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 only be sent if a session has previously been established with 62the server, and the server is known to support it. Additionally these functions 63can be used to send data from the server to the client when the client has not 64yet completed the authentication stage of the handshake. 65 66Early data has weaker security properties than other data sent over an SSL/TLS 67connection. In particular the data does not have forward secrecy. There are also 68additional considerations around replay attacks (see L<REPLAY PROTECTION> 69below). For these reasons extreme care should be exercised when using early 70data. For specific details, consult the TLS 1.3 specification. 71 72When a server receives early data it may opt to immediately respond by sending 73application data back to the client. Data sent by the server at this stage is 74done before the full handshake has been completed. Specifically the client's 75authentication messages have not yet been received, i.e. the client is 76unauthenticated at this point and care should be taken when using this 77capability. 78 79A server or client can determine whether the full handshake has been completed 80or not by calling L<SSL_is_init_finished(3)>. 81 82On the client side, the function SSL_SESSION_get_max_early_data() can be used to 83determine if a session established with a server can be used to send early data. 84If the session cannot be used then this function will return 0. Otherwise it 85will return the maximum number of early data bytes that can be sent. 86 87The function SSL_SESSION_set_max_early_data() sets the maximum number of early 88data bytes that can be sent for a session. This would typically be used when 89creating a PSK session file (see L<SSL_CTX_set_psk_use_session_callback(3)>). If 90using a ticket based PSK then this is set automatically to the value provided by 91the server. 92 93A client uses the function SSL_write_early_data() to send early data. This 94function is similar to the L<SSL_write_ex(3)> function, but with the following 95differences. See L<SSL_write_ex(3)> for information on how to write bytes to 96the underlying connection, and how to handle any errors that may arise. This 97page describes the differences between SSL_write_early_data() and 98L<SSL_write_ex(3)>. 99 100When called by a client, SSL_write_early_data() must be the first IO function 101called on a new connection, i.e. it must occur before any calls to 102L<SSL_write_ex(3)>, L<SSL_read_ex(3)>, L<SSL_connect(3)>, L<SSL_do_handshake(3)> 103or other similar functions. It may be called multiple times to stream data to 104the server, but the total number of bytes written must not exceed the value 105returned from SSL_SESSION_get_max_early_data(). Once the initial 106SSL_write_early_data() call has completed successfully the client may interleave 107calls to L<SSL_read_ex(3)> and L<SSL_read(3)> with calls to 108SSL_write_early_data() as required. 109 110If SSL_write_early_data() fails you should call L<SSL_get_error(3)> to determine 111the correct course of action, as for L<SSL_write_ex(3)>. 112 113When the client no longer wishes to send any more early data then it should 114complete the handshake by calling a function such as L<SSL_connect(3)> or 115L<SSL_do_handshake(3)>. Alternatively you can call a standard write function 116such as L<SSL_write_ex(3)>, which will transparently complete the connection and 117write the requested data. 118 119A server may choose to ignore early data that has been sent to it. Once the 120connection has been completed you can determine whether the server accepted or 121rejected the early data by calling SSL_get_early_data_status(). This will return 122SSL_EARLY_DATA_ACCEPTED if the data was accepted, SSL_EARLY_DATA_REJECTED if it 123was rejected or SSL_EARLY_DATA_NOT_SENT if no early data was sent. This function 124may be called by either the client or the server. 125 126A server uses the SSL_read_early_data() function to receive early data on a 127connection for which early data has been enabled using 128SSL_CTX_set_max_early_data() or SSL_set_max_early_data(). As for 129SSL_write_early_data(), this must be the first IO function 130called on a connection, i.e. it must occur before any calls to 131L<SSL_write_ex(3)>, L<SSL_read_ex(3)>, L<SSL_accept(3)>, L<SSL_do_handshake(3)>, 132or other similar functions. 133 134SSL_read_early_data() is similar to L<SSL_read_ex(3)> with the following 135differences. Refer to L<SSL_read_ex(3)> for full details. 