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
