=pod =head1 NAME SSL_set_max_early_data, SSL_CTX_set_max_early_data, SSL_get_max_early_data, SSL_CTX_get_max_early_data, SSL_SESSION_get_max_early_data, SSL_write_early, SSL_read_early_data, SSL_get_early_data_status - functions for sending and receiving early data =head1 SYNOPSIS #include 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_CTX *s); uint32_t SSL_SESSION_get_max_early_data(const SSL_SESSION *s); int SSL_write_early(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); =head1 DESCRIPTION These functions are used to send and recieve early data. 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 only be sent if a session has previously been established with the server, and the server is known to support it. Early data has weaker security properties than other data sent over an SSL/TLS connection. In particular the data is not forward secret and the server has no guarantees that the same early data was not replayed across multiple connections. For this reason extreme care should be exercised when using early data. 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. A server or client can determine whether the full handshake has been completed or not by calling L. [[TODO(TLS1.3): The server uses SSL_write_ex()/SSL_write() to send data to an unauthenticated client. Should we create a separate function for this to avoid accidents??]] On the client side the function SSL_SESSION_get_max_early_data() can be used to determine whether 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. A client uses the function SSL_write_early() to send early data. This function works in the same way as the L function, but with a few differences. Refer to the L documentation for information on how to write bytes to the underlying connection, and how to handle any errors that may arise. This page will detail the differences between SSL_write_early() and L. SSL_write_early() must be the first IO function called on a new connection, i.e. it must occur before any calls to L, L, L, L 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() call has completed successfully the client may interleave calls to L and L with calls to SSL_write_early() as required. If SSL_write_early() fails you should call L to determine the correct course of action, as for L. When the client no longer wishes to send any more early data then it should complete the handshake by calling a function such as L or L. Alternatively you can call a standard write function such as L, which will transparently complete the connection and write the requested data. Only clients may call SSL_write_early(). 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. As for SSL_write_early() this must be the first IO function called on a connection, i.e. it must occur before any calls to L, L, L, L, or other similar functions. SSL_read_early_data() works in the same way as L except for the differences noted here. Refer to the L documentation for full details. SSL_read_early_data() may return 3 possible values: =over 4 =item SSL_READ_EARLY_DATA_ERROR This indicates an IO or some other error occured. This should be treated in the same way as a 0 return value from L. =item 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 L. You should continue to call SSL_read_early_data() to read more data. =item 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. =back Once the initial SSL_write_early() call has completed successfully the client may interleave calls to L and L with calls to SSL_read_early_data() as required. As noted above data sent via L or L in this way is sent to an unauthenticated client. Servers must not call L or L 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 L or L. Alternatively you can call a standard read function such as L, 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 L. Applications can test for this by calling L. Alternatively, applications may choose to call L anway. 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 this is approximately 16k. A server may override this default value 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. 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. In the event that the current maximum early data setting for the server is different to that originally specified in a session that a client is resuming with then the lower of the two values will apply. =head1 RETURN VALUES SSL_write_early() returns 1 for success or 0 for failure. In the event of a failure call L 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 no more to data be read. In the event of a failure call L 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() and SSL_CTX_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. =head1 SEE ALSO L, L, L, L, L, L, L =head1 HISTORY All of the functions described above were added in OpenSSL 1.1.1. =head1 COPYRIGHT Copyright 2017 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 L. =cut