to stream data to the server, but the total number of
100 bytes written must not exceed the value returned from
101 SSL_SESSION_get_max_early_data(). Once the initial
102 SSL_write_early_data() call has completed successfully the client may
103 interleave calls to SSL_read_ex(3) and SSL_read(3) with calls to
104 SSL_write_early_data() as required.
105 If SSL_write_early_data() fails you should call SSL_get_error(3) to
107 determine the correct course of action, as for SSL_write_ex(3).
108 When the client no longer wishes to send any more early data then it
110 should complete the handshake by calling a function such as
111 SSL_connect(3) or SSL_do_handshake(3). Alternatively you can call a
112 standard write function such as SSL_write_ex(3), which will
113 transparently complete the connection and write the requested data.
114 A server may choose to ignore early data that has been sent to it. Once
116 the connection has been completed you can determine whether the server
117 accepted or rejected the early data by calling
118 SSL_get_early_data_status(). This will return SSL_EARLY_DATA_ACCEPTED
119 if the data was accepted, SSL_EARLY_DATA_REJECTED if it was rejected or
120 SSL_EARLY_DATA_NOT_SENT if no early data was sent. This function may be
121 called by either the client or the server.
122 A server uses the SSL_read_early_data() function to receive early data
124 on a connection for which early data has been enabled using
125 SSL_CTX_set_max_early_data() or SSL_set_max_early_data(). As for
126 SSL_write_early_data(), this must be the first IO function called on a
127 connection, i.e. it must occur before any calls to SSL_write_ex(3),
128 SSL_read_ex(3), SSL_accept(3), SSL_do_handshake(3), or other similar
129 functions.
130 SSL_read_early_data() is similar to SSL_read_ex(3) with the following
132 differences. Refer to SSL_read_ex(3) for full details.
133 SSL_read_early_data() may return 3 possible values:
135 SSL_READ_EARLY_DATA_ERROR
137 This indicates an IO or some other error occurred. This should be
138 treated in the same way as a 0 return value from SSL_read_ex(3).
139 SSL_READ_EARLY_DATA_SUCCESS
141 This indicates that early data was successfully read. This should
142 be treated in the same way as a 1 return value from SSL_read_ex(3).
143 You should continue to call SSL_read_early_data() to read more
144 data.
145 SSL_READ_EARLY_DATA_FINISH
147 This indicates that no more early data can be read. It may be
148 returned on the first call to SSL_read_early_data() if the client
149 has not sent any early data, or if the early data was rejected.
150 Once the initial SSL_read_early_data() call has completed successfully
152 (i.e. it has returned SSL_READ_EARLY_DATA_SUCCESS or
153 SSL_READ_EARLY_DATA_FINISH) then the server may choose to write data
154 immediately to the unauthenticated client using SSL_write_early_data().
155 If SSL_read_early_data() returned SSL_READ_EARLY_DATA_FINISH then in
156 some situations (e.g. if the client only supports TLSv1.2) the
157 handshake may have already been completed and calls to
158 SSL_write_early_data() are not allowed. Call SSL_is_init_finished(3) to
159 determine whether the handshake has completed or not. If the handshake
160 is still in progress then the server may interleave calls to
161 SSL_write_early_data() with calls to SSL_read_early_data() as required.
162 Servers must not call SSL_read_ex(3), SSL_read(3), SSL_write_ex(3) or
164 SSL_write(3) until SSL_read_early_data() has returned with
165 SSL_READ_EARLY_DATA_FINISH. Once it has done so the connection to the
166 client still needs to be completed. Complete the connection by calling
167 a function such as SSL_accept(3) or SSL_do_handshake(3). Alternatively
168 you can call a standard read function such as SSL_read_ex(3), which
169 will transparently complete the connection and read the requested data.
170 Note that it is an error to attempt to complete the connection before
171 SSL_read_early_data() has returned SSL_READ_EARLY_DATA_FINISH.
172 Only servers may call SSL_read_early_data().
174 Calls to SSL_read_early_data() may, in certain circumstances, complete
176 the connection immediately without further need to call a function such
177 as SSL_accept(3). This can happen if the client is using a protocol
178 version less than TLSv1.3. Applications can test for this by calling
179 SSL_is_init_finished(3). Alternatively, applications may choose to call
180 SSL_accept(3) anyway. Such a call will successfully return immediately
181 with no further action taken.