136 137SSL_read_early_data() may return 3 possible values: 138 139=over 4 140 141=item SSL_READ_EARLY_DATA_ERROR 142 143This indicates an IO or some other error occurred. This should be treated in the 144same way as a 0 return value from L<SSL_read_ex(3)>. 145 146=item SSL_READ_EARLY_DATA_SUCCESS 147 148This indicates that early data was successfully read. This should be treated in 149the same way as a 1 return value from L<SSL_read_ex(3)>. You should continue to 150call SSL_read_early_data() to read more data. 151 152=item SSL_READ_EARLY_DATA_FINISH 153 154This indicates that no more early data can be read. It may be returned on the 155first call to SSL_read_early_data() if the client has not sent any early data, 156or if the early data was rejected. 157 158=back 159 160Once the initial SSL_read_early_data() call has completed successfully (i.e. it 161has returned SSL_READ_EARLY_DATA_SUCCESS or SSL_READ_EARLY_DATA_FINISH) then the 162server may choose to write data immediately to the unauthenticated client using 163SSL_write_early_data(). If SSL_read_early_data() returned 164SSL_READ_EARLY_DATA_FINISH then in some situations (e.g. if the client only 165supports TLSv1.2) the handshake may have already been completed and calls 166to SSL_write_early_data() are not allowed. Call L<SSL_is_init_finished(3)> to 167determine whether the handshake has completed or not. If the handshake is still 168in progress then the server may interleave calls to SSL_write_early_data() with 169calls to SSL_read_early_data() as required. 170 171Servers must not call L<SSL_read_ex(3)>, L<SSL_read(3)>, L<SSL_write_ex(3)> or 172L<SSL_write(3)> until SSL_read_early_data() has returned with 173SSL_READ_EARLY_DATA_FINISH. Once it has done so the connection to the client 174still needs to be completed. Complete the connection by calling a function such 175as L<SSL_accept(3)> or L<SSL_do_handshake(3)>. Alternatively you can call a 176standard read function such as L<SSL_read_ex(3)>, which will transparently 177complete the connection and read the requested data. Note that it is an error to 178attempt to complete the connection before SSL_read_early_data() has returned 179SSL_READ_EARLY_DATA_FINISH. 180 181Only servers may call SSL_read_early_data(). 182 183Calls to SSL_read_early_data() may, in certain circumstances, complete the 184connection immediately without further need to call a function such as 185L<SSL_accept(3)>. This can happen if the client is using a protocol version less 186than TLSv1.3. Applications can test for this by calling 187L<SSL_is_init_finished(3)>. Alternatively, applications may choose to call 188L<SSL_accept(3)> anyway. Such a call will successfully return immediately with no 189further action taken. 190 191When a session is created between a server and a client the server will specify 192the maximum amount of any early data that it will accept on any future 193connection attempt. By default the server does not accept early data; a 194server may indicate support for early data by calling 195SSL_CTX_set_max_early_data() or 196SSL_set_max_early_data() to set it for the whole SSL_CTX or an individual SSL 197object respectively. The B<max_early_data> parameter specifies the maximum 198amount of early data in bytes that is permitted to be sent on a single 199connection. Similarly the SSL_CTX_get_max_early_data() and 200SSL_get_max_early_data() functions can be used to obtain the current maximum 201early data settings for the SSL_CTX and SSL objects respectively. Generally a 202server application will either use both of SSL_read_early_data() and 203SSL_CTX_set_max_early_data() (or SSL_set_max_early_data()), or neither of them, 204since there is no practical benefit from using only one of them. If the maximum 205early data setting for a server is non-zero then replay protection is 206automatically enabled (see L</REPLAY PROTECTION> below). 207 208If the server rejects the early data sent by a client then it will skip over 209the data that is sent. The maximum amount of received early data that is skipped 210is controlled by the recv_max_early_data setting. If a client sends more than 211this then the connection will abort. This value can be set by calling 212SSL_CTX_set_recv_max_early_data() or SSL_set_recv_max_early_data(). The current 213value for this setting can be obtained by calling 214SSL_CTX_get_recv_max_early_data() or SSL_get_recv_max_early_data(). The default 215value for this setting is 16,384 bytes. 216 217The recv_max_early_data value also has an impact on early data that is accepted. 