182 When a session is created between a server and a client the server will
184 specify the maximum amount of any early data that it will accept on any
185 future connection attempt. By default the server does not accept early
186 data; a server may indicate support for early data by calling
187 SSL_CTX_set_max_early_data() or SSL_set_max_early_data() to set it for
188 the whole SSL_CTX or an individual SSL object respectively. The
189 max_early_data parameter specifies the maximum amount of early data in
190 bytes that is permitted to be sent on a single connection. Similarly
191 the SSL_CTX_get_max_early_data() and SSL_get_max_early_data() functions
192 can be used to obtain the current maximum early data settings for the
193 SSL_CTX and SSL objects respectively. Generally a server application
194 will either use both of SSL_read_early_data() and
195 SSL_CTX_set_max_early_data() (or SSL_set_max_early_data()), or neither
196 of them, since there is no practical benefit from using only one of
197 them. If the maximum early data setting for a server is nonzero then
198 replay protection is automatically enabled (see "REPLAY PROTECTION"
199 below).
200 If the server rejects the early data sent by a client then it will skip
202 over the data that is sent. The maximum amount of received early data
203 that is skipped is controlled by the recv_max_early_data setting. If a
204 client sends more than this then the connection will abort. This value
205 can be set by calling SSL_CTX_set_recv_max_early_data() or
206 SSL_set_recv_max_early_data(). The current value for this setting can
207 be obtained by calling SSL_CTX_get_recv_max_early_data() or
208 SSL_get_recv_max_early_data(). The default value for this setting is
209 16,384 bytes.
210 The recv_max_early_data value also has an impact on early data that is
212 accepted. The amount of data that is accepted will always be the lower
213 of the max_early_data for the session and the recv_max_early_data
214 setting for the server. If a client sends more data than this then the
215 connection will abort.
216 The configured value for max_early_data on a server may change over
218 time as required. However, clients may have tickets containing the
219 previously configured max_early_data value. The recv_max_early_data
220 should always be equal to or higher than any recently configured
221 max_early_data value in order to avoid aborted connections. The
222 recv_max_early_data should never be set to less than the current
223 configured max_early_data value.
224 Some server applications may wish to have more control over whether
226 early data is accepted or not, for example to mitigate replay risks
227 (see "REPLAY PROTECTION" below) or to decline early_data when the
228 server is heavily loaded. The functions
229 SSL_CTX_set_allow_early_data_cb() and SSL_set_allow_early_data_cb() set
230 a callback which is called at a point in the handshake immediately
231 before a decision is made to accept or reject early data. The callback
232 is provided with a pointer to the user data argument that was provided
233 when the callback was first set. Returning 1 from the callback will
234 allow early data and returning 0 will reject it. Note that the OpenSSL
235 library may reject early data for other reasons in which case this
236 callback will not get called. Notably, the built-in replay protection
237 feature will still be used even if a callback is present unless it has
238 been explicitly disabled using the SSL_OP_NO_ANTI_REPLAY option. See
239 "REPLAY PROTECTION" below.
240
242 The whole purpose of early data is to enable a client to start sending
243 data to the server before a full round trip of network traffic has
244 occurred. Application developers should ensure they consider
245 optimisation of the underlying TCP socket to obtain a performant
246 solution. For example Nagle's algorithm is commonly used by operating
247 systems in an attempt to avoid lots of small TCP packets. In many
248 scenarios this is beneficial for performance, but it does not work well
249 with the early data solution as implemented in OpenSSL. In Nagle's
250 algorithm the OS will buffer outgoing TCP data if a TCP packet has
251 already been sent which we have not yet received an ACK for from the
252 peer. The buffered data will only be transmitted if enough data to fill
253 an entire TCP packet is accumulated, or if the ACK is received from the
254 peer. The initial ClientHello will be sent in the first TCP packet
255 along with any data from the first call to SSL_write_early_data(). If
256 the amount of data written will exceed the size of a single TCP packet,
257 or if there are more calls to SSL_write_early_data() then that
258 additional data will be sent in subsequent TCP packets which will be
259 buffered by the OS and not sent until an ACK is received for the first
260 packet containing the ClientHello. This means the early data is not
261 actually sent until a complete round trip with the server has occurred
262 which defeats the objective of early data.
263 In many operating systems the TCP_NODELAY socket option is available to
265 disable Nagle's algorithm. If an application opts to disable Nagle's
266 algorithm consideration should be given to turning it back on again
267 after the handshake is complete if appropriate.
268 In rare circumstances, it may be possible for a client to have a
270 session that reports a max early data value greater than 0, but where
271 the server does not support this. For example, this can occur if a
272 server has had its configuration changed to accept a lower max early
273 data value such as by calling SSL_CTX_set_recv_max_early_data().