218The amount of data that is accepted will always be the lower of the 219max_early_data for the session and the recv_max_early_data setting for the 220server. If a client sends more data than this then the connection will abort. 221 222The configured value for max_early_data on a server may change over time as 223required. However clients may have tickets containing the previously configured 224max_early_data value. The recv_max_early_data should always be equal to or 225higher than any recently configured max_early_data value in order to avoid 226aborted connections. The recv_max_early_data should never be set to less than 227the current configured max_early_data value. 228 229Some server applications may wish to have more control over whether early data 230is accepted or not, for example to mitigate replay risks (see L</REPLAY PROTECTION> 231below) or to decline early_data when the server is heavily loaded. The functions 232SSL_CTX_set_allow_early_data_cb() and SSL_set_allow_early_data_cb() set a 233callback which is called at a point in the handshake immediately before a 234decision is made to accept or reject early data. The callback is provided with a 235pointer to the user data argument that was provided when the callback was first 236set. Returning 1 from the callback will allow early data and returning 0 will 237reject it. Note that the OpenSSL library may reject early data for other reasons 238in which case this callback will not get called. Notably, the built-in replay 239protection feature will still be used even if a callback is present unless it 240has been explicitly disabled using the SSL_OP_NO_ANTI_REPLAY option. See 241L</REPLAY PROTECTION> below. 242 243=head1 NOTES 244 245The whole purpose of early data is to enable a client to start sending data to 246the server before a full round trip of network traffic has occurred. Application 247developers should ensure they consider optimisation of the underlying TCP socket 248to obtain a performant solution. For example Nagle's algorithm is commonly used 249by operating systems in an attempt to avoid lots of small TCP packets. In many 250scenarios this is beneficial for performance, but it does not work well with the 251early data solution as implemented in OpenSSL. In Nagle's algorithm the OS will 252buffer outgoing TCP data if a TCP packet has already been sent which we have not 253yet received an ACK for from the peer. The buffered data will only be 254transmitted if enough data to fill an entire TCP packet is accumulated, or if 255the ACK is received from the peer. The initial ClientHello will be sent in the 256first TCP packet along with any data from the first call to 257SSL_write_early_data(). If the amount of data written will exceed the size of a 258single TCP packet, or if there are more calls to SSL_write_early_data() then 259that additional data will be sent in subsequent TCP packets which will be 260buffered by the OS and not sent until an ACK is received for the first packet 261containing the ClientHello. This means the early data is not actually 262sent until a complete round trip with the server has occurred which defeats the 263objective of early data. 264 265In many operating systems the TCP_NODELAY socket option is available to disable 266Nagle's algorithm. If an application opts to disable Nagle's algorithm 267consideration should be given to turning it back on again after the handshake is 268complete if appropriate. 269 270In rare circumstances, it may be possible for a client to have a session that 271reports a max early data value greater than 0, but where the server does not 272support this. For example, this can occur if a server has had its configuration 273changed to accept a lower max early data value such as by calling 274SSL_CTX_set_recv_max_early_data(). Another example is if a server used to 275support TLSv1.3 but was later downgraded to TLSv1.2. Sending early data to such 276a server will cause the connection to abort. Clients that encounter an aborted 277connection while sending early data may want to retry the connection without 278sending early data as this does not happen automatically. A client will have to 279establish a new transport layer connection to the server and attempt the SSL/TLS 280connection again but without sending early data. Note that it is inadvisable to 281retry with a lower maximum protocol version. 282 283=head1 REPLAY PROTECTION 284 285When early data is in use the TLS protocol provides no security guarantees that 286the same early data was not replayed across multiple connections. As a 287mitigation for this issue OpenSSL automatically enables replay protection if the 288server is configured with a non-zero max early data value. With replay 289protection enabled sessions are forced to be single use only. If a client 290attempts to reuse a session ticket more than once, then the second and 291subsequent attempts will fall back to a full handshake (and any early data that 292was submitted will be ignored). Note that single use tickets are enforced even 293if a client does not send any early data. 294 295The replay protection mechanism relies on the internal OpenSSL server session 296cache (see L<SSL_CTX_set_session_cache_mode(3)>). When replay protection is 297being used the server will operate as if the SSL_OP_NO_TICKET option had been 298selected (see L<SSL_CTX_set_options(3)>). Sessions will be added to the cache 299whenever a session ticket is issued. When a client attempts to resume the 300session, OpenSSL will check for its presence in the internal cache. If it exists 301then the resumption is allowed and the session is removed from the cache. If it 302does not exist then the resumption is not allowed and a full handshake will 303occur. 304 305Note that some applications may maintain an external cache of sessions (see 306L<SSL_CTX_sess_set_new_cb(3)> and similar functions). It is the application's 307responsibility to ensure that any sessions in the external cache are also 308populated in the internal cache and that once removed from the internal cache 309they are similarly removed from the external cache. Failing to do this could 310result in an application becoming vulnerable to replay attacks. Note that 311OpenSSL will lock the internal cache while a session is removed but that lock is 312not held when the remove session callback (see L<SSL_CTX_sess_set_remove_cb(3)>) 313is called. This could result in a small amount of time where the session has 314been removed from the internal cache but is still available in the external 315cache. Applications should be designed with this in mind in order to minimise 316the possibility of replay attacks. 317 318The OpenSSL replay protection does not apply to external Pre Shared Keys (PSKs) 319(e.g. see SSL_CTX_set_psk_find_session_callback(3)). Therefore extreme caution 320should be applied when combining external PSKs with early data. 321 322Some applications may mitigate the replay risks in other ways. For those 323applications it is possible to turn off the built-in replay protection feature 324using the B<SSL_OP_NO_ANTI_REPLAY> option. See L<SSL_CTX_set_options(3)> for 325details. Applications can also set a callback to make decisions about accepting 326early data or not. See SSL_CTX_set_allow_early_data_cb() above for details. 327 328=head1 RETURN VALUES 329 330SSL_write_early_data() returns 1 for success or 0 for failure. In the event of a 331failure call L<SSL_get_error(3)> to determine the correct course of action. 332 333SSL_read_early_data() returns SSL_READ_EARLY_DATA_ERROR for failure, 334SSL_READ_EARLY_DATA_SUCCESS for success with more data to read and 335SSL_READ_EARLY_DATA_FINISH for success with no more to data be read. In the 336event of a failure call L<SSL_get_error(3)> to determine the correct course of 337action. 338 339SSL_get_max_early_data(), SSL_CTX_get_max_early_data() and 340SSL_SESSION_get_max_early_data() return the maximum number of early data bytes 341that may be sent. 342 343SSL_set_max_early_data(), SSL_CTX_set_max_early_data() and 344SSL_SESSION_set_max_early_data() return 1 for success or 0 for failure. 345 346SSL_get_early_data_status() returns SSL_EARLY_DATA_ACCEPTED if early data was 347accepted by the server, SSL_EARLY_DATA_REJECTED if early data was rejected by 348the server, or SSL_EARLY_DATA_NOT_SENT if no early data was sent. 349 350=head1 SEE ALSO 351 352L<SSL_get_error(3)>, 353L<SSL_write_ex(3)>, 354L<SSL_read_ex(3)>, 355L<SSL_connect(3)>, 356L<SSL_accept(3)>, 357L<SSL_do_handshake(3)>, 358L<SSL_CTX_set_psk_use_session_callback(3)>, 359L<ssl(7)> 360 361=head1 HISTORY 362 363All of the functions described above were added in OpenSSL 1.1.1. 364 365=head1 COPYRIGHT 366 367Copyright 2017-2019 The OpenSSL Project Authors. All Rights Reserved. 368 369Licensed under the OpenSSL license (the "License"). You may not use 370this file except in compliance with the License. You can obtain a copy 371in the file LICENSE in the source distribution or at 372L<https://www.openssl.org/source/license.html>. 373 374=cut 375