274 Another example is if a server used to support TLSv1.3 but was later
275 downgraded to TLSv1.2. Sending early data to such a server will cause
276 the connection to abort. Clients that encounter an aborted connection
277 while sending early data may want to retry the connection without
278 sending early data as this does not happen automatically. A client will
279 have to establish a new transport layer connection to the server and
280 attempt the SSL/TLS connection again but without sending early data.
281 Note that it is inadvisable to retry with a lower maximum protocol
282 version.
283
285 When early data is in use the TLS protocol provides no security
286 guarantees that the same early data was not replayed across multiple
287 connections. As a mitigation for this issue OpenSSL automatically
288 enables replay protection if the server is configured with a nonzero
289 max early data value. With replay protection enabled sessions are
290 forced to be single use only. If a client attempts to reuse a session
291 ticket more than once, then the second and subsequent attempts will
292 fall back to a full handshake (and any early data that was submitted
293 will be ignored). Note that single use tickets are enforced even if a
294 client does not send any early data.
295 The replay protection mechanism relies on the internal OpenSSL server
297 session cache (see SSL_CTX_set_session_cache_mode(3)). When replay
298 protection is being used the server will operate as if the
299 SSL_OP_NO_TICKET option had been selected (see SSL_CTX_set_options(3)).
300 Sessions will be added to the cache whenever a session ticket is
301 issued. When a client attempts to resume the session, OpenSSL will
302 check for its presence in the internal cache. If it exists then the
303 resumption is allowed and the session is removed from the cache. If it
304 does not exist then the resumption is not allowed and a full handshake
305 will occur.
306 Note that some applications may maintain an external cache of sessions
308 (see SSL_CTX_sess_set_new_cb(3) and similar functions). It is the
309 application's responsibility to ensure that any sessions in the
310 external cache are also populated in the internal cache and that once
311 removed from the internal cache they are similarly removed from the
312 external cache. Failing to do this could result in an application
313 becoming vulnerable to replay attacks. Note that OpenSSL will lock the
314 internal cache while a session is removed but that lock is not held
315 when the remove session callback (see SSL_CTX_sess_set_remove_cb(3)) is
316 called. This could result in a small amount of time where the session
317 has been removed from the internal cache but is still available in the
318 external cache. Applications should be designed with this in mind in
319 order to minimise the possibility of replay attacks.
320 The OpenSSL replay protection does not apply to external Pre Shared
322 Keys (PSKs) (e.g. see SSL_CTX_set_psk_find_session_callback(3)).
323 Therefore, extreme caution should be applied when combining external
324 PSKs with early data.
325 Some applications may mitigate the replay risks in other ways. For
327 those applications it is possible to turn off the built-in replay
328 protection feature using the SSL_OP_NO_ANTI_REPLAY option. See
329 SSL_CTX_set_options(3) for details. Applications can also set a
330 callback to make decisions about accepting early data or not. See
331 SSL_CTX_set_allow_early_data_cb() above for details.
332
334 SSL_write_early_data() returns 1 for success or 0 for failure. In the
335 event of a failure call SSL_get_error(3) to determine the correct
336 course of action.
337 SSL_read_early_data() returns SSL_READ_EARLY_DATA_ERROR for failure,
339 SSL_READ_EARLY_DATA_SUCCESS for success with more data to read and
340 SSL_READ_EARLY_DATA_FINISH for success with no more to data be read. In
341 the event of a failure call SSL_get_error(3) to determine the correct
342 course of action.
343 SSL_get_max_early_data(), SSL_CTX_get_max_early_data() and
345 SSL_SESSION_get_max_early_data() return the maximum number of early
346 data bytes that may be sent.
347 SSL_set_max_early_data(), SSL_CTX_set_max_early_data() and
349 SSL_SESSION_set_max_early_data() return 1 for success or 0 for failure.
350 SSL_get_early_data_status() returns SSL_EARLY_DATA_ACCEPTED if early
352 data was accepted by the server, SSL_EARLY_DATA_REJECTED if early data
353 was rejected by the server, or SSL_EARLY_DATA_NOT_SENT if no early data
354 was sent.
355
357 SSL_get_error(3), SSL_write_ex(3), SSL_read_ex(3), SSL_connect(3),
358 SSL_accept(3), SSL_do_handshake(3),
359 SSL_CTX_set_psk_use_session_callback(3), ssl(7)
360
362 All of the functions described above were added in OpenSSL 1.1.1.
363
365 Copyright 2017-2020 The OpenSSL Project Authors. All Rights Reserved.
366 Licensed under the OpenSSL license (the "License"). You may not use
368 this file except in compliance with the License. You can obtain a copy
369 in the file LICENSE in the source distribution or at
370 <https://www.openssl.org/source/license.html>.
3711.1.1i 2021-01-26 SSL_READ_EARLY_DATA(3)